openSUSE 12.2

Security Guide

Publication Date 28 Sep 2012

Copyright © 2006–2012 Novell, Inc. and contributors. All rights reserved.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or (at your option) version 1.3; with the Invariant Section being this copyright notice and license. A copy of the license version 1.2 is included in the section entitled GNU Free Documentation License.

For Novell trademarks, see the Novell Trademark and Service Mark list All other third party trademarks are the property of their respective owners. A trademark symbol (®, ™ etc.) denotes a Novell trademark; an asterisk (*) denotes a third party trademark.

All information found in this book has been compiled with utmost attention to detail. However, this does not guarantee complete accuracy. Neither Novell, Inc., SUSE LINUX Products GmbH, the authors, nor the translators shall be held liable for possible errors or the consequences thereof.


About This Guide
1. Available Documentation
2. Feedback
3. Documentation Conventions
4. About the Making of This Manual
5. Source Code
6. Acknowledgments
1. Security and Confidentiality
1.1. Local Security and Network Security
1.2. Some General Security Tips and Tricks
1.3. Using the Central Security Reporting Address
I. Authentication
2. Authentication with PAM
2.1. What is PAM?
2.2. Structure of a PAM Configuration File
2.3. The PAM Configuration of sshd
2.4. Configuration of PAM Modules
2.5. Configuring PAM Using pam-config
2.6. Manually Configuring PAM
2.7. For More Information
3. Using NIS
3.1. Configuring NIS Servers
3.2. Configuring NIS Clients
4. LDAP—A Directory Service
4.1. LDAP versus NIS
4.2. Structure of an LDAP Directory Tree
4.3. Configuring an LDAP Server with YaST
4.4. Configuring an LDAP Client with YaST
4.5. Configuring LDAP Users and Groups in YaST
4.6. Browsing the LDAP Directory Tree
4.7. Manually Configuring an LDAP Server
4.8. Manually Administering LDAP Data
4.9. For More Information
5. Active Directory Support
5.1. Integrating Linux and AD Environments
5.2. Background Information for Linux AD Support
5.3. Configuring a Linux Client for Active Directory
5.4. Logging In to an AD Domain
5.5. Changing Passwords
6. Network Authentication with Kerberos
6.1. Kerberos Terminology
6.2. How Kerberos Works
6.3. Users' View of Kerberos
6.4. Installing and Administering Kerberos
6.5. For More Information
7. Using the Fingerprint Reader
7.1. Supported Applications and Actions
7.2. Managing Fingerprints with YaST
II. Local Security
8. Configuring Security Settings with YaST
8.1. Security Overview
8.2. Predefined Security Configurations
8.3. Password Settings
8.4. Boot Settings
8.5. Login Settings
8.6. User Addition
8.7. Miscellaneous Settings
9. Access Control Lists in Linux
9.1. Traditional File Permissions
9.2. Advantages of ACLs
9.3. Definitions
9.4. Handling ACLs
9.5. ACL Support in Applications
9.6. For More Information
10. Encrypting Partitions and Files
10.1. Setting Up an Encrypted File System with YaST
10.2. Using Encrypted Home Directories
10.3. Using vi to Encrypt Single ASCII Text Files
11. Intrusion Detection with AIDE
11.1. Why Using AIDE?
11.2. Setting Up an AIDE Database
11.3. Local AIDE Checks
11.4. System Independent Checking
11.5. For More Information
III. Network Security
12. SSH: Secure Network Operations
12.1. ssh—Secure Shell
12.2. scp—Secure Copy
12.3. sftp—Secure File Transfer
12.4. The SSH Daemon (sshd)
12.5. SSH Authentication Mechanisms
12.6. Port Forwarding
12.7. Configuring An SSH Daemon with YaST
12.8. For More Information
13. Masquerading and Firewalls
13.1. Packet Filtering with iptables
13.2. Masquerading Basics
13.3. Firewalling Basics
13.4. SuSEfirewall2
13.5. For More Information
14. Configuring VPN Server
14.1. Conceptual Overview
14.2. Creating the Simplest VPN Example
14.3. Setting Up Your VPN Server Using Certificate Authority
14.4. Changing Nameservers in VPN
14.5. KDE- and GNOME Applets For Clients
14.6. For More Information
15. Managing X.509 Certification
15.1. The Principles of Digital Certification
15.2. YaST Modules for CA Management
15.3. For More Information
IV. Confining Privileges with AppArmor
16. Introducing AppArmor
16.1. Background Information on AppArmor Profiling
17. Getting Started
17.1. Installing AppArmor
17.2. Enabling and Disabling AppArmor
17.3. Choosing the Applications to Profile
17.4. Building and Modifying Profiles
17.5. Updating Your Profiles
18. Immunizing Programs
18.1. Introducing the AppArmor Framework
18.2. Determining Programs to Immunize
18.3. Immunizing cron Jobs
18.4. Immunizing Network Applications
19. Profile Components and Syntax
19.1. Breaking a AppArmor Profile into Its Parts
19.2. Profile Types
19.3. #include Statements
19.4. Capability Entries (POSIX.1e)
19.5. Network Access Control
19.6. Paths and Globbing
19.7. File Permission Access Modes
19.8. Execute Modes
19.9. Resource Limit Control
19.10. Auditing Rules
20. AppArmor Profile Repositories
20.1. Using the Local Repository
21. Building and Managing Profiles with YaST
21.1. Adding a Profile Using the Wizard
21.2. Manually Adding a Profile
21.3. Editing Profiles
21.4. Deleting a Profile
21.5. Updating Profiles from Log Entries
21.6. Managing AppArmor
22. Building Profiles from the Command Line
22.1. Checking the AppArmor Module Status
22.2. Building AppArmor Profiles
22.3. Adding or Creating an AppArmor Profile
22.4. Editing an AppArmor Profile
22.5. Deleting an AppArmor Profile
22.6. Two Methods of Profiling
22.7. Important Filenames and Directories
23. Profiling Your Web Applications Using ChangeHat
23.1. Apache ChangeHat
23.2. Configuring Apache for mod_apparmor
24. Confining Users with pam_apparmor
25. Managing Profiled Applications
25.1. Reacting to Security Event Rejections
25.2. Maintaining Your Security Profiles
26. Support
26.1. Updating AppArmor Online
26.2. Using the Man Pages
26.3. For More Information
26.4. Troubleshooting
26.5. Reporting Bugs for AppArmor
27. AppArmor Glossary
A. GNU Licenses
A.1. GNU General Public License
A.2. GNU Free Documentation License

List of Figures

3.1. NIS Server Setup
3.2. Master Server Setup
3.3. Changing the Directory and Synchronizing Files for a NIS Server
3.4. NIS Server Maps Setup
3.5. Setting Request Permissions for a NIS Server
3.6. Setting Domain and Address of a NIS Server
4.1. Structure of an LDAP Directory
4.2. YaST LDAP Server Configuration
4.3. YaST LDAP Server—New Database
4.4. YaST LDAP Server Configuration
4.5. YaST LDAP Server Database Configuration
4.6. YaST: LDAP Client Configuration
4.7. YaST: Advanced Configuration
4.8. YaST: Module Configuration
4.9. YaST: Configuration of an Object Template
4.10. YaST: Additional LDAP Settings
4.11. Browsing the LDAP Directory Tree
4.12. Browsing the Entry Data
5.1. Active Directory Authentication Schema
5.2. Determining Windows Domain Membership
5.3. Providing Administrator Credentials
6.1. Kerberos Network Topology
6.2. YaST: Basic Configuration of a Kerberos Client
6.3. YaST: Advanced Configuration of a Kerberos Client
8.1. YaST Security Center and Hardening - Security Overview
9.1. Minimum ACL: ACL Entries Compared to Permission Bits
9.2. Extended ACL: ACL Entries Compared to Permission Bits
13.1. iptables: A Packet's Possible Paths
13.2. The YaST Firewall Configuration
14.1. Routed VPN
14.2. Bridged VPN - Scenario 1
14.3. Bridged VPN - Scenario 2
14.4. Bridged VPN - Scenario 3
15.1. YaST CA Module—Basic Data for a Root CA
15.2. YaST CA Module—Using a CA
15.3. Certificates of a CA
15.4. YaST CA Module—Extended Settings
21.1. YaST Controls for AppArmor
21.2. Learning Mode Exception: Controlling Access to Specific Resources
21.3. Learning Mode Exception: Defining Execute Permissions for an Entry

List of Tables

4.1. Commonly Used Object Classes and Attributes
9.1. ACL Entry Types
9.2. Masking Access Permissions
11.1. Important AIDE Checking Options
15.1. X.509v3 Certificate
15.2. X.509 Certificate Revocation List (CRL)
15.3. Passwords during LDAP Export
26.1. Man Pages: Sections and Categories

List of Examples

2.1. PAM Configuration for sshd (/etc/pam.d/sshd)
2.2. Default Configuration for the auth Section (common-auth)
2.3. Default Configuration for the account Section (common-account)
2.4. Default Configuration for the password Section (common-password)
2.5. Default Configuration for the session Section (common-session)
2.6. pam_env.conf
4.1. Excerpt from schema.core
4.2. An LDIF File
4.3. ldapadd with example.ldif
4.4. LDIF Data for Tux
4.5. Modified LDIF File tux.ldif
14.1. VPN Server Configuration File
14.2. VPN Client Configuration File
17.1. Output of aa-unconfined
22.1. Learning Mode Exception: Controlling Access to Specific Resources
22.2. Learning Mode Exception: Defining Execute Permissions for an Entry
23.1. Example phpsysinfo Hat

About This Guide

This manual introduces the basic concepts of system security on openSUSE. It covers extensive documentation about the authentication mechanisms available on Linux, such as NIS or LDAP. It also deals with aspects of local security like access control lists, encryption and intrusion detection. In the network security part you learn how to secure your computers with firewalls and masquerading, and how to set up virtual private networks (VPN). This manual also shows you how to make use of the product's inherent security software like AppArmor (which lets you specify per program which files the program may read, write, and execute).

Many chapters in this manual contain links to additional documentation resources. These include additional documentation that is available on the system, as well as documentation available on the Internet.

For an overview of the documentation available for your product and the latest documentation updates, refer to or to the following section.

1. Available Documentation

We provide HTML and PDF versions of our books in different languages. The following manuals for users and administrators are available on this product:

Start-Up (↑Start-Up)

Guides you step-by-step through the installation of openSUSE from DVD, or from an ISO image, gives short introductions to the GNOME and KDE desktops including some key applications running on it. Also gives an overview of LibreOffice and its modules for writing texts, working with spreadsheets, or creating graphics and presentations.

Reference (↑Reference)

Gives you a general understanding of openSUSE and covers advanced system administration tasks. It is intended mainly for system administrators and home users with basic system administration knowledge. It provides detailed information about advanced deployment scenarios, administration of your system, the interaction of key system components and the set-up of various network and file services openSUSE offers.

Security Guide

Introduces basic concepts of system security, covering both local and network security aspects. Shows how to make use of the product inherent security software like AppArmor (which lets you specify per program which files the program may read, write, and execute) or the auditing system that reliably collects information about any security-relevant events.

System Analysis and Tuning Guide (↑System Analysis and Tuning Guide)

An administrator's guide for problem detection, resolution and optimization. Find how to inspect and optimize your system by means of monitoring tools and how to efficiently manage resources. Also contains an overview of common problems and solutions and of additional help and documentation resources.

Virtualization with KVM (↑Virtualization with KVM)

This manual offers an introduction to setting up and managing virtualization with KVM (Kernel-based Virtual Machine) on openSUSE. Also shows how to manage VM Guests with libvirt and QEMU.

Find HTML versions of most product manuals in your installed system under /usr/share/doc/manual or in the help centers of your desktop. Find the latest documentation updates at where you can download PDF or HTML versions of the manuals for your product.

2. Feedback

Several feedback channels are available:

Bugs and Enhancement Requests

To report bugs for a product component, or to submit enhancement requests, please use For documentation bugs, submit a bug report for the component Documentation of the respective product.

If you are new to Bugzilla, you might find the following articles helpful:

User Comments

We want to hear your comments about and suggestions for this manual and the other documentation included with this product. Use the User Comments feature at the bottom of each page in the online documentation or go to and enter your comments there.


For feedback on the documentation of this product, you can also send a mail to Make sure to include the document title, the product version and the publication date of the documentation. To report errors or suggest enhancements, provide a concise description of the problem and refer to the respective section number and page (or URL).

3. Documentation Conventions

The following typographical conventions are used in this manual:

  • /etc/passwd: directory names and filenames

  • placeholder: replace placeholder with the actual value

  • PATH: the environment variable PATH

  • ls, --help: commands, options, and parameters

  • user: users or groups

  • Alt, Alt+F1: a key to press or a key combination; keys are shown in uppercase as on a keyboard

  • File, File+Save As: menu items, buttons

  • Dancing Penguins (Chapter Penguins, ↑Another Manual): This is a reference to a chapter in another manual.

4. About the Making of This Manual

This book is written in Novdoc, a subset of DocBook (see The XML source files were validated by xmllint, processed by xsltproc, and converted into XSL-FO using a customized version of Norman Walsh's stylesheets. The final PDF is formatted through XEP from RenderX. The open source tools and the environment used to build this manual are available in the package daps that is shipped with openSUSE. The project's home page can be found at

5. Source Code

The source code of openSUSE is publicly available. Refer to for download links and more information.

6. Acknowledgments

With a lot of voluntary commitment, the developers of Linux cooperate on a global scale to promote the development of Linux. We thank them for their efforts—this distribution would not exist without them. Furthermore, we thank Frank Zappa and Pawar. Special thanks, of course, goes to Linus Torvalds.

Have a lot of fun!

Your SUSE Team

Chapter 1. Security and Confidentiality

One of the main characteristics of a Linux or UNIX system is its ability to handle several users at the same time (multiuser) and to allow these users to perform several tasks (multitasking) on the same computer simultaneously. Moreover, the operating system is network transparent. The users often do not know whether the data and applications they are using are provided locally from their machine or made available over the network.

With the multiuser capability, the data of different users must be stored separately, and security and privacy need to be guaranteed. Data security was already an important issue, even before computers could be linked through networks. Just like today, the most important concern was the ability to keep data available in spite of a lost or otherwise damaged data medium (a hard disk in most cases).

This section is primarily focused on confidentiality issues and on ways to protect the privacy of users, but it cannot be stressed enough that a comprehensive security concept should always include procedures to have a regularly updated, workable, and tested backup in place. Without this, you could have a very hard time getting your data back—not only in the case of some hardware defect, but also in the case that someone has gained unauthorized access and tampered with files.

1.1. Local Security and Network Security

There are several ways of accessing data:

  • personal communication with people who have the desired information or access to the data on a computer

  • directly through physical access from the console of a computer

  • over a serial line

  • using a network link

In all these cases, a user should be authenticated before accessing the resources or data in question. A Web server might be less restrictive in this respect, but you still would not want it to disclose your personal data to an anonymous user.

In the list above, the first case is the one where the highest amount of human interaction is involved (such as when you are contacting a bank employee and are required to prove that you are the person owning that bank account). Then, you are asked to provide a signature, a PIN, or a password to prove that you are the person you claim to be. In some cases, it might be possible to elicit some intelligence from an informed person just by mentioning known bits and pieces to win the confidence of that person. The victim could be led to reveal gradually more information, maybe without even being aware of it. Among hackers, this is called social engineering. You can only guard against this by educating people and by dealing with language and information in a conscious way. Before breaking into computer systems, attackers often try to target receptionists, service people working with the company, or even family members. In many cases, such an attack based on social engineering is only discovered at a much later time.

A person wanting to obtain unauthorized access to your data could also use the traditional way and try to get at your hardware directly. Therefore, the machine should be protected against any tampering so that no one can remove, replace, or cripple its components. This also applies to backups and even any network cables or power cords. Also secure the boot procedure, because there are some well-known key combinations that might provoke unusual behavior. Protect yourself against this by setting passwords for the BIOS and the boot loader.

Serial terminals connected to serial ports are still used in many places. Unlike network interfaces, they do not rely on network protocols to communicate with the host. A simple cable or an infrared port is used to send plain characters back and forth between the devices. The cable itself is the weakest point of such a system: with an older printer connected to it, it is easy to record any data being transferred thusly. What can be achieved with a printer can also be accomplished in other ways, depending on the effort that goes into the attack.

Reading a file locally on a host requires additional access rules than opening a network connection with a server on a different host. There is a distinction between local security and network security. The line is drawn where data must be put into packets to be sent somewhere else.

1.1.1. Local Security

Local security starts with the physical environment at the location in which computer is running. Set up your machine in a place where security is in line with your expectations and needs. The main goal of local security is to keep users separate from each other, so no user can assume the permissions or the identity of another. This is a general rule to be observed, but it is especially true for the user root, who holds system administration privileges. root can take on the identity of any other local user and read any locally-stored file without being prompted for the password. Passwords

On a Linux system, passwords are not stored as plain text and the entered text string is not simply matched with the saved pattern. If this were the case, all accounts on your system would be compromised as soon as someone got access to the corresponding file. Instead, the stored password is encrypted and, each time it is entered, is encrypted again and the two encrypted strings are compared. This only provides more security if the encrypted password cannot be reverse-computed into the original text string.

This is actually achieved by a special kind of algorithm, also called trapdoor algorithm, because it only works in one direction. An attacker who has obtained the encrypted string is not able to get your password by simply applying the same algorithm again. Instead, it would be necessary to test all the possible character combinations until a combination is found that looks like your password when encrypted. With passwords eight characters long, there are quite a number of possible combinations to calculate.

In the seventies, it was argued that this method would be more secure than others due to the relative slowness of the algorithm used, which took a few seconds to encrypt just one password. In the meantime, however, PCs have become powerful enough to do several hundred thousand or even millions of encryptions per second. Because of this, encrypted passwords should not be visible to regular users (/etc/shadow cannot be read by normal users). It is even more important that passwords are not easy to guess, in case the password file becomes visible due to some error. Consequently, it is not really useful to translate a password like tantalize into t@nt@1lz3.

Replacing some letters of a word with similar looking numbers (like writing the password tantalize as t@nt@1lz3) is not sufficient. Password cracking programs that use dictionaries to guess words also play with substitutions like that. A better way is to make up a word with no common meaning, something that only makes sense to you personally, like the first letters of the words of a sentence or the title of a book, such as The Name of the Rose by Umberto Eco. This would give the following safe password: TNotRbUE9. In contrast, passwords like beerbuddy or jasmine76 are easily guessed even by someone who has only some casual knowledge about you. The Boot Procedure

Configure your system so it cannot be booted from a floppy or from a CD, either by removing the drives entirely or by setting a BIOS password and configuring the BIOS to allow booting from a hard disk only. Normally, a Linux system is started by a boot loader, allowing you to pass additional options to the booted kernel. Prevent others from using such parameters during boot by setting an additional password in /boot/grub/menu.lst (see Chapter 9, The Boot Loader GRUB (↑Reference)). This is crucial to your system's security. Not only does the kernel itself run with root permissions, but it is also the first authority to grant root permissions at system start-up. File Permissions

As a general rule, always work with the most restrictive privileges possible for a given task. For example, it is definitely not necessary to be root to read or write e-mail. If the mail program has a bug, this bug could be exploited for an attack that acts with exactly the permissions of the program when it was started. By following the above rule, minimize the possible damage.

The permissions of all files included in the openSUSE distribution are carefully chosen. A system administrator who installs additional software or other files should take great care when doing so, especially when setting the permission bits. Experienced and security-conscious system administrators always use the -l option with the command ls to get an extensive file list, which allows them to detect any incorrect file permissions immediately. An incorrect file attribute does not only mean that files could be changed or deleted. These modified files could be executed by root or, in the case of configuration files, programs could use such files with the permissions of root. This significantly increases the possibilities of an attack. Attacks like these are called cuckoo eggs, because the program (the egg) is executed (hatched) by a different user (bird), just like a cuckoo tricks other birds into hatching its eggs.

An openSUSE® system includes the files permissions, permissions.easy,, and permissions.paranoid, all in the directory /etc. The purpose of these files is to define special permissions, such as world-writable directories or, for files, the setuser ID bit (programs with the setuser ID bit set do not run with the permissions of the user that has launched it, but with the permissions of the file owner, in most cases root). An administrator can use the file /etc/permissions.local to add his own settings.

To define which of the above files is used by openSUSE's configuration programs to set permissions, select Local Security in the Security and Users section of YaST. To learn more about the topic, read the comments in /etc/permissions or consult the manual page of chmod (man chmod). Buffer Overflows and Format String Bugs

Special care must be taken whenever a program needs to process data that could be changed by a user, but this is more of an issue for the programmer of an application than for regular users. The programmer must make sure that his application interprets data in the correct way, without writing it into memory areas that are too small to hold it. Also, the program should hand over data in a consistent manner, using interfaces defined for that purpose.

A buffer overflow can happen if the actual size of a memory buffer is not taken into account when writing to that buffer. There are cases where this data (as generated by the user) uses up more space than what is available in the buffer. As a result, data is written beyond the end of that buffer area, which, under certain circumstances, makes it possible for a program to execute program sequences influenced by the user (and not by the programmer), rather than just processing user data. A bug of this kind may have serious consequences, especially if the program is being executed with special privileges (see Section, “File Permissions”).

Format string bugs work in a slightly different way, but again it is the user input that could lead the program astray. In most cases, these programming errors are exploited with programs executed with special permissions—setuid and setgid programs—which also means that you can protect your data and your system from such bugs by removing the corresponding execution privileges from programs. Again, the best way is to apply a policy of using the lowest possible privileges (see Section, “File Permissions”).

Given that buffer overflows and format string bugs are bugs related to the handling of user data, they are not only exploitable if access has been given to a local account. Many of the bugs that have been reported can also be exploited over a network link. Accordingly, buffer overflows and format string bugs should be classified as being relevant for both local and network security. Viruses

Contrary to popular opinion, there are viruses that run on Linux. However, the viruses that are known were released by their authors as a proof of concept that the technique works as intended. None of these viruses have been spotted in the wild so far.

Viruses cannot survive and spread without a host on which to live. In this case, the host would be a program or an important storage area of the system, such as the master boot record, which needs to be writable for the program code of the virus. Owing to its multiuser capability, Linux can restrict write access to certain files (this is especially important with system files). Therefore, if you did your normal work with root permissions, you would increase the chance of the system being infected by a virus. In contrast, if you follow the principle of using the lowest possible privileges as mentioned above, chances of getting a virus are slim.

Apart from that, you should never rush into executing a program from some Internet site that you do not really know. openSUSE's RPM packages carry a cryptographic signature, as a digital label that the necessary care was taken to build them. Viruses are a typical sign that the administrator or the user lacks the required security awareness, putting at risk even a system that should be highly secure by its very design.

Viruses should not be confused with worms, which belong entirely to the world of networks. Worms do not need a host to spread.

1.1.2. Network Security

Network security is important for protecting from an attack that is started outside the network. The typical login procedure requiring a username and a password for user authentication is still a local security issue. In the particular case of logging in over a network, differentiate between the two security aspects. What happens until the actual authentication is network security and anything that happens afterwards is local security. X Window System and X Authentication

As mentioned at the beginning, network transparency is one of the central characteristics of a UNIX system. X, the windowing system of UNIX operating systems, can make use of this feature in an impressive way. With X, it is basically no problem to log in at a remote host and start a graphical program that is then sent over the network to be displayed on your computer.

When an X client needs to be displayed remotely using an X server, the latter should protect the resource managed by it (the display) from unauthorized access. In more concrete terms, certain permissions must be given to the client program. With the X Window System, there are two ways to do this, called host-based access control and cookie-based access control. The former relies on the IP address of the host where the client should run. The program to control this is xhost. xhost enters the IP address of a legitimate client into a database belonging to the X server. However, relying on IP addresses for authentication is not very secure. For example, if there were a second user working on the host sending the client program, that user would have access to the X server as well—just like someone stealing the IP address. Because of these shortcomings, this authentication method is not described in more detail here, but you can learn about it with man xhost.

In the case of cookie-based access control, a character string is generated that is only known to the X server and to the legitimate user, just like an ID card of some kind. This cookie is stored on login in the file .Xauthority in the user's home directory and is available to any X client wanting to use the X server to display a window. The file .Xauthority can be examined by the user with the tool xauth. If you rename .Xauthority, or if you delete the file from your home directory by accident, you would not be able to open any new windows or X clients.

SSH (secure shell) can be used to encrypt a network connection completely and forward it to an X server transparently, without the encryption mechanism being perceived by the user. This is also called X forwarding. X forwarding is achieved by simulating an X server on the server side and setting a DISPLAY variable for the shell on the remote host. Further details about SSH can be found in Chapter 12, SSH: Secure Network Operations.


If you do not consider the host where you log in to be a secure host, do not use X forwarding. With X forwarding enabled, an attacker could authenticate via your SSH connection to intrude on your X server and perpetrate various actions (reading, or sniffing, your keyboard input, for instance). Buffer Overflows and Format String Bugs

As discussed in Section, “Buffer Overflows and Format String Bugs”, buffer overflows and format string bugs should be classified as issues applying to both local and network security. As with the local variants of such bugs, buffer overflows in network programs, when successfully exploited, are mostly used to obtain root permissions. Even if that is not the case, an attacker could use the bug to gain access to an unprivileged local account to exploit other vulnerabilities that might exist on the system.

Buffer overflows and format string bugs exploitable over a network link are certainly the most frequent form of remote attacks, in general. Exploits for these—programs to exploit these newly-found security holes—are often posted on security mailing lists. They can be used to target the vulnerability without knowing the details of the code. Over the years, experience has shown that the availability of exploit codes has contributed to more secure operating systems, obviously due to the fact that operating system makers were forced to fix the problems in their software. With free software, anyone has access to the source code (openSUSE comes with all available source codes) and anyone who finds a vulnerability and its exploit code can submit a patch to fix the corresponding bug. Denial of Service

The purpose of a denial of service (DoS) attack is to block a server program or even an entire system, something that could be achieved by various means: overloading the server, keeping it busy with garbage packets, or exploiting a remote buffer overflow. Often, a DoS attack is made with the sole purpose of making the service disappear. However, once a given service has become unavailable, communications could become vulnerable to man-in-the-middle attacks (sniffing, TCP connection hijacking, spoofing) and DNS poisoning. Man in the Middle: Sniffing, Hijacking, Spoofing

In general, any remote attack performed by an attacker who puts himself between the communicating hosts is called a man-in-the-middle attack. What almost all types of man-in-the-middle attacks have in common is that the victim is usually not aware that there is something happening. There are many possible variants. For example, the attacker could pick up a connection request and forward that to the target machine. Now the victim has unwittingly established a connection with the wrong host, because the other end is posing as the legitimate destination machine.

The simplest form of a man-in-the-middle attack is called sniffer (the attacker is just listening to the network traffic passing by). As a more complex attack, the man in the middle could try to take over an already established connection (hijacking). To do so, the attacker would need to analyze the packets for some time to be able to predict the TCP sequence numbers belonging to the connection. When the attacker finally seizes the role of the target host, the victims notice this, because they get an error message saying the connection was terminated due to a failure. The fact that there are protocols not secured against hijacking through encryption (which only perform a simple authentication procedure upon establishing the connection) makes it easier for attackers.

Spoofing is an attack where packets are modified to contain counterfeit source data, usually the IP address. Most active forms of attack rely on sending out such fake packets (something that, on a Linux machine, can only be done by the superuser (root)).

Many of the attacks mentioned are carried out in combination with a DoS. If an attacker sees an opportunity to bring down a certain host abruptly, even if only for a short time, it makes it easier for him to push the active attack, because the host will not be able to interfere with the attack for some time. DNS Poisoning

DNS poisoning means that the attacker corrupts the cache of a DNS server by replying to it with spoofed DNS reply packets, trying to get the server to send certain data to a victim who is requesting information from that server. Many servers maintain a trust relationship with other hosts, based on IP addresses or hostnames. The attacker needs a good understanding of the actual structure of the trust relationships among hosts to disguise itself as one of the trusted hosts. Usually, the attacker analyzes some packets received from the server to get the necessary information. The attacker often needs to target a well-timed DoS attack at the name server as well. Protect yourself by using encrypted connections that are able to verify the identity of the hosts to which to connect. Worms

Worms are often confused with viruses, but there is a clear difference between the two. Unlike viruses, worms do not need to infect a host program to live. Instead, they are specialized to spread as quickly as possible on network structures. The worms that appeared in the past, such as Ramen, Lion, or Adore, make use of well-known security holes in server programs like bind8 or lprNG. Protection against worms is relatively easy. Given that some time elapses between the discovery of a security hole and the moment the worm hits your server, there is a good chance that an updated version of the affected program is available on time. That is only useful if the administrator actually installs the security updates on the systems in question.

1.2. Some General Security Tips and Tricks

To handle security competently, it is important to observe some recommendations. You may find the following list of rules useful in dealing with basic security concerns:

  • Get and install the updated packages recommended by security announcements as quickly as possible.

  • Stay informed about the latest security issues:

  • Discuss any security issues of interest on our mailinglist

  • According to the rule of using the most restrictive set of permissions possible for every job, avoid doing your regular jobs as root. This reduces the risk of getting a cuckoo egg or a virus and protects you from your own mistakes.

  • If possible, always try to use encrypted connections to work on a remote machine. Using ssh (secure shell) to replace telnet, ftp, rsh, and rlogin should be standard practice.

  • Avoid using authentication methods based solely on IP addresses.

  • Try to keep the most important network-related packages up-to-date and subscribe to the corresponding mailing lists to receive announcements on new versions of such programs (bind, postfix, ssh, etc.). The same should apply to software relevant to local security.

  • Change the /etc/permissions file to optimize the permissions of files crucial to your system's security. If you remove the setuid bit from a program, it might well be that it cannot do its job anymore in the intended way. On the other hand, consider that, in most cases, the program will also have ceased to be a potential security risk. You might take a similar approach with world-writable directories and files.

  • Disable any network services you do not absolutely require for your server to work properly. This makes your system safer. Open ports, with the socket state LISTEN, can be found with the program netstat. As for the options, it is recommended to use netstat -ap or netstat -anp. The -p option allows you to see which process is occupying a port under which name.

    Compare the netstat results with those of a thorough port scan done from outside your host. An excellent program for this job is nmap, which not only checks out the ports of your machine, but also draws some conclusions as to which services are waiting behind them. However, port scanning may be interpreted as an aggressive act, so do not do this on a host without the explicit approval of the administrator. Finally, remember that it is important not only to scan TCP ports, but also UDP ports (options -sS and -sU).

  • To monitor the integrity of the files of your system in a reliable way, use the program AIDE (Advanced Intrusion Detection Environment), available on openSUSE. Encrypt the database created by AIDE to prevent someone from tampering with it. Furthermore, keep a backup of this database available outside your machine, stored on an external data medium not connected to it by a network link.

  • Take proper care when installing any third-party software. There have been cases where a hacker had built a trojan horse into the tar archive of a security software package, which was fortunately discovered very quickly. If you install a binary package, have no doubts about the site from which you downloaded it.

    SUSE's RPM packages are gpg-signed. The key used by SUSE for signing is:

    ID:9C800ACA 2000-10-19 SUSE Package Signing Key <>
         Key fingerprint = 79C1 79B2 E1C8 20C1 890F 9994 A84E DAE8 9C80 0ACA

    The command rpm --checksig package.rpm shows whether the checksum and the signature of an uninstalled package are correct. Find the key on the first CD of the distribution and on most key servers worldwide.

  • Check backups of user and system files regularly. Consider that if you do not test whether the backup works, it might actually be worthless.

  • Check your log files. Whenever possible, write a small script to search for suspicious entries. Admittedly, this is not exactly a trivial task. In the end, only you can know which entries are unusual and which are not.

  • Use tcp_wrapper to restrict access to the individual services running on your machine, so you have explicit control over which IP addresses can connect to a service. For further information regarding tcp_wrapper, consult the manual pages of tcpd and hosts_access (man 8 tcpd, man hosts_access).

  • Use SuSEfirewall to enhance the security provided by tcpd (tcp_wrapper).

  • Design your security measures to be redundant: a message seen twice is much better than no message at all.

  • If you use suspend to disk, consider configuring the suspend image encryption using the script. The program creates the key, copies it to /etc/suspend.key, and modifies /etc/suspend.conf to use encryption for suspend images.

1.3. Using the Central Security Reporting Address

If you discover a security-related problem (please check the available update packages first), write an e-mail to Please include a detailed description of the problem and the version number of the package concerned. SUSE will try to send a reply as soon as possible. You are encouraged to pgp-encrypt your e-mail messages. SUSE's pgp key is:

ID:3D25D3D9 1999-03-06 SUSE Security Team <>
Key fingerprint = 73 5F 2E 99 DF DB 94 C4 8F 5A A3 AE AF 22 F2 D5

This key is also available for download from

Part I. Authentication

Chapter 2. Authentication with PAM

Linux uses PAM (pluggable authentication modules) in the authentication process as a layer that mediates between user and application. PAM modules are available on a systemwide basis, so they can be requested by any application. This chapter describes how the modular authentication mechanism works and how it is configured.

2.1. What is PAM?

System administrators and programmers often want to restrict access to certain parts of the system or to limit the use of certain functions of an application. Without PAM, applications must be adapted every time a new authentication mechanism, such as LDAP, Samba, or Kerberos, is introduced. This process, however, is rather time-consuming and error-prone. One way to avoid these drawbacks is to separate applications from the authentication mechanism and delegate authentication to centrally managed modules. Whenever a newly required authentication scheme is needed, it is sufficient to adapt or write a suitable PAM module for use by the program in question.

The PAM concept consists of:

  • PAM modules, which are a set of shared libraries for a specific authentication mechanism.

  • A module stack with of one or more PAM modules.

  • A PAM-aware service which needs authentication by using a module stack or PAM modules. Usually a service is a familiar name of the corresponding application, like login or su. The service name other is a reserved word for default rules.

  • Module arguments, with which the execution of a single PAM module can be influenced.

  • A mechanism evaluating each result of a single PAM module execution. A positive value executes the next PAM module. The way a negative value is dealt with, depends on the configuration— no influence, proceed up to terminate immediately and anything in between are valid options.

2.2. Structure of a PAM Configuration File

PAM can be configured in two ways:

File based configuration (/etc/pam.conf)

The configuration of each service is stored in /etc/pam.conf. However, for maintenance and usability reasons, this configuration scheme is not used in openSUSE.

Directory based configuration (/etc/pam.d/)

Every service (or program) that relies on the PAM mechanism has its own configuration file in the /etc/pam.d/ directory. For example, the service for sshd can be found in the /etc/pam.d/sshd file.

The files under /etc/pam.d/ define the PAM modules used for authentication. Each file consists of lines, which define a service, and each line consists of a maximum of four components:


The components have the following meaning:


Declares the type of the service. PAM modules are processed as stacks. Different types of modules have different purposes. For example, one module checks the password, another verifies the location from which the system is accessed, and yet another reads user-specific settings. PAM knows about four different types of modules:


Check the user's authenticity, traditionally by querying a password. However, this can also be achieved with the help of a chip card or through biometrics (for example, fingerprints or iris scan).


Modules of this type check if the user has general permission to use the requested service. As an example, such a check should be performed to ensure that no one can log in with the username of an expired account.


The purpose of this type of module is to enable the change of an authentication token. In most cases, this is a password.


Modules of this type are responsible for managing and configuring user sessions. They are started before and after authentication to log login attempts and configure the user's specific environment (mail accounts, home directory, system limits, etc.).


Indicates the behavior of a PAM module. Each module can have the following control flags:


A module with this flag must be successfully processed before the authentication may proceed. After the failure of a module with the required flag, all other modules with the same flag are processed before the user receives a message about the failure of the authentication attempt.


Modules having this flag must also be processed successfully, in much the same way as a module with the required flag. However, in case of failure a module with this flag gives immediate feedback to the user and no further modules are processed. In case of success, other modules are subsequently processed, just like any modules with the required flag. The requisite flag can be used as a basic filter checking for the existence of certain conditions that are essential for a correct authentication.


After a module with this flag has been successfully processed, the requesting application receives an immediate message about the success and no further modules are processed, provided there was no preceding failure of a module with the required flag. The failure of a module with the sufficient flag has no direct consequences, in the sense that any subsequent modules are processed in their respective order.


The failure or success of a module with this flag does not have any direct consequences. This can be useful for modules that are only intended to display a message (for example, to tell the user that mail has arrived) without taking any further action.


If this flag is given, the file specified as argument is inserted at this place.


Contains a full filename of a PAM module. It does not need to be specified explicitly, as long as the module is located in the default directory /lib/security (for all 64-bit platforms supported by openSUSE®, the directory is /lib64/security).


Contains a space-separated list of options to influence the behavior of a PAM module, such as debug (enables debugging) or nullok (allows the use of empty passwords).

In addition, there are global configuration files for PAM modules under /etc/security, which define the exact behavior of these modules (examples include pam_env.conf and time.conf). Every application that uses a PAM module actually calls a set of PAM functions, which then process the information in the various configuration files and return the result to the requesting application.

To facilitate the creation and maintenance of PAM modules, common default configuration files for the types auth, account, password, and session modules have been introduced. These are retrieved from every application's PAM configuration. Updates to the global PAM configuration modules in common-* are thus propagated across all PAM configuration files without requiring the administrator to update every single PAM configuration file.

The global PAM configuration files are maintained using the pam-config tool. This tool automatically adds new modules to the configuration, changes the configuration of existing ones or deletes modules (or options) from the configurations. Manual intervention in maintaining PAM configurations is minimized or no longer required.

[Note]64-Bit and 32-Bit Mixed Installations

When using a 64-bit operating system, it is possible to also include a runtime environment for 32-bit applications. In this case, make sure that you install both versions of the PAM modules.

2.3. The PAM Configuration of sshd

Consider the PAM configuration of sshd as an example:

Example 2.1. PAM Configuration for sshd (/etc/pam.d/sshd)

#%PAM-1.0                               1
auth     requisite 2
auth     include        common-auth 3
account  requisite 2
account  include        common-account 3
password include        common-password 3
session  required 4
session  include        common-session 3


Declares the version of this configuration file for PAM 1.0. This is merely a convention, but could be used in the future to check the version.


Checks, if /etc/nologin exists. If it does, no user other than root may log in.


Refers to the configuration files of four module types: common-auth, common-account, common-password, and common-session. These four files hold the default configuration for each module type.


Sets the login uid process attribute for the process that was authenticated.

By including the configuration files instead of adding each module separately to the respective PAM configuration, you automatically get an updated PAM configuration when an administrator changes the defaults. Formerly, you had to adjust all configuration files manually for all applications when changes to PAM occurred or a new application was installed. Now the PAM configuration is made with central configuration files and all changes are automatically inherited by the PAM configuration of each service.

The first include file (common-auth) calls three modules of the auth type:, and See Example 2.2, “Default Configuration for the auth Section (common-auth)”.

Example 2.2. Default Configuration for the auth Section (common-auth)

auth    required           1
auth    required         2

1 loads /etc/security/pam_env.conf to set the environment variables as specified in this file. It can be used to set the DISPLAY variable to the correct value, because the pam_env module knows about the location from which the login is taking place.


pam_unix2 checks the user's login and password against /etc/passwd and /etc/shadow.

The whole stack of auth modules is processed before sshd gets any feedback about whether the login has succeeded. All modules of the stack having the required control flag must be processed successfully before sshd receives a message about the positive result. If one of the modules is not successful, the entire module stack is still processed and only then is sshd notified about the negative result.

As soon as all modules of the auth type have been successfully processed, another include statement is processed, in this case, that in Example 2.3, “Default Configuration for the account Section (common-account)”. common-account contains just one module, pam_unix2. If pam_unix2 returns the result that the user exists, sshd receives a message announcing this success and the next stack of modules (password) is processed, shown in Example 2.4, “Default Configuration for the password Section (common-password)”.

Example 2.3. Default Configuration for the account Section (common-account)

account required

Example 2.4. Default Configuration for the password Section (common-password)

password requisite  nullok cracklib
password required    nullok use_authtok

Again, the PAM configuration of sshd involves just an include statement referring to the default configuration for password modules located in common-password. These modules must successfully be completed (control flags requisite and required) whenever the application requests the change of an authentication token.

Changing a password or another authentication token requires a security check. This is achieved with the pam_pwcheck module. The pam_unix2 module used afterwards carries over any old and new passwords from pam_pwcheck, so the user does not need to authenticate again after changing the password. This procedure makes it impossible to circumvent the checks carried out by pam_pwcheck. Whenever the account or the auth type are configured to complain about expired passwords, the password modules should also be used.

Example 2.5. Default Configuration for the session Section (common-session)

session required
session required
session optional

As the final step, the modules of the session type (bundled in the common-session file) are called to configure the session according to the settings for the user in question. The pam_limits module loads the file /etc/security/limits.conf, which may define limits on the use of certain system resources. The pam_unix2 module is processed again. The pam_umask module can be used to set the file mode creation mask. Since this module carries the optional flag, a failure of this module would not affect the successful completion of the entire session module stack. The session modules are called a second time when the user logs out.

2.4. Configuration of PAM Modules

Some of the PAM modules are configurable. The configuration files are located in /etc/security. This section briefly describes the configuration files relevant to the sshd example—pam_env.conf and limits.conf.

2.4.1. pam_env.conf

pam_env.conf can be used to define a standardized environment for users that is set whenever the pam_env module is called. With it, preset environment variables using the following syntax:


Name of the environment variable to set.


Default value the administrator wants to set.


Values that may be queried and set by pam_env, overriding the default value.

A typical example of how pam_env can be used is the adaptation of the DISPLAY variable, which is changed whenever a remote login takes place. This is shown in Example 2.6, “pam_env.conf”.

Example 2.6. pam_env.conf


The first line sets the value of the REMOTEHOST variable to localhost, which is used whenever pam_env cannot determine any other value. The DISPLAY variable in turn contains the value of REMOTEHOST. Find more information in the comments in /etc/security/pam_env.conf.

2.4.2. pam_mount.conf

The purpose of pam_mount is to mount user home directories during the login process, and to unmount them during logout in an environment where a central file server keeps all the home directories of users. With this method, it is not necessary to mount a complete /home directory where all the user home directories would be accessible. Instead, only the home directory of the user who is about to log in, is mounted.

After installing pam_mount, a template of pam_mount.conf.xml is available in /etc/security. The description of the various elements can be found in the manual page man 5 pam_mount.conf.

A basic configuration of this feature can be done with YaST. Select Network Settings+Windows Domain Membership+Expert Settings to add the file server; see Section “Configuring Clients” (Chapter 19, Samba, ↑Reference).

2.4.3. limits.conf

System limits can be set on a user or group basis in limits.conf, which is read by the pam_limits module. The file allows you to set hard limits, which may not be exceeded at all, and soft limits, which may be exceeded temporarily. For more information about the syntax and the options, see the comments in /etc/security/limits.conf.

2.5. Configuring PAM Using pam-config

The pam-config tool helps you configure the global PAM configuration files (/etc/pam.d/common-*-pc) as well as several selected application configurations. For a list of supported modules, use the pam-config --list-modules command. Use the pam-config command to maintain your PAM configuration files. Add new modules to your PAM configurations, delete other modules or modify options to these modules. When changing global PAM configuration files, no manual tweaking of the PAM setup for individual applications is required.

A simple use case for pam-config involves the following:

  1. Auto-generate a fresh Unix-style PAM configuration.  Let pam-config create the simplest possible setup which you can extend later on. The pam-config --create command creates a simple UNIX authentication configuration. Pre-existing configuration files not maintained by pam-config are overwritten, but backup copies are kept as *.pam-config-backup.

  2. Add a new authentication method.  Adding a new authentication method (for example, LDAP) to your stack of PAM modules comes down to a simple pam-config --add --ldap command. LDAP is added wherever appropriate across all common-*-pc PAM configuration files.

  3. Add debugging for test purposes.  To make sure the new authentication procedure works as planned, turn on debugging for all PAM-related operations. The pam-config --add --ldap-debug turns on debugging for LDAP-related PAM operations. Find the debugging output in /var/log/messages.

  4. Query your setup.  Before you finally apply your new PAM setup, check if it contains all the options you wanted to add. The pam-config --query --module lists both the type and the options for the queried PAM module.

  5. Remove the debug options.  Finally, remove the debug option from your setup when you are entirely satisfied with the performance of it. The pam-config --delete --ldap-debug command turns off debugging for LDAP authentication. In case you had debugging options added for other modules, use similar commands to turn these off.

For more information on the pam-config command and the options available, refer to the manual page of pam-config(8).

2.6. Manually Configuring PAM

If you prefer to manually create or maintain your PAM configuration files, you need to make sure to disable pam-config for these files.

When you create your PAM configuration files from scratch using the pam-config --create command, it creates symbolic links from the common-* to the common-*-pc files. pam-config only modifies the common-*-pc configuration files. Removing these symbolic links effectively disables pam-config, because pam-config only operates on the common-*-pc files and these files are not put into effect without the symbolic links.

2.7. For More Information

In the /usr/share/doc/packages/pam directory after installing the pam-doc package, find the following additional documentation:


In the top level of this directory, there is the modules subdirectory holding README files about the available PAM modules.

The Linux-PAM System Administrators' Guide

This document comprises everything that the system administrator should know about PAM. It discusses a range of topics, from the syntax of configuration files to the security aspects of PAM.

The Linux-PAM Module Writers' Manual

This document summarizes the topic from the developer's point of view, with information about how to write standard-compliant PAM modules.

The Linux-PAM Application Developers' Guide

This document comprises everything needed by an application developer who wants to use the PAM libraries.

The PAM Manual Pages

PAM in general as well as the individual modules come with manual pages that provide a good overview of the functionality of all the components.

Chapter 3. Using NIS

As soon as multiple UNIX systems in a network access common resources, it becomes imperative that all user and group identities are the same for all machines in that network. The network should be transparent to users: their environments should not vary, regardless of which machine they are actually using. This can be done by means of NIS and NFS services. NFS distributes file systems over a network and is discussed in Chapter 18, Sharing File Systems with NFS (↑Reference).

NIS (Network Information Service) can be described as a database-like service that provides access to the contents of /etc/passwd, /etc/shadow, and /etc/group across networks. NIS can also be used for other purposes (making the contents of files like /etc/hosts or /etc/services available, for example), but this is beyond the scope of this introduction. People often refer to NIS as YP, because it works like the network's yellow pages.

3.1. Configuring NIS Servers

To distribute NIS information across networks, either install one single server (a master) that serves all clients, or NIS slave servers requesting this information from the master and relaying it to their respective clients.

3.1.1. Configuring a NIS Master Server

To configure a NIS master server for your network, proceed as follows:

  1. To check whether the YaST NIS server configuration module is already installed, start YaST and select Software+Software Management. Search for yast2-nis-server and install it, if needed.

  2. Start YaST+Network Services+NIS Server.

  3. If you need just one NIS server in your network or if this server is to act as the master for further NIS slave servers, select Install and Set Up NIS Master Server. YaST installs the required packages.


    If NIS server software is already installed on your machine, initiate the creation of a NIS master server by clicking Create NIS Master Server.

    Figure 3.1. NIS Server Setup

    NIS Server Setup

  4. Determine basic NIS setup options:

    1. Enter the NIS domain name.

    2. Define whether the host should also be a NIS client (enabling users to log in and access data from the NIS server) by selecting This Host is also a NIS Client.

    3. If your NIS server needs to act as a master server to NIS slave servers in other subnets, select Active Slave NIS Server Exists.

      The option Fast Map Distribution is only useful in conjunction with Active Slave NIS Servers Exist. It speeds up the transfer of maps to the slaves.

    4. Select Allow Changes to Passwords to allow users in your network (both local users and those managed through the NIS server) to change their passwords on the NIS server (with the command yppasswd). This makes the options Allow Changes to GECOS Field and Allow Changes to Login Shell available. GECOS means that the users can also change their names and address settings with the command ypchfn. Shell allows users to change their default shell with the command ypchsh (for example, to switch from bash to sh). The new shell must be one of the predefined entries in /etc/shells.

    5. Select Open Port in Firewall to have YaST adapt the firewall settings for the NIS server.

      Figure 3.2. Master Server Setup

      Master Server Setup

    6. Leave this dialog with Next or click Other Global Settings to make additional settings.

      Other Global Settings include changing the source directory of the NIS server (/etc by default). In addition, passwords can be merged here. The setting should be Yes to create the user database from the system authentication files /etc/passwd, /etc/shadow, and /etc/group. Also, determine the smallest user and group ID that should be offered by NIS. Click OK to confirm your settings and return to the previous screen.

      Figure 3.3. Changing the Directory and Synchronizing Files for a NIS Server

      Changing the Directory and Synchronizing Files for a NIS Server

  5. If you previously enabled Active Slave NIS Server Exists, enter the hostnames used as slaves and click Next. If no slave servers exist, this configuration step is skipped.

  6. Continue to the dialog for the database configuration. Specify the NIS Server Maps, the partial databases to transfer from the NIS server to the client. The default settings are usually adequate. Leave this dialog with Next.

  7. Check which maps should be available and click Next to continue.

    Figure 3.4. NIS Server Maps Setup

    NIS Server Maps Setup

  8. Determine which hosts are allowed to query the NIS server. You can add, edit, or delete hosts by clicking the appropriate button. Specify from which networks requests can be sent to the NIS server. Normally, this is your internal network. In this case, there should be the following two entries:

    The first entry enables connections from your own host, which is the NIS server. The second one allows all hosts to send requests to the server.

    Figure 3.5. Setting Request Permissions for a NIS Server

    Setting Request Permissions for a NIS Server

  9. Click Finish to save your changes and exit the setup.

3.1.2. Configuring a NIS Slave Server

To configure additional NIS slave servers in your network, proceed as follows:

  1. Start YaST+Network Services+NIS Server.

  2. Select Install and Set Up NIS Slave Server and click Next.


    If NIS server software is already installed on your machine, initiate the creation of a NIS slave server by clicking Create NIS Slave Server.

  3. Complete the basic setup of your NIS slave server:

    1. Enter the NIS domain.

    2. Enter hostname or IP address of the master server.

    3. Set This Host is also a NIS Client if you want to enable user logins on this server.

    4. Adapt the firewall settings with Open Ports in Firewall.

    5. Click Next.

  4. Enter the hosts that are allowed to query the NIS server. You can add, edit, or delete hosts by clicking the appropriate button. Specify all networks from which requests can be sent to the NIS server. If it applies to all networks, use the following configuration:

    The first entry enables connections from your own host, which is the NIS server. The second one allows all hosts with access to the same network to send requests to the server.

  5. Click Finish to save changes and exit the setup.

3.2. Configuring NIS Clients

To use NIS on a workstation, do the following:

  1. Start YaST+Network Services+NIS Client.

  2. Activate the Use NIS button.

  3. Enter the NIS domain. This is usually a domain name given by your administrator or a static IP address received by DHCP. For information about DHCP, see Chapter 16, DHCP (↑Reference).

    Figure 3.6. Setting Domain and Address of a NIS Server

    Setting Domain and Address of a NIS Server

  4. Enter your NIS servers and separate their addresses by spaces. If you do not know your NIS server, click on Find to let YaST search for any NIS servers in your domain. Depending on the size of your local network, this may be a time-consuming process. Broadcast asks for a NIS server in the local network after the specified servers fail to respond.

  5. Depending on your local installation, you may also want to activate the automounter. This option also installs additional software if required.

  6. If you do not want other hosts to be able to query which server your client is using, go to the Expert settings and disable Answer Remote Hosts. By checking Broken Server, the client is enabled to receive replies from a server communicating through an unprivileged port. For further information, see man ypbind.

  7. Click Finish to save them and return to the YaST control center. Your client is now configured with NIS.

Chapter 4. LDAP—A Directory Service

The Lightweight Directory Access Protocol (LDAP) is a set of protocols designed to access and maintain information directories. LDAP can be used for user and group management, system configuration management, address management, and more. This chapter provides a basic understanding of how OpenLDAP works and how to manage LDAP data with YaST.

In a network environment it is crucial to keep important information structured and to serve it quickly. A directory service—like the common yellow pages, keeps information available in a well-structured and readily-searchable form.

Ideally, a central server stores the data in a directory and distributes it to all clients using a well-defined protocol. The structured data allow a wide range of applications to access them. A central repository reduces the necessary administrative effort. The use of an open and standardized protocol like LDAP ensures that as many different client applications as possible can access such information.

A directory in this context is a type of database optimized for quick and effective reading and searching:

  • To make multiple concurrent reading accesses possible, the number of updates is usually very low. The number of read and write accesses is often limited to a few users with administrative privileges. In contrast, conventional databases are optimized for accepting the largest possible data volume in a short time.

  • When static data is administered, updates of the existing data sets are very rare. When working with dynamic data, especially when data sets like bank accounts or accounting are concerned, the consistency of the data is of primary importance. If an amount should be subtracted from one place to be added to another, both operations must happen concurrently, within one transaction, to ensure balance over the data stock. Traditional relational databases usually have a very strong focus on data consistency, such as the referential integrity support of transactions. Conversely, short-term inconsistencies are usually acceptable in LDAP directories. LDAP directories often do not have such strong consistency requirements as relational databases.

The design of a directory service like LDAP is not laid out to support complex update or query mechanisms. All applications are guaranteed to access this service quickly and easily.

4.1. LDAP versus NIS

Unix system administrators traditionally use NIS (Network Information Service) for name resolution and data distribution in a network. The configuration data contained in the files group, hosts, mail, netgroup, networks, passwd, printcap, protocols, rpc, and services in the /etc directory is distributed to clients all over the network. These files can be maintained without major effort because they are simple text files. The handling of larger amounts of data, however, becomes increasingly difficult due to nonexistent structuring. NIS is only designed for Unix platforms, and is not suitable as a centralized data administration tool in heterogeneous networks.

Unlike NIS, the LDAP service is not restricted to pure Unix networks. Windows servers (from 2000) support LDAP as a directory service. The application tasks mentioned above are additionally supported in non-Unix systems.

The LDAP principle can be applied to any data structure that needs to be centrally administered. A few application examples are:

  • Replacement for the NIS service

  • Mail routing (postfix, sendmail)

  • Address books for mail clients, like Mozilla, Evolution, and Outlook

  • Administration of zone descriptions for a BIND9 name server

  • User authentication with Samba in heterogeneous networks

This list can be extended because LDAP is extensible, unlike NIS. The clearly-defined hierarchical structure of the data eases the administration of large amounts of data, as it can be searched more easily.

4.2. Structure of an LDAP Directory Tree

To get background knowledge on how a LDAP server works and how the data is stored, it is vital to understand the way the data is organized on the server and how this structure enables LDAP to provide fast access to the data. To successfully operate an LDAP setup, you also need to be familiar with some basic LDAP terminology. This section introduces the basic layout of an LDAP directory tree and provides the basic terminology used with respect to LDAP. Skip this introductory section if you already have some LDAP background knowledge and just want to learn how to set up an LDAP environment in openSUSE. Read on at Section 4.3, “Configuring an LDAP Server with YaST” or Section 4.7, “Manually Configuring an LDAP Server”.

An LDAP directory has a tree structure. All entries (called objects) of the directory have a defined position within this hierarchy. This hierarchy is called the directory information tree (DIT). The complete path to the desired entry, which unambiguously identifies it, is called the distinguished name or DN. A single node along the path to this entry is called relative distinguished name or RDN.

The relations within an LDAP directory tree become more evident in the following example, shown in Figure 4.1, “Structure of an LDAP Directory”.

Figure 4.1. Structure of an LDAP Directory

Structure of an LDAP Directory

The complete diagram is a fictional directory information tree. The entries on three levels are depicted. Each entry corresponds to one box in the image. The complete, valid distinguished name for the fictional employee Geeko Linux, in this case, is cn=Geeko Linux,ou=doc,dc=example,dc=com. It is composed by adding the RDN cn=Geeko Linux to the DN of the preceding entry ou=doc,dc=example,dc=com.

The types of objects that can be stored in the DIT are globally determined following a Schema. The type of an object is determined by the object class. The object class determines what attributes the relevant object must or can be assigned. The Schema, therefore, must contain definitions of all object classes and attributes used in the desired application scenario. There are a few common Schemas (see RFC 2252 and 2256). The LDAP RFC defines a few commonly used Schemas (see e.g., RFC4519). Additionally there are Schemas available for many other use cases (e.g., Samba, NIS replacement, etc.). It is, however, possible to create custom Schemas or to use multiple Schemas complementing each other (if this is required by the environment in which the LDAP server should operate).

Table 4.1, “Commonly Used Object Classes and Attributes” offers a small overview of the object classes from core.schema and inetorgperson.schema used in the example, including required attributes (Req. Attr.) and valid attribute values.

Table 4.1. Commonly Used Object Classes and Attributes

Object Class


Example Entry

Req. Attr.


domainComponent (name components of the domain)




organizationalUnit (organizational unit)




inetOrgPerson (person-related data for the intranet or Internet)

Geeko Linux

sn and cn

Example 4.1, “Excerpt from schema.core” shows an excerpt from a Schema directive with explanations.

Example 4.1. Excerpt from schema.core

attributetype ( NAME ( 'ou' 'organizationalUnitName') 1
       DESC 'RFC2256: organizational unit this object belongs to' 2
       SUP name ) 3

objectclass ( NAME 'organizationalUnit' 4
       DESC 'RFC2256: an organizational unit' 5
       SUP top STRUCTURAL 6
       MUST ou 7
MAY (userPassword $ searchGuide $ seeAlso $ businessCategory 8
  $ x121Address $ registeredAddress $ destinationIndicator 
  $ preferredDeliveryMethod $ telexNumber 
  $ teletexTerminalIdentifier $ telephoneNumber 
  $ internationaliSDNNumber $ facsimileTelephoneNumber 
  $ street $ postOfficeBox $ postalCode $ postalAddress 
  $ physicalDeliveryOfficeName
  $ st $ l $ description) )

The attribute type organizationalUnitName and the corresponding object class organizationalUnit serve as an example here.


The name of the attribute, its unique OID (object identifier) (numerical), and the abbreviation of the attribute.


A brief description of the attribute with DESC. The corresponding RFC, on which the definition is based, is also mentioned here.


SUP indicates a superordinate attribute type to which this attribute belongs.


The definition of the object class organizationalUnit begins—the same as in the definition of the attribute—with an OID and the name of the object class.


A brief description of the object class.


The SUP top entry indicates that this object class is not subordinate to another object class.


With MUST list all attribute types that must be used in conjunction with an object of the type organizationalUnit.


With MAY list all attribute types that are permitted in conjunction with this object class.

A very good introduction to the use of Schemas can be found in the OpenLDAP documentation (openldap2-doc). When installed, find it in /usr/share/doc/packages/openldap2/adminguide/guide.html.

4.3. Configuring an LDAP Server with YaST

Use YaST to set up an LDAP server. Typical use cases for LDAP servers include the management of user account data and the configuration of mail, DNS, and DHCP servers.

Figure 4.2. YaST LDAP Server Configuration

YaST LDAP Server Configuration

Figure 4.3. YaST LDAP Server—New Database

YaST LDAP Server—New Database

To set up an LDAP server for user account data, make sure the yast2-ldap-server and openldap2 packages are installed. Then proceed as follows:

  1. Start YaST as root and select Network Services+LDAP Server to invoke the configuration wizard.

  2. Configure the Global Settings of your LDAP server (you can change these settings later)—see Figure 4.2, “YaST LDAP Server Configuration”:

    1. Set LDAP to be started.

    2. If the LDAP server should announce its services via SLP, check Register at an SLP Daemon.

    3. Configure Firewall Settings.

    4. Click Next.

  3. Select the server type: stand-alone server, master server in a replication setup, or replication (slave) server.

  4. Select security options (TLS Settings).

    It is strongly recommended to Enable TLS. For more information, see Step 4.

    [Warning]Password Encryption

    Enabling TLS ensures passwords are sent encrypted over the network. When this option is not enabled, passwords are sent unencrypted.

    Also consider to use LDAP over SSL and certificates.

  5. Confirm Basic Database Settings with entering an LDAP Administrator Password and then clicking Next—see Figure 4.3, “YaST LDAP Server—New Database”.

  6. Check the LDAP Server Configuration Summary and click Finish to exit the configuration wizard.

Figure 4.4. YaST LDAP Server Configuration

YaST LDAP Server Configuration

For changes or additional configuration start the LDAP server module again and in the left pane expand Global Settings to make subentries visible—see Figure 4.4, “YaST LDAP Server Configuration”:

  1. With Log Level Settings, configure the degree of logging activity (verbosity) of the LDAP server. From the predefined list, select or deselect the logging options according to your needs. The more options are enabled, the larger your log files grow.

  2. Configure which connection types the server should offer under Allow/Disallow Features. Choose from:

    LDAPv2 Bind Requests

    This option enables connection requests (bind requests) from clients using the previous version of the protocol (LDAPv2).

    Anonymous Bind When Credentials Not Empty

    Normally the LDAP server denies any authentication attempts with empty credentials (DN or password). Enabling this option, however, makes it possible to connect with a password and no DN to establish an anonymous connection.

    Unauthenticated Bind When DN Not Empty

    Enabling this option makes it possible to connect without authentication (anonymously) using a DN but no password.

    Unauthenticated Update Options to Process

    Enabling this option allows non-authenticated (anonymous) update operations. Access is restricted according to ACLs and other rules.

  3. Allow/Disallow Features also lets you configure the server flags. Choose from:

    Disable Acceptance of Anonymous Bind Requests

    The server will no longer accept anonymous bind requests. Note, that this does not generally prohibit anonymous directory access.

    Disable Simple Bind Authentication

    Completely disable Simple Bind authentication.

    Disable Forcing Session to Anonymous Status upon StartTLS Operation Receipt

    The server will no longer force an authenticated connection back to the anonymous state when receiving the StartTLS operation.

    Disallow the StartTLS Operation if Authenticated

    The server will disallow the StartTLS operation on already authenticated connections.

  4. To configure secure communication between client and server, proceed with TLS Settings:

    1. Activate Enable TLS to enable TLS and SSL encryption of the client/server communication.

    2. Either Import Certificate by specifying the exact path to its location or enable the Use Common Server Certificate. If the Use Common Server Certificate is not available because it has not been created during installation, go for Launch CA Management Module first— for more information, see Section 15.2, “YaST Modules for CA Management”.

Add Schema files to be included in the server's configuration by selecting Schema Files in the left part of the dialog. The default selection of schema files applies to the server providing a source of YaST user account data.

YaST allows to add traditional Schema files (usually with a name ending in .schema) or LDIF files containing Schema definitions in OpenLDAP's LDIF Schema format.

Figure 4.5. YaST LDAP Server Database Configuration

YaST LDAP Server Database Configuration

To configure the databases managed by your LDAP server, proceed as follows:

  1. Select the Databases item in the left part of the dialog.

  2. Click Add Database to add a new database.

  3. Enter the requested data:

    Base DN

    Enter the base DN of your LDAP server.

    Administrator DN

    Enter the DN of the administrator in charge of the server. If you check Append Base DN, only provide the cn of the administrator and the system fills in the rest automatically.

    LDAP Administrator Password

    Enter the password for the database administrator.

    Use This Database as the Default for OpenLDAP Clients

    For convenience, check this option if wanted.

  4. In the next dialog configure replication settings.

  5. In the next dialog, enable enforcement of password policies to provide extra security to your LDAP server:

    1. Check Enable Password Policies to be able to specify a password policy.

    2. Activate Hash Clear Text Passwords to have clear text passwords be hashed before they are written to the database whenever they are added or modified.

    3. Disclose "Account Locked" Status provides a relevant error message for bind requests to locked accounts.

      [Warning]Locked Accounts in Security Sensitive Environments

      Do not use the Disclose "Account Locked" Status option if your environment is sensitive to security issues, because the Locked Account error message provides security-sensitive information that can be exploited by a potential attacker.

    4. Enter the DN of the default policy object. To use a DN other than the one suggested by YaST, enter your choice. Otherwise, accept the default settings.

  6. Complete the database configuration by clicking Finish.

If you have not opted for password policies, your server is ready to run at this point. If you have chosen to enable password policies, proceed with the configuration of the password policy in detail. If you have chosen a password policy object that does not yet exist, YaST creates one:

  1. Enter the LDAP server password. In the navigation tree below Databases expand your database object and activate the Password Policy Configuration item.

  2. Make sure Enable Password Policies is activated. Then click Edit Policy.

  3. Configure the password change policies:

    1. Determine the number of passwords stored in the password history. Saved passwords may not be reused by the user.

    2. Determine if users will be able to change their passwords and if they will need to change their passwords after a reset by the administrator. Require the old password for password changes (optional).

    3. Determine whether and to what extent passwords should be subject to quality checking. Set the minimum password length that must be met before a password is valid. If you select Accept Uncheckable Passwords, users are allowed to use encrypted passwords, even though the quality checks cannot be performed. If you opt for Only Accept Checked Passwords only those passwords that pass the quality tests are accepted as valid.

  4. Configure the password time-limit policies:

    1. Determine the minimum password time-limit (the time that needs to pass between two valid password changes) and the maximum password time-limit.

    2. Determine the time between a password expiration warning and the actual password expiration.

    3. Set the number of postponement uses of an expired password before the password expires permanently.

  5. Configure the lockout policies:

    1. Enable password locking.

    2. Determine the number of bind failures that trigger a password lock.

    3. Determine the duration of the password lock.

    4. Determine the length of time that password failures are kept in the cache before they are purged.

  6. Apply your password policy settings with OK.

To edit a previously created database, select its base DN in the tree to the left. In the right part of the window, YaST displays a dialog similar to the one used for the creation of a new database (with the main difference that the base DN entry is grayed out and cannot be changed).

After leaving the LDAP server configuration by selecting Finish, you are ready to go with a basic working configuration for your LDAP server. To fine-tune this setup, make use of OpenLDAP's dynamic configuration backend.

The OpenLDAP's dynamic configuration backend stores the configuration in an LDAP database. That database consists of a set of .ldif files in /etc/openldap/slapd.d. There is no need to access these files directly. To access the settings you can either use the YaST LDAP server module (the yast2-ldap-server package) or an LDAP client such as ldapmodify or ldapsearch. For more information on the dynamic configuration of OpenLDAP, see the OpenLDAP Administration Guide.

4.4. Configuring an LDAP Client with YaST

YaST includes a module to set up LDAP-based user management. If you did not enable this feature during the installation, start the module by selecting Network Services+LDAP Client. YaST automatically enables any PAM and NSS-related changes as required by LDAP and installs the necessary files. Simply connect your client to the server and let YaST manage users over LDAP. This basic setup is described in Section 4.4.1, “Configuring Basic Settings”.

Use the YaST LDAP client to further configure the YaST group and user configuration modules. This includes manipulating the default settings for new users and groups and the number and nature of the attributes assigned to a user or group. LDAP user management allows you to assign far more and different attributes to users and groups than traditional user or group management solutions. This is described in Section 4.4.2, “Configuring the YaST Group and User Administration Modules”.

4.4.1. Configuring Basic Settings

The basic LDAP client configuration dialog (Figure 4.6, “YaST: LDAP Client Configuration”) opens during installation if you choose LDAP user management or when you select Network Services+LDAP Client in the YaST Control Center in the installed system.

Figure 4.6. YaST: LDAP Client Configuration

YaST: LDAP Client Configuration

To authenticate users of your machine against an OpenLDAP server and to enable user management via OpenLDAP, proceed as follows:

  1. Click Use LDAP to enable the use of LDAP. Select Use LDAP but Disable Logins instead if you want to use LDAP for authentication, but do not want other users to log in to this client.

  2. Enter the IP address of the LDAP server to use.

  3. Enter the LDAP Base DN to select the search base on the LDAP server. To retrieve the base DN automatically, click Fetch DN. YaST then checks for any LDAP database on the server address specified above. Choose the appropriate base DN from the search results given by YaST.

  4. If TLS or SSL-protected communication with the server is required, select LDAP TLS/SSL. Click Download CA Certificate to download a certificate in PEM format from a URL.

  5. Select Start Automounter to mount remote directories on your client, such as a remotely managed /home.

  6. Select Create Home Directory on Login to have a user's home automatically created on the first user login.

  7. Click OK to apply your settings.

To modify data on the server as administrator, click Advanced Configuration. The following dialog is split into two tabs. See Figure 4.7, “YaST: Advanced Configuration”.

Figure 4.7. YaST: Advanced Configuration

YaST: Advanced Configuration

  1. In the Client Settings tab, adjust the following settings according to your needs:

    1. If the search base for users, passwords, and groups differs from the global search base specified in the LDAP base DN, enter these different naming contexts in User Map, Password Map, and Group Map.

    2. Specify the password change protocol. The standard method to use whenever a password is changed is crypt, meaning that password hashes generated by crypt are used. For details on this and other options, refer to the pam_ldap man page.

    3. Specify the LDAP group to use with Group Member Attribute. The default value for this is member.

    4. If a secure connection requires certificate checking, specify where your CA Certificate File in PEM format is located. Or specify a directory with certificates.

    5. If the LDAP server still uses LDAPv2, enable the use of this protocol version by selecting LDAP Version 2.

  2. In Administration Settings, adjust the following settings:

    1. Set the base for storing your user management data via Configuration Base DN.

    2. Enter the appropriate value for Administrator DN. This DN must be identical with the rootdn value specified in /etc/openldap/slapd.conf to enable this particular user to manipulate data stored on the LDAP server. Enter the full DN (such as cn=Administrator,dc=example,dc=com) or activate Append Base DN to have the base DN added automatically when you enter cn=Administrator.

    3. Check Create Default Configuration Objects to create the basic configuration objects on the server to enable user management via LDAP.

    4. If your client machine needs to act as a file server for home directories across your network, check Home Directories on This Machine.

    5. Use the Password Policy section to select, add, delete, or modify the password policy settings to use. The configuration of password policies with YaST is part of the LDAP server setup.

    6. Click OK to leave the Advanced Configuration, then Finish to apply your settings.

Use Configure User Management Settings to edit entries on the LDAP server. Access to the configuration modules on the server is then granted according to the ACLs and ACIs stored on the server. Follow the procedures outlined in Section 4.4.2, “Configuring the YaST Group and User Administration Modules”.

4.4.2. Configuring the YaST Group and User Administration Modules

Use the YaST LDAP client to adapt the YaST modules for user and group administration and to extend them as needed. Define templates with default values for the individual attributes to simplify the data registration. The presets created here are stored as LDAP objects in the LDAP directory. The registration of user data is still done with the regular YaST modules for user and group management. The registered data is stored as LDAP objects on the server.

Figure 4.8. YaST: Module Configuration

YaST: Module Configuration

The dialog for module configuration (Figure 4.8, “YaST: Module Configuration”) allows the creation of new modules, selection and modification of existing configuration modules, and design and modification of templates for such modules.

To create a new configuration module, proceed as follows:

  1. In the LDAP Client Configuration click Advanced Configuration, then open the Administration Settings tab. Click Configure User Management Settings and enter the LDAP server credentials.

  2. Click New and select the type of module to create. For a user configuration module, select suseUserConfiguration and for a group configuration choose suseGroupConfiguration.

  3. Choose a name for the new template (e.g., userConfig). The content view shows a table listing all attributes allowed in this module and their assigned values.

  4. Accept the preset values or adjust the defaults to use in group and user configurations by selecting the relevant attribute, pressing Edit, and entering the new value. Rename a module by changing the cn attribute of the module. Clicking Delete deletes the currently selected module.

  5. After you click OK, the new module is added to the selection menu.

The YaST modules for group and user administration embed templates with standard values. To edit a template associated with a configuration module, start the object template configuration (Figure 4.9, “YaST: Configuration of an Object Template”):

  1. In the Module Configuration dialog, click Configure Template.

  2. Determine the values of the general attributes assigned to this template according to your needs or leave them empty. Empty attributes are deleted on the LDAP server.

  3. Modify, delete, or add new default values for new objects (user or group configuration objects in the LDAP tree).

Figure 4.9. YaST: Configuration of an Object Template

YaST: Configuration of an Object Template

Connect the template to its module by setting the susedefaulttemplate attribute value of the module to the DN of the adapted template.


The default values for an attribute can be created from other attributes by using a variable instead of an absolute value. For example, when creating a new user, cn=%sn %givenName is created automatically from the attribute values for sn and givenName.

Once all modules and templates are configured correctly and ready to run, new groups and users can be registered in the usual way with YaST.

4.5. Configuring LDAP Users and Groups in YaST

The actual registration of user and group data differs only slightly from the procedure when not using LDAP. The following instructions relate to the administration of users. The procedure for administering groups is analogous.

  1. Access the YaST user administration with Security and Users+User and Group Management.

  2. Use Set Filter to limit the view of users to the LDAP users and enter the password for Root DN.

  3. Click Add to enter the user configuration. A dialog with four tabs opens:

    1. Specify username, login, and password in the User Data tab.

    2. Check the Details tab for the group membership, login shell, and home directory of the new user. If necessary, change the default to values that better suit your needs. The default values (as well as those of the password settings) can be defined with the procedure described in Section 4.4.2, “Configuring the YaST Group and User Administration Modules”.

    3. Modify or accept the default Password Settings.

    4. Enter the Plug-Ins tab, select the LDAP plug-in, and click Launch to configure additional LDAP attributes assigned to the new user (see Figure 4.10, “YaST: Additional LDAP Settings”).

  4. Click OK to apply your settings and leave the user configuration.

Figure 4.10. YaST: Additional LDAP Settings

YaST: Additional LDAP Settings

The initial input form of user administration offers LDAP Options. This allows you to apply LDAP search filters to the set of available users. Alternatively open the module for configuring LDAP users and groups by selecting LDAP User and Group Configuration.

4.6. Browsing the LDAP Directory Tree

To conveniently browse the LDAP directory tree and all its entries, use the YaST LDAP Browser:

  1. Log in as root.

  2. Start YaST+Network Services+LDAP Browser.

  3. Enter the address of the LDAP server, the Administrator DN, and the password for the Root DN of this server (if you need both to read and write the data stored on the server).

    Alternatively, choose Anonymous Access and do not provide the password to gain read access to the directory.

    The LDAP Tree tab displays the content of the LDAP directory to which your machine connected. Click to expand each item's submenu.

    Figure 4.11. Browsing the LDAP Directory Tree

    Browsing the LDAP Directory Tree

  4. To view any entry in detail, select it in the LDAP Tree view and open the Entry Data tab.

    All attributes and values associated with this entry are displayed.

    Figure 4.12. Browsing the Entry Data

    Browsing the Entry Data

  5. To change the value of any of these attributes, select the attribute, click Edit, enter the new value, click Save, and provide the Root DN password when prompted.

  6. Leave the LDAP browser with Close.

4.7. Manually Configuring an LDAP Server

YaST uses OpenLDAP's dynamic configuration database (back-config) to store the LDAP server's configuration. For details about the dynamic configuration backend please see the slapd-config(5) man page or the OpenLDAP Software 2.4 Administrator's Guide located at /usr/share/doc/packages/openldap2/guide/admin/guide.html on your system if the openldap2 package is installed.

[Tip]Upgrading an Old OpenLDAP Installation

YaST does not use /etc/openldap/slapd.conf to store the OpenLDAP configuration anymore. In case of a system upgrade, a copy of the original /etc/openldap/slapd.conf file will get created as /etc/openldap/slapd.conf.YaSTsave.

To conveniently access the configuration backend, you use SASL external authentication. For example, the following ldapsearch command executed as root can be used to show the complete slapd configuration:

ldapsearch -Y external -H ldapi:/// -b cn=config

4.7.1. Starting and Stopping the Servers

Once the LDAP server is fully configured and all desired entries have been made according to the pattern described in Section 4.8, “Manually Administering LDAP Data”, start the LDAP server as root by entering rcldap start. To stop the server manually, enter the command rcldap stop. Query the status of the running LDAP server with rcldap status.

4.8. Manually Administering LDAP Data

OpenLDAP offers a series of tools for the administration of data in the LDAP directory. The four most important tools for adding to, deleting from, searching through and modifying the data stock are explained in this section.

4.8.1. Inserting Data into an LDAP Directory

Once your LDAP server is correctly configured (it features appropriate entries for suffix, directory, rootdn, rootpw and index), proceed to entering records. OpenLDAP offers the ldapadd command for this task. If possible, add the objects to the database in bundles (for practical reasons). LDAP is able to process the LDIF format (LDAP data interchange format) for this. An LDIF file is a simple text file that can contain an arbitrary number of attribute and value pairs. The LDIF file for creating a rough framework for the example in Figure 4.1, “Structure of an LDAP Directory” would look like the one in Example 4.2, “An LDIF File”.

[Important]Encoding of LDIF Files

LDAP works with UTF-8 (Unicode). Umlauts must be encoded correctly. Otherwise, avoid umlauts and other special characters or use iconv to convert the input to UTF-8.

Example 4.2. An LDIF File

# The Organization
dn: dc=example,dc=com
objectClass: dcObject
objectClass: organization
o: Example dc: example

# The organizational unit development (devel)
dn: ou=devel,dc=example,dc=com
objectClass: organizationalUnit
ou: devel

# The organizational unit documentation (doc)
dn: ou=doc,dc=example,dc=com
objectClass: organizationalUnit
ou: doc

# The organizational unit internal IT (it)
dn: ou=it,dc=example,dc=com
objectClass: organizationalUnit
ou: it

Save the file with the .ldif suffix then pass it to the server with the following command:

ldapadd -x -D dn_of_the_administrator -W -f file.ldif

-x switches off the authentication with SASL in this case. -D declares the user that calls the operation. The valid DN of the administrator is entered here just like it has been configured in slapd.conf. In the current example, this is cn=Administrator,dc=example,dc=com. -W circumvents entering the password on the command line (in clear text) and activates a separate password prompt. The -f option passes the filename. See the details of running ldapadd in Example 4.3, “ldapadd with example.ldif”.

Example 4.3. ldapadd with example.ldif

ldapadd -x -D cn=Administrator,dc=example,dc=com -W -f example.ldif 

Enter LDAP password: 
adding new entry "dc=example,dc=com" 
adding new entry "ou=devel,dc=example,dc=com" 
adding new entry "ou=doc,dc=example,dc=com" 
adding new entry "ou=it,dc=example,dc=com"

The user data of individuals can be prepared in separate LDIF files. Example 4.4, “LDIF Data for Tux” adds Tux to the new LDAP directory.

Example 4.4. LDIF Data for Tux

# coworker Tux
dn: cn=Tux Linux,ou=devel,dc=example,dc=com
objectClass: inetOrgPerson
cn: Tux Linux
givenName: Tux
sn: Linux
uid: tux
telephoneNumber: +49 1234 567-8

An LDIF file can contain an arbitrary number of objects. It is possible to pass directory branches (entirely or in part) to the server in one go, as shown in the example of individual objects. If it is necessary to modify some data relatively often, a fine subdivision of single objects is recommended.

4.8.2. Modifying Data in the LDAP Directory

The tool ldapmodify is provided for modifying the data stock. The easiest way to do this is to modify the corresponding LDIF file and pass the modified file to the LDAP server. To change the telephone number of colleague Tux from +49 1234 567-8 to +49 1234 567-10, edit the LDIF file like in Example 4.5, “Modified LDIF File tux.ldif”.

Example 4.5. Modified LDIF File tux.ldif

# coworker Tux
dn: cn=Tux Linux,ou=devel,dc=example,dc=com 
changetype: modify
replace: telephoneNumber 
telephoneNumber: +49 1234 567-10

Import the modified file into the LDAP directory with the following command:

ldapmodify -x -D cn=Administrator,dc=example,dc=com -W -f tux.ldif

Alternatively, pass the attributes to change directly to ldapmodify as follows:

  1. Start ldapmodify and enter your password:

    ldapmodify -x -D cn=Administrator,dc=example,dc=com -W 
    Enter LDAP password:
  2. Enter the changes while carefully complying with the syntax in the order presented below:

    dn: cn=Tux Linux,ou=devel,dc=example,dc=com
    changetype: modify
    replace: telephoneNumber
    telephoneNumber: +49 1234 567-10

For more information about ldapmodify and its syntax, see the ldapmodify man page.

4.8.3. Searching or Reading Data from an LDAP Directory

OpenLDAP provides, with ldapsearch, a command line tool for searching data within an LDAP directory and reading data from it. This is a simple query:

ldapsearch -x -b dc=example,dc=com "(objectClass=*)"

The -b option determines the search base (the section of the tree within which the search should be performed). In the current case, this is dc=example,dc=com. To perform a more finely-grained search in specific subsections of the LDAP directory (for example, only within the devel department), pass this section to ldapsearch with -b. -x requests activation of simple authentication. (objectClass=*) declares that all objects contained in the directory should be read. This command option can be used after the creation of a new directory tree to verify that all entries have been recorded correctly and the server responds as desired. For more information about the use of ldapsearch, see the ldapsearch(1) man page.

4.8.4. Deleting Data from an LDAP Directory

Delete unwanted entries with ldapdelete. The syntax is similar to that of the other commands. To delete, for example, the complete entry for Tux Linux, issue the following command:

ldapdelete -x -D cn=Administrator,dc=example,dc=com -W cn=Tux \

4.9. For More Information

More complex subjects (like SASL configuration or establishment of a replicating LDAP server that distributes the workload among multiple slaves) were omitted from this chapter. Find detailed information about both subjects in the OpenLDAP 2.4 Administrator's Guide—see at OpenLDAP 2.4 Administrator's Guide.

The Web site of the OpenLDAP project offers exhaustive documentation for beginner and advanced LDAP users:

OpenLDAP Faq-O-Matic

A detailed question and answer collection applying to the installation, configuration, and use of OpenLDAP. Find it at

Quick Start Guide

Brief step-by-step instructions for installing your first LDAP server. Find it at or on an installed system in Section 2 of /usr/share/doc/packages/openldap2/guide/admin/guide.html.

OpenLDAP 2.4 Administrator's Guide

A detailed introduction to all important aspects of LDAP configuration, including access controls and encryption. See or, on an installed system, /usr/share/doc/packages/openldap2/guide/admin/guide.html.

Understanding LDAP

A detailed general introduction to the basic principles of LDAP:

Printed literature about LDAP:

  • LDAP System Administration by Gerald Carter (ISBN 1-56592-491-6)

  • Understanding and Deploying LDAP Directory Services by Howes, Smith, and Good (ISBN 0-672-32316-8)

The ultimate reference material for the subject of LDAP are the corresponding RFCs (request for comments), 2251 to 2256.

Chapter 5. Active Directory Support

Active Directory* (AD) is a directory-service based on LDAP, Kerberos, and other services that is used by Microsoft Windows to manage resources, services, and people. In an MS Windows network, AD provides information about these objects, restricts access to them, and enforces policies. openSUSE® lets you join existing AD domains and integrate your Linux machine into a Windows environment.

5.1. Integrating Linux and AD Environments

With a Linux client (configured as an Active Directory client) that is joined to an existing Active Directory domain, benefit from various features not available on a pure openSUSE Linux client:

Browsing Shared Files and Folders with SMB

Both Nautilus (the GNOME file manager) and Dolphin or Konqueror (its KDE counterparts) support browsing shared resources through SMB.

Sharing Files and Folders with SMB

Both Nautilus, Dolphin, and Konqueror support sharing folders and files as in Windows.

Accessing and Manipulating User Data on the Windows Server

Through Nautilus and Konqueror, users are able to access their Windows user data and can edit, create, and delete files and folders on the Windows server. Users can access their data without having to enter their password multiple times.

Offline Authentication

Users are able to log in and access their local data on the Linux machine even if they are offline or the AD server is unavailable for other reasons.

Windows Password Change

This port of AD support in Linux enforces corporate password policies stored in Active Directory. The display managers and console support password change messages and accept your input. You can even use the Linux passwd command to set Windows passwords.

Single-Sign-On through Kerberized Applications

Many applications of both desktops are Kerberos-enabled (kerberized), which means they can transparently handle authentication for the user without the need for password reentry at Web servers, proxies, groupware applications, or other locations.

A brief technical background for most of these features is given in the following section.

5.2. Background Information for Linux AD Support

Many system components need to interact flawlessly in order to integrate a Linux client into an existing Windows Active Directory domain. Figure 5.1, “Active Directory Authentication Schema” highlights the most prominent ones. The following sections focus on the underlying processes of the key events in AD server and client interaction.

Figure 5.1. Active Directory Authentication Schema

Active Directory Authentication Schema

To communicate with the directory service, the client needs to share at least two protocols with the server:


LDAP is a protocol optimized for managing directory information. A Windows domain controller with AD can use the LDAP protocol to exchange directory information with the clients. To learn more about LDAP in general and about the open source port of it, OpenLDAP, refer to Chapter 4, LDAP—A Directory Service.


Kerberos is a third-party trusted authentication service. All its clients trust Kerberos's authorization of another client's identity, enabling kerberized single-sign-on (SSO) solutions. Windows supports a Kerberos implementation, making Kerberos SSO possible even with Linux clients.

The following client components process account and authentication data:


The most central part of this solution is the winbind daemon that is a part of the Samba project and handles all communication with the AD server.

NSS (Name Service Switch)

NSS routines provide name service information. Naming service for both users and groups is provided by nss_winbind. This module directly interacts with the winbind daemon.

PAM (Pluggable Authentication Modules)

User authentication for AD users is done by the pam_winbind module. The creation of user homes for the AD users on the Linux client is handled by pam_mkhomedir. The pam_winbind module directly interacts with winbindd. To learn more about PAM in general, refer to Chapter 2, Authentication with PAM.

Applications that are PAM-aware, like the login routines and the GNOME and KDE display managers, interact with the PAM and NSS layer to authenticate against the Windows server. Applications supporting Kerberos authentication (such as file managers, Web browsers, or e-mail clients) use the Kerberos credential cache to access user's Kerberos tickets, making them part of the SSO framework.

5.2.1. Domain Join

During domain join, the server and the client establish a secure relation. On the client, the following tasks need to be performed to join the existing LDAP and Kerberos SSO environment provided by the Window domain controller. The entire join process is handled by the YaST Domain Membership module, which can be run during installation or in the installed system:

  1. The Windows domain controller providing both LDAP and KDC (Key Distribution Center) services is located.

  2. A machine account for the joining client is created in the directory service.

  3. An initial ticket granting ticket (TGT) is obtained for the client and stored in its local Kerberos credential cache. The client needs this TGT to get further tickets allowing it to contact other services, like contacting the directory server for LDAP queries.

  4. NSS and PAM configurations are adjusted to enable the client to authenticate against the domain controller.

During client boot, the winbind daemon is started and retrieves the initial Kerberos ticket for the machine account. winbindd automatically refreshes the machine's ticket to keep it valid. To keep track of the current account policies, winbindd periodically queries the domain controller.

5.2.2. Domain Login and User Homes

The login managers of GNOME and KDE (GDM and KDM) have been extended to allow the handling of AD domain login. Users can choose to log into the primary domain the machine has joined or to one of the trusted domains with which the domain controller of the primary domain has established a trust relationship.

User authentication is mediated by a number of PAM modules as described in Section 5.2, “Background Information for Linux AD Support”. The pam_winbind module used to authenticate clients against Active Directory or NT4 domains is fully aware of Windows error conditions that might prohibit a user's login. The Windows error codes are translated into appropriate user-readable error messages that PAM gives at login through any of the supported methods (GDM, KDM, console, and SSH):

Password has expired

The user sees a message stating that the password has expired and needs to be changed. The system prompts for a new password and informs the user if the new password does not comply with corporate password policies (for example the password is too short, too simple, or already in the history). If a user's password change fails, the reason is shown and a new password prompt is given.

Account disabled

The user sees an error message stating that the account has been disabled and to contact the system administrator.

Account locked out

The user sees an error message stating that the account has been locked and to contact the system administrator.

Password has to be changed

The user can log in but receives a warning that the password needs to be changed soon. This warning is sent three days before that password expires. After expiration, the user cannot log in.

Invalid workstation

When a user is restricted to specific workstations and the current openSUSE machine is not among them, a message appears that this user cannot log in from this workstation.

Invalid logon hours

When a user is only allowed to log in during working hours and tries to log in outside working hours, a message informs the user that logging in is not possible at that time.

Account expired

An administrator can set an expiration time for a specific user account. If that user tries to log in after expiration, the user gets a message that the account has expired and cannot be used to log in.

During a successful authentication, pam_winbind acquires a ticket granting ticket (TGT) from the Kerberos server of Active Directory and stores it in the user's credential cache. It also renews the TGT in the background, requiring no user interaction.

openSUSE supports local home directories for AD users. If configured through YaST as described in Section 5.3, “Configuring a Linux Client for Active Directory”, user homes are created at the first login of a Windows (AD) user into the Linux client. These home directories look and feel entirely the same as standard Linux user home directories and work independently of the AD domain controller. Using a local user home, it is possible to access a user's data on this machine (even when the AD server is disconnected) as long as the Linux client has been configured to perform offline authentication.

5.2.3. Offline Service and Policy Support

Users in a corporate environment must have the ability to become roaming users (for example, to switch networks or even work disconnected for some time). To enable users to log in to a disconnected machine, extensive caching was integrated into the winbind daemon. The winbind daemon enforces password policies even in the offline state. It tracks the number of failed login attempts and reacts according to the policies configured in Active Directory. Offline support is disabled by default and must be explicitly enabled in the YaST Domain Membership module.

When the domain controller has become unavailable, the user can still access network resources (other than the AD server itself) with valid Kerberos tickets that have been acquired before losing the connection (as in Windows). Password changes cannot be processed unless the domain controller is online. While disconnected from the AD server, a user cannot access any data stored on this server. When a workstation has become disconnected from the network entirely and connects to the corporate network again later, openSUSE acquires a new Kerberos ticket as soon as the user has locked and unlocked the desktop (for example, using a desktop screen saver).

5.3. Configuring a Linux Client for Active Directory

Before your client can join an AD domain, some adjustments must be made to your network setup to ensure the flawless interaction of client and server.


Configure your client machine to use a DNS server that can forward DNS requests to the AD DNS server. Alternatively, configure your machine to use the AD DNS server as the name service data source.


To succeed with Kerberos authentication, the client must have its time set accurately. It is highly recommended to use a central NTP time server for this purpose (this can be also the NTP server running on your Active Directory domain controller). If the clock skew between your Linux host and the domain controller exceeds a certain limit, Kerberos authentication fails and the client is logged in using the weaker NTLM (NT LAN Manager) authentication. For more details about using active directory for time synchronization, see Procedure 5.1, “Joining an AD Domain”.


If your client uses dynamic network configuration with DHCP, configure DHCP to provide the same IP and hostname to the client. If possible, use static IP addresses.


To browse your network neighborhood, either disable the firewall entirely or mark the interface used for browsing as part of the internal zone.

To change the firewall settings on your client, log in as root and start the YaST firewall module. Select Interfaces. Select your network interface from the list of interfaces and click Change. Select Internal Zone and apply your settings with OK. Leave the firewall settings with Next+Finish. To disable the firewall, just check the Disable Firewall Automatic Starting option, and leave the firewall module with Next+Finish.

AD Account

You cannot log in to an AD domain unless the AD administrator has provided you with a valid user account for that domain. Use the AD username and password to log in to the AD domain from your Linux client.

Join an existing AD domain during installation (or by later activating SMB user authentication with YaST in the installed system).


Currently only a domain administrator account, such as Administrator, can join openSUSE into Active Directory.

To join an AD domain in a running system, proceed as follows:

Procedure 5.1. Joining an AD Domain

  1. Log in as root and start YaST.

  2. Start Network Services+Windows Domain Membership.

  3. Enter the domain to join at Domain or Workgroup in the Windows Domain Membership screen (see Figure 5.2, “Determining Windows Domain Membership”). If the DNS settings on your host are properly integrated with the Windows DNS server, enter the AD domain name in its DNS format ( If you enter the short name of your domain (also known as the pre–Windows 2000 domain name), YaST must rely on NetBIOS name resolution instead of DNS to find the correct domain controller.

    Figure 5.2. Determining Windows Domain Membership

    Determining Windows Domain Membership

  4. Check Also Use SMB Information for Linux Authentication to use the SMB source for Linux authentication.

  5. Check Create Home Directory on Login to automatically create a local home directory for your AD user on the Linux machine.

  6. Check Offline Authentication to allow your domain users to log in even if the AD server is temporarily unavailable, or if you do not have a network connection.

  7. Select Expert Settings, if you want to change the UID and GID ranges for the Samba users and groups. Let DHCP retrieve the WINS server only if you need it. This is the case when some of your machines are resolved only by the WINS system.

  8. Configure NTP time synchronization for your AD environment by selecting NTP Configuration and entering an appropriate server name or IP address. This step is obsolete if you have already entered the appropriate settings in the standalone YaST NTP configuration module.

  9. Click OK and confirm the domain join when prompted for it.

  10. Provide the password for the Windows administrator on the AD server and click OK (see Figure 5.3, “Providing Administrator Credentials”).

    Figure 5.3. Providing Administrator Credentials

    Providing Administrator Credentials

After you have joined the AD domain, you can log in to it from your workstation using the display manager of your desktop or the console.

5.4. Logging In to an AD Domain

Provided your machine has been configured to authenticate against Active Directory and you have a valid Windows user identity, you can log in to your machine using the AD credentials. Login is supported for both desktop environments (GNOME and KDE), the console, SSH, and any other PAM-aware application.

[Important]Offline Authentication

openSUSE supports offline authentication, allowing you to remain logged in to your client machine even if the client machine is disconnected from the network.

5.4.1. GDM and KDM

To authenticate a GNOME client machine against an AD server, proceed as follows:

  1. Select the domain.

  2. Enter your Windows username and press Enter.

  3. Enter your Windows password and press Enter.

To authenticate a KDE client machine against an AD server, proceed as follows:

  1. Select the domain.

  2. Enter your Windows username.

  3. Enter your Windows password and press Enter.

If configured to do so, openSUSE creates a user home directory on the local machine on the first login of each AD authenticated user. This allows you to benefit from the AD support of openSUSE while still having a fully functional Linux machine at your disposal.

5.4.2. Console Login

As well as logging into the AD client machine using a graphical front-end, you can log in using the text-based console login or even remotely using SSH.

To log in to your AD client from a console, enter DOMAIN\user at the login: prompt and provide the password.

To remotely log in to your AD client machine using SSH, proceed as follows:

  1. At the login prompt, enter:

    ssh DOMAIN\\user@hostname

    The \ domain and login delimiter is escaped with another \ sign.

  2. Provide the user's password.

5.5. Changing Passwords

openSUSE has the ability to help a user choose a suitable new password that meets the corporate security policy. The underlying PAM module retrieves the current password policy settings from the domain controller, informing the user about the specific password quality requirements a user account typically has by means of a message on login. Like its Windows counterpart, openSUSE presents a message describing:

  • Password history settings

  • Minimum password length requirements

  • Minimum password age

  • Password complexity

The password change process cannot succeed unless all requirements have been successfully met. Feedback about the password status is given both through the display managers and the console.

GDM and KDM provide feedback about password expiration and the prompt for new passwords in an interactive mode. To change passwords in the display managers, just provide the password information when prompted.

To change your Windows password, you can use the standard Linux utility, passwd, instead of having to manipulate this data on the server. To change your Windows password, proceed as follows:

  1. Log in at the console.

  2. Enter passwd.

  3. Enter your current password when prompted.

  4. Enter the new password.

  5. Reenter the new password for confirmation. If your new password does not comply with the policies on the Windows server, this feedback is given to you and you are prompted for another password.

To change your Windows password from the GNOME desktop, proceed as follows:

  1. Click the Computer icon on the left edge of the panel.

  2. Select Control Center.

  3. From the Personal section, select About Me+Change Password.

  4. Enter your old password.

  5. Enter and confirm the new password.

  6. Leave the dialog with Close to apply your settings.

To change your Windows password from the KDE desktop, proceed as follows:

  1. Select Configure Desktop from the main menu.

  2. Select About Me from the Personal section.

  3. Click Password & User Account.

  4. Click Change Password.

  5. Enter your current password.

  6. Enter and confirm the new password and apply your settings with OK.

  7. Leave the configuration dialog with File+Quit.

Chapter 6. Network Authentication with Kerberos

An open network provides no means of ensuring that a workstation can identify its users properly, except through the usual password mechanisms. In common installations, the user must enter the password each time a service inside the network is accessed. Kerberos provides an authentication method with which a user registers only once and is trusted in the complete network for the rest of the session. To have a secure network, the following requirements must be met:

  • Have all users prove their identity for each desired service and make sure that no one can take the identity of someone else.

  • Make sure that each network server also proves its identity. Otherwise an attacker might be able to impersonate the server and obtain sensitive information transmitted to the server. This concept is called mutual authentication, because the client authenticates to the server and vice versa.

Kerberos helps you meet these requirements by providing strongly encrypted authentication. Only the basic principles of Kerberos are discussed here. For detailed technical instruction, refer to the Kerberos documentation.

6.1. Kerberos Terminology

The following glossary defines some Kerberos terminology.


Users or clients need to present some kind of credentials that authorize them to request services. Kerberos knows two kinds of credentials—tickets and authenticators.


A ticket is a per-server credential used by a client to authenticate at a server from which it is requesting a service. It contains the name of the server, the client's name, the client's Internet address, a time stamp, a lifetime, and a random session key. All this data is encrypted using the server's key.


Combined with the ticket, an authenticator is used to prove that the client presenting a ticket is really the one it claims to be. An authenticator is built using the client's name, the workstation's IP address, and the current workstation's time, all encrypted with the session key known only to the client and the relevant server. An authenticator can only be used once, unlike a ticket. A client can build an authenticator itself.


A Kerberos principal is a unique entity (a user or service) to which it can assign a ticket. A principal consists of the following components:

  • Primary—the first part of the principal, which can be the same as your username in the case of a user.

  • Instance—some optional information characterizing the primary. This string is separated from the primary by a /.

  • Realm—this specifies your Kerberos realm. Normally, your realm is your domain name in uppercase letters.

mutual authentication

Kerberos ensures that both client and server can be sure of each other's identity. They share a session key, which they can use to communicate securely.

session key

Session keys are temporary private keys generated by Kerberos. They are known to the client and used to encrypt the communication between the client and the server for which it requested and received a ticket.


Almost all messages sent in a network can be eavesdropped, stolen, and resent. In the Kerberos context, this would be most dangerous if an attacker manages to obtain your request for a service containing your ticket and authenticator. The attacker could then try to resend it (replay) to impersonate you. However, Kerberos implements several mechanisms to deal with this problem.

server or service

Service is used to refer to a specific action to perform. The process behind this action is referred to as a server.

6.2. How Kerberos Works

Kerberos is often called a third party trusted authentication service, which means all its clients trust Kerberos's judgment of another client's identity. Kerberos keeps a database of all its users and their private keys.

To ensure Kerberos is working correctly, run both the authentication and ticket-granting server on a dedicated machine. Make sure that only the administrator can access this machine physically and over the network. Reduce the (networking) services running on it to the absolute minimum—do not even run sshd.

6.2.1. First Contact

Your first contact with Kerberos is quite similar to any login procedure at a normal networking system. Enter your username. This piece of information and the name of the ticket-granting service are sent to the authentication server (Kerberos). If the authentication server knows you, it generates a random session key for further use between your client and the ticket-granting server. Now the authentication server prepares a ticket for the ticket-granting server. The ticket contains the following information—all encrypted with a session key only the authentication server and the ticket-granting server know:

  • The names of both, the client and the ticket-granting server

  • The current time

  • A lifetime assigned to this ticket

  • The client's IP address

  • The newly-generated session key

This ticket is then sent back to the client together with the session key, again in encrypted form, but this time the private key of the client is used. This private key is only known to Kerberos and the client, because it is derived from your user password. Now that the client has received this response, you are prompted for your password. This password is converted into the key that can decrypt the package sent by the authentication server. The package is unwrapped and password and key are erased from the workstation's memory. As long as the lifetime given to the ticket used to obtain other tickets does not expire, your workstation can prove your identity.

6.2.2. Requesting a Service

To request a service from any server in the network, the client application needs to prove its identity to the server. Therefore, the application generates an authenticator. An authenticator consists of the following components:

  • The client's principal

  • The client's IP address

  • The current time

  • A checksum (chosen by the client)

All this information is encrypted using the session key that the client has already received for this special server. The authenticator and the ticket for the server are sent to the server. The server uses its copy of the session key to decrypt the authenticator, which gives it all the information needed about the client requesting its service, to compare it to that contained in the ticket. The server checks if the ticket and the authenticator originate from the same client.

Without any security measures implemented on the server side, this stage of the process would be an ideal target for replay attacks. Someone could try to resend a request stolen off the net some time before. To prevent this, the server does not accept any request with a time stamp and ticket received previously. In addition to that, a request with a time stamp differing too much from the time the request is received is ignored.

6.2.3. Mutual Authentication

Kerberos authentication can be used in both directions. It is not only a question of the client being the one it claims to be. The server should also be able to authenticate itself to the client requesting its service. Therefore, it sends an authenticator itself. It adds one to the checksum it received in the client's authenticator and encrypts it with the session key, which is shared between it and the client. The client takes this response as a proof of the server's authenticity and they both start cooperating.

6.2.4. Ticket Granting—Contacting All Servers

Tickets are designed to be used for one server at a time. This implies that you have to get a new ticket each time you request another service. Kerberos implements a mechanism to obtain tickets for individual servers. This service is called the ticket-granting service. The ticket-granting service is a service (like any other service mentioned before) and uses the same access protocols that have already been outlined. Any time an application needs a ticket that has not already been requested, it contacts the ticket-granting server. This request consists of the following components:

  • The requested principal

  • The ticket-granting ticket

  • An authenticator

Like any other server, the ticket-granting server now checks the ticket-granting ticket and the authenticator. If they are considered valid, the ticket-granting server builds a new session key to be used between the original client and the new server. Then the ticket for the new server is built, containing the following information:

  • The client's principal

  • The server's principal

  • The current time

  • The client's IP address

  • The newly-generated session key

The new ticket has a lifetime, which is either the remaining lifetime of the ticket-granting ticket or the default for the service. The lesser of both values is assigned. The client receives this ticket and the session key, which are sent by the ticket-granting service, but this time the answer is encrypted with the session key that came with the original ticket-granting ticket. The client can decrypt the response without requiring the user's password when a new service is contacted. Kerberos can thus acquire ticket after ticket for the client without bothering the user.

6.2.5. Compatibility to Windows 2000

Windows 2000 contains a Microsoft implementation of Kerberos 5. openSUSE® uses the MIT implementation of Kerberos 5, find useful information and guidance in the MIT documentation at Section 6.5, “For More Information”.

6.3. Users' View of Kerberos

Ideally, a user's one and only contact with Kerberos happens during login at the workstation. The login process includes obtaining a ticket-granting ticket. At logout, a user's Kerberos tickets are automatically destroyed, which makes it difficult for anyone else to impersonate this user. The automatic expiration of tickets can lead to a somewhat awkward situation when a user's login session lasts longer than the maximum lifespan given to the ticket-granting ticket (a reasonable setting is 10 hours). However, the user can get a new ticket-granting ticket by running kinit. Enter the password again and Kerberos obtains access to desired services without additional authentication. To get a list of all the tickets silently acquired for you by Kerberos, run klist.

Here is a short list of some applications that use Kerberos authentication. These applications can be found under /usr/lib/mit/bin or /usr/lib/mit/sbin after installing the package krb5-apps-clients. They all have the full functionality of their common UNIX and Linux brothers plus the additional bonus of transparent authentication managed by Kerberos:

  • telnet, telnetd

  • rlogin

  • rsh, rcp, rshd

  • ftp, ftpd

  • ksu

You no longer have to enter your password for using these applications because Kerberos has already proven your identity. ssh, if compiled with Kerberos support, can even forward all the tickets acquired for one workstation to another one. If you use ssh to log in to another workstation, ssh makes sure that the encrypted contents of the tickets are adjusted to the new situation. Simply copying tickets between workstations is not sufficient because the ticket contains workstation-specific information (the IP address). XDM, GDM, and KDM offer Kerberos support, too. Read more about the Kerberos network applications in Kerberos V5 UNIX User's Guide at

6.4. Installing and Administering Kerberos

A Kerberos environment consists of several different components. A key distribution center (KDC) holds the central database with all Kerberos-relevant data. All clients rely on the KDC for proper authentication across the network. Both the KDC and the clients need to be configured to match your setup:

General Preparations

Check your network setup and make sure it meets the minimum requirements outlined in Section 6.4.1, “Kerberos Network Topology”. Choose an appropriate realm for your Kerberos setup, see Section 6.4.2, “Choosing the Kerberos Realms”. Carefully set up the machine that is to serve as the KDC and apply tight security, see Section 6.4.3, “Setting Up the KDC Hardware”. Set up a reliable time source in your network to make sure all tickets contain valid timestamps, see Section 6.4.4, “Configuring Time Synchronization”.

Basic Configuration

Configure the KDC and the clients, see Section 6.4.5, “Configuring the KDC” and Section 6.4.6, “Configuring Kerberos Clients”. Enable remote administration for your Kerberos service, so you do not need physical access to your KDC machine, see Section 6.4.7, “Configuring Remote Kerberos Administration”. Create service principals for every service in your realm, see Section 6.4.8, “Creating Kerberos Service Principals”.

Enabling Kerberos Authentication

Various services in your network can make use of Kerberos. To add Kerberos password-checking to applications using PAM, proceed as outlined in Section 6.4.9, “Enabling PAM Support for Kerberos”. To configure SSH or LDAP with Kerberos authentication, proceed as outlined in Section 6.4.10, “Configuring SSH for Kerberos Authentication” and Section 6.4.11, “Using LDAP and Kerberos”.

6.4.1. Kerberos Network Topology

Any Kerberos environment must meet the following requirements to be fully functional:

  • Provide a DNS server for name resolution across your network, so clients and servers can locate each other. Refer to Chapter 15, The Domain Name System (↑Reference) for information on DNS setup.

  • Provide a time server in your network. Using exact time stamps is crucial to a Kerberos setup, because valid Kerberos tickets must contain correct time stamps. Refer to Chapter 17, Time Synchronization with NTP (↑Reference) for information on NTP setup.

  • Provide a key distribution center (KDC) as the center piece of the Kerberos architecture. It holds the Kerberos database. Use the tightest possible security policy on this machine to prevent any attacks on this machine compromising your entire infrastructure.

  • Configure the client machines to use Kerberos authentication.

The following figure depicts a simple example network with just the minimum components needed to build a Kerberos infrastructure. Depending on the size and topology of your deployment, your setup may vary.

Figure 6.1. Kerberos Network Topology

Kerberos Network Topology

[Tip]Configuring Subnet Routing

For a setup similar to the one in Figure 6.1, “Kerberos Network Topology”, configure routing between the two subnets ( and Refer to Section “Configuring Routing” (Chapter 13, Basic Networking, ↑Reference) for more information on configuring routing with YaST.

6.4.2. Choosing the Kerberos Realms

The domain of a Kerberos installation is called a realm and is identified by a name, such as EXAMPLE.COM or simply ACCOUNTING. Kerberos is case-sensitive, so is actually a different realm than EXAMPLE.COM. Use the case you prefer. It is common practice, however, to use uppercase realm names.

It is also a good idea to use your DNS domain name (or a subdomain, such as ACCOUNTING.EXAMPLE.COM). As shown below, your life as an administrator can be much easier if you configure your Kerberos clients to locate the KDC and other Kerberos services via DNS. To do so, it is helpful if your realm name is a subdomain of your DNS domain name.

Unlike the DNS name space, Kerberos is not hierarchical. You cannot set up a realm named EXAMPLE.COM, have two subrealms named DEVELOPMENT and ACCOUNTING underneath it, and expect the two subordinate realms to somehow inherit principals from EXAMPLE.COM. Instead, you would have three separate realms for which you would have to configure crossrealm authentication for users from one realm to interact with servers or other users from another realm.

For the sake of simplicity, let us assume you are setting up just one realm for your entire organization. For the remainder of this section, the realm name EXAMPLE.COM is used in all examples.

6.4.3. Setting Up the KDC Hardware

The first thing required to use Kerberos is a machine that acts as the key distribution center, or KDC for short. This machine holds the entire Kerberos user database with passwords and all information.

The KDC is the most important part of your security infrastructure—if someone breaks into it, all user accounts and all of your infrastructure protected by Kerberos is compromised. An attacker with access to the Kerberos database can impersonate any principal in the database. Tighten security for this machine as much as possible:

  1. Put the server machine into a physically secured location, such as a locked server room to which only a very few people have access.

  2. Do not run any network applications on it except the KDC. This includes servers and clients—for example, the KDC should not import any file systems via NFS or use DHCP to retrieve its network configuration.

  3. Install a minimal system first then check the list of installed packages and remove any unneeded packages. This includes servers, such as inetd, portmap, and cups, as well as anything X-based. Even installing an SSH server should be considered a potential security risk.

  4. No graphical login is provided on this machine as an X server is a potential security risk. Kerberos provides its own administration interface.

  5. Configure /etc/nsswitch.conf to use only local files for user and group lookup. Change the lines for passwd and group to look like this:

    passwd:         files 
    group:          files

    Edit the passwd, group, and shadow files in /etc and remove the lines that start with a + character (these are for NIS lookups).

  6. Disable all user accounts except root's account by editing /etc/shadow and replacing the hashed passwords with * or ! characters.

6.4.4. Configuring Time Synchronization

To use Kerberos successfully, make sure that all system clocks within your organization are synchronized within a certain range. This is important because Kerberos protects against replayed credentials. An attacker might be able to observe Kerberos credentials on the network and reuse them to attack the server. Kerberos employs several defenses to prevent this. One of them is that it puts time stamps into its tickets. A server receiving a ticket with a time stamp that differs from the current time rejects the ticket.

Kerberos allows a certain leeway when comparing time stamps. However, computer clocks can be very inaccurate in keeping time—it is not unheard of for PC clocks to lose or gain half an hour over the course of a week. For this reason, configure all hosts on the network to synchronize their clocks with a central time source.

A simple way to do so is by installing an NTP time server on one machine and having all clients synchronize their clocks with this server. Do this either by running an NTP daemon in client mode on all these machines or by running ntpdate once a day from all clients (this solution probably works for a small number of clients only). The KDC itself needs to be synchronized to the common time source as well. Because running an NTP daemon on this machine would be a security risk, it is probably a good idea to do this by running ntpdate via a cron entry. To configure your machine as an NTP client, proceed as outlined in Section “Configuring an NTP Client with YaST” (Chapter 17, Time Synchronization with NTP, ↑Reference).

A different way to secure the time service and still use the NTP daemon is to attach a hardware reference clock to a dedicated NTP server as well as an additional hardware reference clock to the KDC.

It is also possible to adjust the maximum deviation Kerberos allows when checking time stamps. This value (called clock skew) can be set in the krb5.conf file as described in Section, “Adjusting the Clock Skew”.

6.4.5. Configuring the KDC

This section covers the initial configuration and installation of the KDC, including the creation of an administrative principal. This procedure consists of several steps:

  1. Install the RPMs.  On a machine designated as the KDC, install the following software packages: krb5, krb5-server and krb5-client packages.

  2. Adjust the Configuration Files.  The /etc/krb5.conf and /var/lib/kerberos/krb5kdc/kdc.conf configuration files must be adjusted for your scenario. These files contain all information on the KDC.

  3. Create the Kerberos Database.  Kerberos keeps a database of all principal identifiers and the secret keys of all principals that need to be authenticated. Refer to Section, “Setting Up the Database” for details.

  4. Adjust the ACL Files: Add Administrators.  The Kerberos database on the KDC can be managed remotely. To prevent unauthorized principals from tampering with the database, Kerberos uses access control lists. You must explicitly enable remote access for the administrator principal to enable him to manage the database. The Kerberos ACL file is located under /var/lib/kerberos/krb5kdc/kadm5.acl. Refer to Section 6.4.7, “Configuring Remote Kerberos Administration” for details.

  5. Adjust the Kerberos Database: Add Administrators.  You need at least one administrative principal to run and administer Kerberos. This principal must be added before starting the KDC. Refer to Section, “Creating a Principal” for details.

  6. Start the Kerberos Daemon.  Once the KDC software is installed and properly configured, start the Kerberos daemon to provide Kerberos service for your realm. Refer to Section, “Starting the KDC” for details.

  7. Create a Principal for Yourself.  You need a principal for yourself. Refer to Section, “Creating a Principal” for details. Setting Up the Database

Your next step is to initialize the database where Kerberos keeps all information about principals. Set up the database master key, which is used to protect the database from accidental disclosure (in particular if it is backed up to tape). The master key is derived from a pass phrase and is stored in a file called the stash file. This is so you do not need to enter the password every time the KDC is restarted. Make sure that you choose a good pass phrase, such as a sentence from a book opened to a random page.

When you make tape backups of the Kerberos database (/var/lib/kerberos/krb5kdc/principal), do not back up the stash file (which is in /var/lib/kerberos/krb5kdc/.k5.EXAMPLE.COM). Otherwise, everyone able to read the tape could also decrypt the database. Therefore, keep a copy of the pass phrase in a safe or some other secure location, because you will need it to restore your database from backup tape after a crash.

To create the stash file and the database, run:

kdb5_util create -r EXAMPLE.COM -s

You will see the following output:

Initializing database '/var/lib/kerberos/krb5kdc/principal' for realm 'EXAMPLE.COM',
master key name 'K/M@EXAMPLE.COM'
You will be prompted for the database Master Password.
It is important that you NOT FORGET this password.
Enter KDC database master key:  1
Re-enter KDC database master key to verify:  2


Type the master password.


Type the password again.

To verify, use the list command:


kadmin> listprincs

You will see several principals in the database, which are for internal use by Kerberos:

krbtgt/EXAMPLE.COM@EXAMPLE.COM Creating a Principal

Create two Kerberos principals for yourself: one normal principal for everyday work and one for administrative tasks relating to Kerberos. Assuming your login name is geeko, proceed as follows:


kadmin> ank geeko

You will see the following output:

geeko@EXAMPLE.COM's Password: 1
Verifying password: 2


Type geeko's password.


Type geeko's password again.

Next, create another principal named geeko/admin by typing ank geeko/admin at the kadmin prompt. The admin suffixed to your username is a role. Later, use this role when administering the Kerberos database. A user can have several roles for different purposes. Roles are basically completely different accounts with similar names. Starting the KDC

Start the KDC daemon and the kadmin daemon. To start the daemons manually, enter rckrb5kdc start and rckadmind start. Also make sure that KDC and kadmind are started by default when the server machine is rebooted with the command insserv krb5kdc and insserv kadmind or use the YaST runlevel editor.

6.4.6. Configuring Kerberos Clients

Once the supporting infrastructure is in place (DNS, NTP) and the KDC has been properly configured and started, configure the client machines. You can either use YaST to configure a Kerberos client or use one of the two manual approaches described below. Configuring a Kerberos Client with YaST

Rather than manually editing all relevant configuration files when configuring a Kerberos client, let YaST do the job for you. You can either perform the client configuration during the installation of your machine or in the installed system as follows:

  1. Log in as root and select Network Services+Kerberos Client (Figure 6.2, “YaST: Basic Configuration of a Kerberos Client”).

  2. Select Use Kerberos.

  3. To configure a DNS-based Kerberos client, proceed as follows:

    1. DNS-Based Static Kerberos Client

      [Note]Using DNS Support

      The Use DNS option cannot be selected if the DNS server does not provide such data.

    2. Click Advanced Settings to configure details on ticket-related issues, OpenSSH support, time synchronization, and extended PAM configurations.

  4. To configure a static Kerberos client, proceed as follows:

    1. Set Default Domain, Default Realm, and KDC Server Address to the values that match your setup.

    2. Click Advanced Settings to configure details on ticket-related issues, OpenSSH support, time synchronization, and extended PAM configurations.

Figure 6.2. YaST: Basic Configuration of a Kerberos Client

YaST: Basic Configuration of a Kerberos Client

To configure ticket-related options in the Advanced Settings dialog (Figure 6.3, “YaST: Advanced Configuration of a Kerberos Client”), choose from the following options:

  • Specify the Default Ticket Lifetime and the Default Renewable Lifetime in days, hours, or minutes (using the units of measurement d, h, and m, with no blank space between the value and the unit).

  • To forward your complete identity (to use your tickets on other hosts), select Forwardable.

  • Enable the transfer of certain tickets by selecting Proxiable.

  • Enable Kerberos authentication support for your OpenSSH client by selecting the corresponding check box. The client then uses Kerberos tickets to authenticate with the SSH server.

  • Exclude a range of user accounts from using Kerberos authentication by providing a value for the Minimum UID that a user of this feature must have. For instance, you may want to exclude the system administrator (root).

  • Use Clock Skew to set a value for the allowable difference between the time stamps and your host's system time.

  • To keep the system time in sync with an NTP server, you can also set up the host as an NTP client by selecting NTP Configuration, which opens the YaST NTP client dialog that is described in Section “Configuring an NTP Client with YaST” (Chapter 17, Time Synchronization with NTP, ↑Reference). After finishing the configuration, YaST performs all the necessary changes and the Kerberos client is ready to use.

Figure 6.3. YaST: Advanced Configuration of a Kerberos Client

YaST: Advanced Configuration of a Kerberos Client

For more information about the configuration of Expert PAM Settings and PAM Services tabs, see the official documentation referenced in Section 6.5, “For More Information” and the manual page man 5 krb5.conf, which is part of the krb5-doc package. Manually Configuring Kerberos Clients

When configuring Kerberos, there are basically two approaches you can take—static configuration in the /etc/krb5.conf file or dynamic configuration with DNS. With DNS configuration, Kerberos applications try to locate the KDC services using DNS records. With static configuration, add the hostnames of your KDC server to krb5.conf (and update the file whenever you move the KDC or reconfigure your realm in other ways).

DNS-based configuration is generally a lot more flexible and the amount of configuration work per machine is a lot less. However, it requires that your realm name is either the same as your DNS domain or a subdomain of it. Configuring Kerberos via DNS also creates a minor security issue—an attacker can seriously disrupt your infrastructure through your DNS (by shooting down the name server, spoofing DNS records, etc.). However, this amounts to a denial of service at worst. A similar scenario applies to the static configuration case unless you enter IP addresses in krb5.conf instead of hostnames. Static Configuration

One way to configure Kerberos is to edit /etc/krb5.conf. The file installed by default contains various sample entries. Erase all of these entries before starting. krb5.conf is made up of several sections (stanzas), each introduced by the section name in brackets like [this].

To configure your Kerberos clients, add the following stanza to krb5.conf (where is the hostname of the KDC):

        default_realm = EXAMPLE.COM

        EXAMPLE.COM = {
                kdc =
                admin_server =

The default_realm line sets the default realm for Kerberos applications. If you have several realms, just add additional statements to the [realms] section.

Also add a statement to this file that tells applications how to map hostnames to a realm. For example, when connecting to a remote host, the Kerberos library needs to know in which realm this host is located. This must be configured in the [domain_realms] section:

[domain_realm] = EXAMPLE.COM = EXAMPLE.COM

This tells the library that all hosts in the DNS domains are in the EXAMPLE.COM Kerberos realm. In addition, one external host named should also be considered a member of the EXAMPLE.COM realm. DNS-Based Configuration

DNS-based Kerberos configuration makes heavy use of SRV records. See (RFC2052) A DNS RR for specifying the location of services at

The name of an SRV record, as far as Kerberos is concerned, is always in the format _service._proto.realm, where realm is the Kerberos realm. Domain names in DNS are case insensitive, so case-sensitive Kerberos realms would break when using this configuration method. _service is a service name (different names are used when trying to contact the KDC or the password service, for example). _proto can be either _udp or _tcp, but not all services support both protocols.

The data portion of SRV resource records consists of a priority value, a weight, a port number, and a hostname. The priority defines the order in which hosts should be tried (lower values indicate a higher priority). The weight value is there to support some sort of load balancing among servers of equal priority. You probably do not need any of this, so it is okay to set these to zero.

MIT Kerberos currently looks up the following names when looking for services:


This defines the location of the KDC daemon (the authentication and ticket granting server). Typical records look like this:

_kerberos._udp.EXAMPLE.COM.  IN  SRV    0 0 88 
_kerberos._tcp.EXAMPLE.COM.  IN  SRV    0 0 88

This describes the location of the remote administration service. Typical records look like this:

_kerberos-adm._tcp.EXAMPLE.COM. IN  SRV    0 0 749

Because kadmind does not support UDP, there should be no _udp record.

As with the static configuration file, there is a mechanism to inform clients that a specific host is in the EXAMPLE.COM realm, even if it is not part of the DNS domain. This can be done by attaching a TXT record to _kerberos.hostname, as shown here:  IN TXT "EXAMPLE.COM" Adjusting the Clock Skew

The clock skew is the tolerance for accepting tickets with time stamps that do not exactly match the host's system clock. Usually, the clock skew is set to 300 seconds (five minutes). This means a ticket can have a time stamp somewhere between five minutes behind and five minutes ahead of the server's clock.

When using NTP to synchronize all hosts, you can reduce this value to about one minute. The clock skew value can be set in /etc/krb5.conf like this:

        clockskew = 60

6.4.7. Configuring Remote Kerberos Administration

To be able to add and remove principals from the Kerberos database without accessing the KDC's console directly, tell the Kerberos administration server which principals are allowed to do what by editing /var/lib/kerberos/krb5kdc/kadm5.acl. The ACL (access control list) file allows you to specify privileges with a precise degree of control. For details, refer to the manual page with man 8 kadmind.

For now, just grant yourself the privilege to administer the database by putting the following line into the file:

geeko/admin              *

Replace the username geeko with your own. Restart kadmind for the change to take effect.

You should now be able to perform Kerberos administration tasks remotely using the kadmin tool. First, obtain a ticket for your admin role and use that ticket when connecting to the kadmin server:

kadmin -p geeko/admin
Authenticating as principal geeko/admin@EXAMPLE.COM with password.
Password for geeko/admin@EXAMPLE.COM:
kadmin:  getprivs
current privileges: GET ADD MODIFY DELETE

Using the getprivs command, verify which privileges you have. The list shown above is the full set of privileges.

As an example, modify the principal geeko:

kadmin -p geeko/admin
Authenticating as principal geeko/admin@EXAMPLE.COM with password.
Password for geeko/admin@EXAMPLE.COM:

kadmin:  getprinc geeko
Principal: geeko@EXAMPLE.COM
Expiration date: [never]
Last password change: Wed Jan 12 17:28:46 CET 2005
Password expiration date: [none]
Maximum ticket life: 0 days 10:00:00
Maximum renewable life: 7 days 00:00:00
Last modified: Wed Jan 12 17:47:17 CET 2005 (admin/admin@EXAMPLE.COM)
Last successful authentication: [never]
Last failed authentication: [never]
Failed password attempts: 0
Number of keys: 2
Key: vno 1, Triple DES cbc mode with HMAC/sha1, no salt
Key: vno 1, DES cbc mode with CRC-32, no salt
Policy: [none]

kadmin:  modify_principal -maxlife "8 hours" geeko
Principal "geeko@EXAMPLE.COM" modified.
kadmin:  getprinc joe
Principal: geeko@EXAMPLE.COM
Expiration date: [never]
Last password change: Wed Jan 12 17:28:46 CET 2005
Password expiration date: [none]
Maximum ticket life: 0 days 08:00:00
Maximum renewable life: 7 days 00:00:00
Last modified: Wed Jan 12 17:59:49 CET 2005 (geeko/admin@EXAMPLE.COM)
Last successful authentication: [never]
Last failed authentication: [never]
Failed password attempts: 0
Number of keys: 2
Key: vno 1, Triple DES cbc mode with HMAC/sha1, no salt
Key: vno 1, DES cbc mode with CRC-32, no salt
Policy: [none]

This changes the maximum ticket life time to eight hours. For more information about the kadmin command and the options available, see the krb5-doc package, refer to, or the man8 kadmin manual page.

6.4.8. Creating Kerberos Service Principals

So far, only user credentials have been discussed. However, Kerberos-compatible services usually need to authenticate themselves to the client user, too. Therefore, special service principals must be present in the Kerberos database for each service offered in the realm. For example, if offers an LDAP service, you need a service principal, ldap/, to authenticate this service to all clients.

The naming convention for service principals is service/hostname@REALM, where hostname is the host's fully qualified hostname.

Valid service descriptors are:

Service Descriptor



Telnet, RSH, SSH


NFSv4 (with Kerberos support)


HTTP (with Kerberos authentication)







Service principals are similar to user principals, but have significant differences. The main difference between a user principal and a service principal is that the key of the former is protected by a password—when a user obtains a ticket-granting ticket from the KDC, he needs to type his password so Kerberos can decrypt the ticket. It would be quite inconvenient for the system administrator if he had to obtain new tickets for the SSH daemon every eight hours or so.

Instead, the key required to decrypt the initial ticket for the service principal is extracted by the administrator from the KDC only once and stored in a local file called the keytab. Services such as the SSH daemon read this key and use it to obtain new tickets automatically, when needed. The default keytab file resides in /etc/krb5.keytab.

To create a host service principal for enter the following commands during your kadmin session:

kadmin -p geeko/admin
Authenticating as principal geeko/admin@EXAMPLE.COM with password.
Password for geeko/admin@EXAMPLE.COM:
kadmin:  addprinc -randkey host/
WARNING: no policy specified for host/;
defaulting to no policy
Principal "host/" created.

Instead of setting a password for the new principal, the -randkey flag tells kadmin to generate a random key. This is used here because no user interaction is wanted for this principal. It is a server account for the machine.

Finally, extract the key and store it in the local keytab file /etc/krb5.keytab. This file is owned by the superuser, so you must be root to execute the next command in the kadmin shell:

kadmin:  ktadd host/
Entry for principal host/ with kvno 3, encryption type Triple
DES cbc mode with HMAC/sha1 added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/ with kvno 3, encryption type DES
cbc mode with CRC-32 added to keytab WRFILE:/etc/krb5.keytab.

When completed, make sure that you destroy the admin ticket obtained with kinit above with kdestroy.

6.4.9. Enabling PAM Support for Kerberos

openSUSE® comes with a PAM module named pam_krb5, which supports Kerberos login and password update. This module can be used by applications such as console login, su, and graphical login applications like KDM (where the user presents a password and would like the authenticating application to obtain an initial Kerberos ticket on his behalf). To configure PAM support for Kerberos, use the following command:

pam-config --add --krb5

The above command adds the pam_krb5 module to the existing PAM configuration files and makes sure it is called in the right order. To make precise adjustments to the way in which pam_krb5 is used, edit the file /etc/krb5.conf and add default applications to pam. For details, refer to the manual page with man 5 pam_krb5.

The pam_krb5 module was specifically not designed for network services that accept Kerberos tickets as part of user authentication. This is an entirely different matter, and is discussed below.

6.4.10. Configuring SSH for Kerberos Authentication

OpenSSH supports Kerberos authentication in both protocol version 1 and 2. In version 1, there are special protocol messages to transmit Kerberos tickets. Version 2 does not use Kerberos directly anymore, but relies on GSSAPI, the General Security Services API. This is a programming interface that is not specific to Kerberos—it was designed to hide the peculiarities of the underlying authentication system, be it Kerberos, a public-key authentication system like SPKM, or others. However, the included GSSAPI library only supports Kerberos.

To use sshd with Kerberos authentication, edit /etc/ssh/sshd_config and set the following options:

# These are for protocol version 1 
# KerberosAuthentication yes
# KerberosTicketCleanup yes

# These are for version 2 - better to use this
GSSAPIAuthentication yes
GSSAPICleanupCredentials yes

Then restart your SSH daemon using rcsshd restart.

To use Kerberos authentication with protocol version 2, enable it on the client side as well. Do this either in the systemwide configuration file /etc/ssh/ssh_config or on a per-user level by editing ~/.ssh/config. In both cases, add the option GSSAPIAuthentication yes.

You should now be able to connect using Kerberos authentication. Use klist to verify that you have a valid ticket, then connect to the SSH server. To force SSH protocol version 1, specify the -1 option on the command line.

[Tip]Additional Information

The file /usr/share/doc/packages/openssh/README.kerberos discusses the interaction of OpenSSH and Kerberos in more detail.

6.4.11. Using LDAP and Kerberos

When using Kerberos, one way to distribute the user information (such as user ID, groups, and home directory) in your local network is to use LDAP. This requires a strong authentication mechanism that prevents packet spoofing and other attacks. One solution is to use Kerberos for LDAP communication, too.

OpenLDAP implements most authentication flavors through SASL, the simple authentication session layer. SASL is basically a network protocol designed for authentication. The SASL implementation is cyrus-sasl, which supports a number of different authentication flavors. Kerberos authentication is performed through GSSAPI (General Security Services API). By default, the SASL plug-in for GSSAPI is not installed. Install the cyrus-sasl-gssapi with YaST.

To enable Kerberos to bind to the OpenLDAP server, create a principal ldap/ and add that to the keytab.

By default, the LDAP server slapd runs as user and group ldap, while the keytab file is readable by root only. Therefore, either change the LDAP configuration so the server runs as root or make the keytab file readable by the group ldap. The latter is done automatically by the OpenLDAP start script (/etc/init.d/ldap) if the keytab file has been specified in the OPENLDAP_KRB5_KEYTAB variable in /etc/sysconfig/openldap and the OPENLDAP_CHOWN_DIRS variable is set to yes, which is the default setting. If OPENLDAP_KRB5_KEYTAB is left empty, the default keytab under /etc/krb5.keytab is used and you must adjust the privileges yourself as described below.

To run slapd as root, edit /etc/sysconfig/openldap. Disable the OPENLDAP_USER and OPENLDAP_GROUP variables by putting a comment character in front of them.

To make the keytab file readable by group LDAP, execute

chgrp ldap /etc/krb5.keytab
chmod 640 /etc/krb5.keytab

A third (and maybe the best) solution is to tell OpenLDAP to use a special keytab file. To do this, start kadmin, and enter the following command after you have added the principal ldap/

ktadd -k /etc/openldap/ldap.keytab ldap/

Then in the shell run:

chown ldap.ldap /etc/openldap/ldap.keytab 
chmod 600 /etc/openldap/ldap.keytab

To tell OpenLDAP to use a different keytab file, change the following variable in /etc/sysconfig/openldap:


Finally, restart the LDAP server using rcldap restart. Using Kerberos Authentication with LDAP

You are now able to automatically use tools such as ldapsearch with Kerberos authentication.

ldapsearch -b ou=people,dc=example,dc=com '(uid=geeko)'

SASL/GSSAPI authentication started
SASL installing layers

# geeko, people,
dn: uid=geeko,ou=people,dc=example,dc=com
uid: geeko
cn: Olaf Kirch

As you can see, ldapsearch prints a message that it started GSSAPI authentication. The next message is very cryptic, but it shows that the security strength factor (SSF for short) is 56 (The value 56 is somewhat arbitrary. Most likely it was chosen because this is the number of bits in a DES encryption key). What this tells you is that GSSAPI authentication was successful and that encryption is being used to protect integrity and provide confidentiality for the LDAP connection.

In Kerberos, authentication is always mutual. This means that not only have you authenticated yourself to the LDAP server, but also the LDAP server has authenticated itself to you. In particular, this means communication is with the desired LDAP server, rather than some bogus service set up by an attacker. Kerberos Authentication and LDAP Access Control

Now, allow each user to modify the login shell attribute of their LDAP user record. Assuming you have a schema where the LDAP entry of user joe is located at uid=joe,ou=people,dc=example,dc=com, set up the following access controls in /etc/openldap/slapd.conf:

# This is required for things to work _at all_
access to dn.base="" by * read
# Let each user change their login shell
access to dn="*,ou=people,dc=example,dc=com" attrs=loginShell
       by self write
# Every user can read everything
access to *
       by users read

The second statement gives authenticated users write access to the loginShell attribute of their own LDAP entry. The third statement gives all authenticated users read access to the entire LDAP directory.

There is one minor piece of the puzzle missing—how the LDAP server can find out that the Kerberos user joe@EXAMPLE.COM corresponds to the LDAP distinguished name uid=joe,ou=people,dc=example,dc=com. This sort of mapping must be configured manually using the saslExpr directive. In this example, add the following to slapd.conf:


To understand how this works, you need to know that when SASL authenticates a user, OpenLDAP forms a distinguished name from the name given to it by SASL (such as joe) and the name of the SASL flavor (GSSAPI). The result would be uid=joe,cn=GSSAPI,cn=auth.

If a authz-regexp has been configured, it checks the DN formed from the SASL information using the first argument as a regular expression. If this regular expression matches, the name is replaced with the second argument of the authz-regexp statement. The placeholder $1 is replaced with the substring matched by the (.*) expression.

More complicated match expressions are possible. If you have a more complicated directory structure or a schema in which the username is not part of the DN, you can even use search expressions to map the SASL DN to the user DN.

6.5. For More Information

The official site of the MIT Kerberos is There, find links to any other relevant resource concerning Kerberos, including Kerberos installation, user, and administration guides.

The paper at gives quite an extensive insight to the basic principles of Kerberos, without being too difficult to read. It also provides a lot of opportunities for further investigation and reading about Kerberos.

The official Kerberos FAQ is available at The book Kerberos—A Network Authentication System by Brian Tung (ISBN 0-201-37924-4) offers extensive information.

Chapter 7. Using the Fingerprint Reader

If your system includes a fingerprint reader, you can use biometric authentication in addition to standard authentication via login and password. After registering their fingerprint, users can log in to the system either by swiping a finger on the fingerprint reader or by typing in a password. openSUSE® supports most available fingerprint readers. For a list of supported devices, please refer to

If the hardware check detects the fingerprint reader integrated with your laptop (or connected to your system), the packages libfprint, pam_fp, and yast2-fingerprint-reader are automatically installed.

Currently, only one fingerprint per user can be registered. The user's fingerprint data is stored to /home/login/.fprint/.

7.1. Supported Applications and Actions

The PAM module pam_fp supports fingerprint authentication for the following applications and actions (although you may not be prompted to swipe your finger in all cases):

  • Logging in to GDM/KDM or a login shell

  • Unlocking your screen on the GNOME/KDE desktop

  • Starting YaST and the YaST modules

  • Starting an application with root permission: sudo or gnomesu

  • Changing to a different user identity with su or su - username

[Note]Fingerprint Reader Devices and Encrypted Home Directories

If you want to use a fingerprint reader device, you must not use encrypted home directories (see Chapter 10, Managing Users with YaST (↑Start-Up) for more information). Otherwise logging in will fail, because decrypting during login is not possible in combination with an active fingerprint reader device.

7.2. Managing Fingerprints with YaST

Procedure 7.1. Enabling Fingerprint Authentication

You can only use biometric authentication if PAM is configured accordingly. Usually, this is done automatically during installation of the packages when the hardware check detects a supported fingerprint reader. If not, manually enable the fingerprint support in YaST as follows:

  1. Start YaST and select Hardware+Fingerprint Reader.

  2. In the configuration dialog, activate Use Fingerprint Reader and click Finish to save the changes and close the dialog.

Now you can register a fingerprint for various users.

Procedure 7.2. Registering a Fingerprint

  1. In YaST, click Security and Users+User Management to open the User and Group Administration dialog. A list of users or groups in the system is displayed.

  2. Select the user for whom you want to register a fingerprint and click Edit.

  3. On the Plug-Ins tab, select the fingerprint entry and click Launch to open the Fingerprint Configuration dialog.

  4. YaST prompts the user to swipe his finger until three readable fingerprints have been gathered.

  5. After the fingerprint has been acquired successfully, click Accept to close the Fingerprint Configuration dialog and the dialog for the user.

  6. If you also want to use fingerprint authentication for starting YaST or the YaST modules, you need to register a fingerprint for root, too.

    To do so, set the filter in the User and Group Administration dialog to System Users, select the root entry and register a fingerprint for root as described above.

  7. After you have registered fingerprints for the desired users, click Finish to close the administration dialog and to save the changes.

As soon as the user's fingerprint has been successfully registered, the user can choose to authenticate with either fingerprint or password for the actions and applications listed in Section 7.1, “Supported Applications and Actions”.

Currently, YaST does not offer verification or removal of fingerprints, but you remove fingerprints by deleting the directory /home/login/.fprint.

For more technical details, refer to

Part II. Local Security

Chapter 8. Configuring Security Settings with YaST

The YaST module Security Center and Hardening offers a central clearinghouse to configure security-related settings for openSUSE. Use it to configure security aspects such as settings for the login procedure and for password creation, for boot permissions, user creation or for default file permissions. Launch it from the YaST Control Center by Security and Users+Security Center and Hardening. The Security Center dialog always starts with the Security Overview, and other configuration dialogs are available from the right pane.

8.1. Security Overview

The Security Overview displays a comprehensive list of the most important security settings for your system. The security status of each entry in the list is clearly visible. A green check mark indicates a secure setting while a red cross indicates an entry as being insecure. Clicking on Help presents an overview of the setting and information on how to make it secure. To change a setting, click on the corresponding link in the Status column. Depending on the setting, the following entries are available:


Clicking on this entry will toggle the status of the setting to either enabled or disabled.


Clicking on this entry will launch another YaST module for configuration. You will return to the Security Overview when leaving the module.


A setting's status is set to unknown when the associated service is not installed. Such a setting does not represent a potential security risk.

Figure 8.1. YaST Security Center and Hardening - Security Overview

YaST Security Center and Hardening - Security Overview

8.2. Predefined Security Configurations

openSUSE comes with three Predefined Security Configurations. These configurations affect all the settings available in the Security Center module. Each configuration can be modified to your needs using the dialogs available from the right pane. Choose between the following sets:

Home Workstation

This setting is designed for a computer that has no network connection at all (including a connection to the Internet). It provides the least secure configuration of the predefined settings.

Networked Workstation

A configuration for a workstation with any kind of network connection (including a connection to the Internet).

Network Server

Security settings designed for a machine providing network services such as a web server, file server, name server, etc. This set provides the most secure configuration of the predefined settings.

Custom Settings

A pre-selected Custom Settings (when opening the Predefined Security Configurations dialog) indicates that one of the predefined sets has been modified. Actively choosing this option does not change the current configuration - you will have to change it using the Security Overview.

8.3. Password Settings

Passwords that are easy to guess are a major security issue. The Password Settings dialog provides the means to ensure that only secure passwords can be used.

Check New Passwords

By activating this option, a warning will be issued if new passwords appear in a dictionary, or if they are proper names (proper nouns). In order to also check for a minimum length, enter the desired length into the field Minimum Acceptable Password Length after having activated Check New Passwords.

Number of Passwords to Remember

When password expiration is activated (via Password Age), this setting stores the given number of a user's previous passwords, preventing their reuse.

Password Encryption Method

Choose a password encryption algorithm. Normally there is no need to change the default (Blowfish).

Minimum Acceptable Password Length

If the user chooses a password with a length shorter than specified here, a warning will be issued.

Password Age

Activate password expiration by specifying a minimum and a maximum time limit (in days). By setting the minimum age to a value greater than 0 days, you can prevent users from immediately changing their passwords again (and in doing so circumventing the password expiration). Use the values 0 and 99999 to deactivate password expiration.

Days Before Password Expires Warning

When a password expires, the user receives a warning in advance. Specify the number of days prior to the expiration date that the warning should be issued.

8.4. Boot Settings

Configure which users will be able to shutdown the machine via the graphical login manager in this dialog. You can also specify how Ctrl+Alt+Del will be interpreted.

8.5. Login Settings

This dialog lets you configure security-related login settings:

Delay after Incorrect Login Attempt

In order to make it difficult to guess a user's password by repeatedly logging in, it is recommended to delay the display of the login prompt that follows an incorrect login. Specify the value in seconds. Make sure that users who have mistyped their passwords do not need to wait too long.

Record Successful Login Attempts

With this option turned on, the last successful login attempt is recorded in /var/log/lastlog and displayed when logging in. This data is also used by the command finger.


Note that logging to /var/log/wtmp is not affected by this option. This file collects login dates, login times and reboot dates. The content of /var/log/wtmp can be displayed by using the command last.

Allow Remote Graphical Login

When checked, the graphical login manager (e.g. gdm or kdm) can be accessed from the network. This is a potential security risk.

8.6. User Addition

Set minimum and maximum values for user and group IDs. These default settings would rarely need to be changed.

8.7. Miscellaneous Settings

Other security settings that don't fit the above-mentioned categories are listed here:

File Permissions

openSUSE comes with three predefined sets of file permissions for system files. These permission sets define whether a regular user may read log files or start certain programs. Easy file permissions are suitable for standalone machines. This settings allows regular users, for example, to read most system files. See the file /etc/permissions.easy for the complete configuration. The Secure file permissions are designed for multi-user machines with network access. A thorough explanation of these settings can be found in /etc/ The Paranoid settings are the most restrictive ones and should be used with care. See /etc/permissions.paranoid for more information.

User Launching updatedb

The program updatedb scans the system and creates a database of all file locations which can be queried with the command locate. When updatedb is run as user nobody, only world-readable files will be added to the database. When run as user root, almost all files (except the ones root is not allowed to read) will be added.

Current Directory in root's Path / Current Directory in Path of Regular Users

Whenever a program is called without specifying the full path to the executable, the system looks in the user's search path (defined by the variable $PATH) for the executable. By default the current directory is not added to the search path. This setting ensures that, for example, /bin/ls and not the trojan horse /current directory/ls is executed when entering ls. In order to start a program in the current directory the command must be prefixed with ./. When activating these options, the current directory (.) is appended to the search path. It is recommended you not change the default.

Enable Magic SysRq Keys

The magic SysRq key is a keycombo that enables you to have some control over the system even when it has crashed. The complete documentation can be found at /usr/src/linux/Documentation/sysrq.txt (requires installation of the kernel-source package).

Chapter 9. Access Control Lists in Linux

POSIX ACLs (access control lists) can be used as an expansion of the traditional permission concept for file system objects. With ACLs, permissions can be defined more flexibly than with the traditional permission concept.

The term POSIX ACL suggests that this is a true POSIX (portable operating system interface) standard. The respective draft standards POSIX 1003.1e and POSIX 1003.2c have been withdrawn for several reasons. Nevertheless, ACLs (as found on many systems belonging to the UNIX family) are based on these drafts and the implementation of file system ACLs (as described in this chapter) follows these two standards, as well.

9.1. Traditional File Permissions

Find detailed information about the traditional file permissions in the GNU Coreutils Info page, Node File permissions (info coreutils "File permissions"). More advanced features are the setuid, setgid, and sticky bit.

9.1.1. The setuid Bit

In certain situations, the access permissions may be too restrictive. Therefore, Linux has additional settings that enable the temporary change of the current user and group identity for a specific action. For example, the passwd program normally requires root permissions to access /etc/passwd. This file contains some important information, like the home directories of users and user and group IDs. Thus, a normal user would not be able to change passwd, because it would be too dangerous to grant all users direct access to this file. A possible solution to this problem is the setuid mechanism. setuid (set user ID) is a special file attribute that instructs the system to execute programs marked accordingly under a specific user ID. Consider the passwd command:

-rwsr-xr-x  1 root shadow 80036 2004-10-02 11:08 /usr/bin/passwd

You can see the s that denotes that the setuid bit is set for the user permission. By means of the setuid bit, all users starting the passwd command execute it as root.

9.1.2. The setgid Bit

The setuid bit applies to users. However, there is also an equivalent property for groups: the setgid bit. A program for which this bit was set runs under the group ID under which it was saved, no matter which user starts it. Therefore, in a directory with the setgid bit, all newly created files and subdirectories are assigned to the group to which the directory belongs. Consider the following example directory:

drwxrws--- 2 tux archive 48 Nov 19 17:12  backup

You can see the s that denotes that the setgid bit is set for the group permission. The owner of the directory and members of the group archive may access this directory. Users that are not members of this group are mapped to the respective group. The effective group ID of all written files will be archive. For example, a backup program that runs with the group ID archive is able to access this directory even without root privileges.

9.1.3. The Sticky Bit

There is also the sticky bit. It makes a difference whether it belongs to an executable program or a directory. If it belongs to a program, a file marked in this way is loaded to RAM to avoid needing to get it from the hard disk each time it is used. This attribute is used rarely, because modern hard disks are fast enough. If this bit is assigned to a directory, it prevents users from deleting each other's files. Typical examples include the /tmp and /var/tmp directories:

drwxrwxrwt 2 root root 1160 2002-11-19 17:15 /tmp

9.2. Advantages of ACLs

Traditionally, three permission sets are defined for each file object on a Linux system. These sets include the read (r), write (w), and execute (x) permissions for each of three types of users—the file owner, the group, and other users. In addition to that, it is possible to set the set user id, the set group id, and the sticky bit. This lean concept is fully adequate for most practical cases. However, for more complex scenarios or advanced applications, system administrators formerly had to use a number of workarounds to circumvent the limitations of the traditional permission concept.

ACLs can be used as an extension of the traditional file permission concept. They allow the assignment of permissions to individual users or groups even if these do not correspond to the original owner or the owning group. Access control lists are a feature of the Linux kernel and are currently supported by ReiserFS, Ext2, Ext3, JFS, and XFS. Using ACLs, complex scenarios can be realized without implementing complex permission models on the application level.

The advantages of ACLs are evident if you want to replace a Windows server with a Linux server. Some of the connected workstations may continue to run under Windows even after the migration. The Linux system offers file and print services to the Windows clients with Samba. With Samba supporting access control lists, user permissions can be configured both on the Linux server and in Windows with a graphical user interface (only Windows NT and later). With winbindd, part of the samba suite, it is even possible to assign permissions to users only existing in the Windows domain without any account on the Linux server.

9.3. Definitions

User Class

The conventional POSIX permission concept uses three classes of users for assigning permissions in the file system: the owner, the owning group, and other users. Three permission bits can be set for each user class, giving permission to read (r), write (w), and execute (x).


The user and group access permissions for all kinds of file system objects (files and directories) are determined by means of ACLs.

Default ACL

Default ACLs can only be applied to directories. They determine the permissions a file system object inherits from its parent directory when it is created.

ACL Entry

Each ACL consists of a set of ACL entries. An ACL entry contains a type, a qualifier for the user or group to which the entry refers, and a set of permissions. For some entry types, the qualifier for the group or users is undefined.

9.4. Handling ACLs

Table 9.1, “ACL Entry Types” summarizes the six possible types of ACL entries, each defining permissions for a user or a group of users. The owner entry defines the permissions of the user owning the file or directory. The owning group entry defines the permissions of the file's owning group. The superuser can change the owner or owning group with chown or chgrp, in which case the owner and owning group entries refer to the new owner and owning group. Each named user entry defines the permissions of the user specified in the entry's qualifier field. Each named group entry defines the permissions of the group specified in the entry's qualifier field. Only the named user and named group entries have a qualifier field that is not empty. The other entry defines the permissions of all other users.

The mask entry further limits the permissions granted by named user, named group, and owning group entries by defining which of the permissions in those entries are effective and which are masked. If permissions exist in one of the mentioned entries as well as in the mask, they are effective. Permissions contained only in the mask or only in the actual entry are not effective—meaning the permissions are not granted. All permissions defined in the owner and owning group entries are always effective. The example in Table 9.2, “Masking Access Permissions” demonstrates this mechanism.

There are two basic classes of ACLs: A minimum ACL contains only the entries for the types owner, owning group, and other, which correspond to the conventional permission bits for files and directories. An extended ACL goes beyond this. It must contain a mask entry and may contain several entries of the named user and named group types.

Table 9.1. ACL Entry Types


Text Form



named user


owning group


named group






Table 9.2. Masking Access Permissions

Entry Type

Text Form


named user






effective permissions:


9.4.1. ACL Entries and File Mode Permission Bits

Figure 9.1, “Minimum ACL: ACL Entries Compared to Permission Bits” and Figure 9.2, “Extended ACL: ACL Entries Compared to Permission Bits” illustrate the two cases of a minimum ACL and an extended ACL. The figures are structured in three blocks—the left block shows the type specifications of the ACL entries, the center block displays an example ACL, and the right block shows the respective permission bits according to the conventional permission concept (for example, as displayed by ls -l). In both cases, the owner class permissions are mapped to the ACL entry owner. Other class permissions are mapped to the respective ACL entry. However, the mapping of the group class permissions is different in the two cases.

Figure 9.1. Minimum ACL: ACL Entries Compared to Permission Bits

Minimum ACL: ACL Entries Compared to Permission Bits

In the case of a minimum ACL—without mask—the group class permissions are mapped to the ACL entry owning group. This is shown in Figure 9.1, “Minimum ACL: ACL Entries Compared to Permission Bits”. In the case of an extended ACL—with mask—the group class permissions are mapped to the mask entry. This is shown in Figure 9.2, “Extended ACL: ACL Entries Compared to Permission Bits”.

Figure 9.2. Extended ACL: ACL Entries Compared to Permission Bits

Extended ACL: ACL Entries Compared to Permission Bits

This mapping approach ensures the smooth interaction of applications, regardless of whether they have ACL support. The access permissions that were assigned by means of the permission bits represent the upper limit for all other fine adjustments made with an ACL. Changes made to the permission bits are reflected by the ACL and vice versa.

9.4.2. A Directory with an ACL

With getfacl and setfacl on the command line, you can access ACLs. The usage of these commands is demonstrated in the following example.

Before creating the directory, use the umask command to define which access permissions should be masked each time a file object is created. The command umask 027 sets the default permissions by giving the owner the full range of permissions (0), denying the group write access (2), and giving other users no permissions at all (7). umask actually masks the corresponding permission bits or turns them off. For details, consult the umask man page.

mkdir mydir creates the mydir directory with the default permissions as set by umask. Use ls -dl mydir to check whether all permissions were assigned correctly. The output for this example is:

drwxr-x--- ... tux project3 ... mydir

With getfacl mydir, check the initial state of the ACL. This gives information like:

# file: mydir 
# owner: tux 
# group: project3 

The first three output lines display the name, owner, and owning group of the directory. The next three lines contain the three ACL entries owner, owning group, and other. In fact, in the case of this minimum ACL, the getfacl command does not produce any information you could not have obtained with ls.

Modify the ACL to assign read, write, and execute permissions to an additional user geeko and an additional group mascots with:

setfacl -m user:geeko:rwx,group:mascots:rwx mydir

The option -m prompts setfacl to modify the existing ACL. The following argument indicates the ACL entries to modify (multiple entries are separated by commas). The final part specifies the name of the directory to which these modifications should be applied. Use the getfacl command to take a look at the resulting ACL.

# file: mydir 
# owner: tux 
# group: project3 

In addition to the entries initiated for the user geeko and the group mascots, a mask entry has been generated. This mask entry is set automatically so that all permissions are effective. setfacl automatically adapts existing mask entries to the settings modified, unless you deactivate this feature with -n. mask defines the maximum effective access permissions for all entries in the group class. This includes named user, named group, and owning group. The group class permission bits displayed by ls -dl mydir now correspond to the mask entry.

drwxrwx---+ ... tux project3 ... mydir

The first column of the output contains an additional + to indicate that there is an extended ACL for this item.

According to the output of the ls command, the permissions for the mask entry include write access. Traditionally, such permission bits would mean that the owning group (here project3) also has write access to the directory mydir. However, the effective access permissions for the owning group correspond to the overlapping portion of the permissions defined for the owning group and for the mask—which is r-x in our example (see Table 9.2, “Masking Access Permissions”). As far as the effective permissions of the owning group in this example are concerned, nothing has changed even after the addition of the ACL entries.

Edit the mask entry with setfacl or chmod. For example, use chmod g-w mydir. ls -dl mydir then shows:

drwxr-x---+ ... tux project3 ... mydir

getfacl mydir provides the following output:

# file: mydir 
# owner: tux 
# group: project3 
user:geeko:rwx          # effective: r-x
group:mascots:rwx       # effective: r-x 

After executing the chmod command to remove the write permission from the group class bits, the output of the ls command is sufficient to see that the mask bits must have changed accordingly: write permission is again limited to the owner of mydir. The output of the getfacl confirms this. This output includes a comment for all those entries in which the effective permission bits do not correspond to the original permissions, because they are filtered according to the mask entry. The original permissions can be restored at any time with chmod g+w mydir.

9.4.3. A Directory with a Default ACL

Directories can have a default ACL, which is a special kind of ACL defining the access permissions that objects in the directory inherit when they are created. A default ACL affects both subdirectories and files. Effects of a Default ACL

There are two ways in which the permissions of a directory's default ACL are passed to the files and subdirectories:

  • A subdirectory inherits the default ACL of the parent directory both as its default ACL and as an ACL.

  • A file inherits the default ACL as its ACL.

All system calls that create file system objects use a mode parameter that defines the access permissions for the newly created file system object. If the parent directory does not have a default ACL, the permission bits as defined by the umask are subtracted from the permissions as passed by the mode parameter, with the result being assigned to the new object. If a default ACL exists for the parent directory, the permission bits assigned to the new object correspond to the overlapping portion of the permissions of the mode parameter and those that are defined in the default ACL. The umask is disregarded in this case. Application of Default ACLs

The following three examples show the main operations for directories and default ACLs:

  1. Add a default ACL to the existing directory mydir with:

    setfacl -d -m group:mascots:r-x mydir

    The option -d of the setfacl command prompts setfacl to perform the following modifications (option -m) in the default ACL.

    Take a closer look at the result of this command:

    getfacl mydir
    # file: mydir 
    # owner: tux 
    # group: project3 

    getfacl returns both the ACL and the default ACL. The default ACL is formed by all lines that start with default. Although you merely executed the setfacl command with an entry for the mascots group for the default ACL, setfacl automatically copied all other entries from the ACL to create a valid default ACL. Default ACLs do not have an immediate effect on access permissions. They only come into play when file system objects are created. These new objects inherit permissions only from the default ACL of their parent directory.

  2. In the next example, use mkdir to create a subdirectory in mydir, which inherits the default ACL.

    mkdir mydir/mysubdir
    getfacl mydir/mysubdir 
    # file: mydir/mysubdir 
    # owner: tux 
    # group: project3 

    As expected, the newly-created subdirectory mysubdir has the permissions from the default ACL of the parent directory. The ACL of mysubdir is an exact reflection of the default ACL of mydir. The default ACL that this directory will hand down to its subordinate objects is also the same.

  3. Use touch to create a file in the mydir directory, for example, touch mydir/myfile. ls -l mydir/myfile then shows:

    -rw-r-----+ ... tux project3 ... mydir/myfile

    The output of getfacl mydir/myfile is:

    # file: mydir/myfile 
    # owner: tux 
    # group: project3
    group::r-x          # effective:r-- 
    group:mascots:r-x   # effective:r-- 

    touch uses a mode with the value 0666 when creating new files, which means that the files are created with read and write permissions for all user classes, provided no other restrictions exist in umask or in the default ACL (see Section, “Effects of a Default ACL”). In effect, this means that all access permissions not contained in the mode value are removed from the respective ACL entries. Although no permissions were removed from the ACL entry of the group class, the mask entry was modified to mask permissions not set in mode.

    This approach ensures the smooth interaction of applications (such as compilers) with ACLs. You can create files with restricted access permissions and subsequently mark them as executable. The mask mechanism guarantees that the right users and groups can execute them as desired.

9.4.4. The ACL Check Algorithm

A check algorithm is applied before any process or application is granted access to an ACL-protected file system object. As a basic rule, the ACL entries are examined in the following sequence: owner, named user, owning group or named group, and other. The access is handled in accordance with the entry that best suits the process. Permissions do not accumulate.

Things are more complicated if a process belongs to more than one group and would potentially suit several group entries. An entry is randomly selected from the suitable entries with the required permissions. It is irrelevant which of the entries triggers the final result access granted. Likewise, if none of the suitable group entries contain the required permissions, a randomly selected entry triggers the final result access denied.

9.5. ACL Support in Applications

ACLs can be used to implement very complex permission scenarios that meet the requirements of modern applications. The traditional permission concept and ACLs can be combined in a smart manner. The basic file commands (cp, mv, ls, etc.) support ACLs, as do Samba and Konqueror.

Unfortunately, many editors and file managers still lack ACL support. When copying files with Emacs, for instance, the ACLs of these files are lost. When modifying files with an editor, the ACLs of files are sometimes preserved and sometimes not, depending on the backup mode of the editor used. If the editor writes the changes to the original file, the ACL is preserved. If the editor saves the updated contents to a new file that is subsequently renamed to the old filename, the ACLs may be lost, unless the editor supports ACLs. Except for the star archiver, there are currently no backup applications that preserve ACLs.

9.6. For More Information

For more information about ACLs, see the man pages for getfacl(1), acl(5), and setfacl(1).

Chapter 10. Encrypting Partitions and Files

Most users have some confidential data on their computer that third parties should not be able to access. The more you rely on mobile computing and on working in different environments and networks, the more carefully you should handle your data. The encryption of files or entire partitions is recommended if others have network or physical access to your system. Laptops or removable media, such as external hard disks or USB sticks, are prone to being lost or stolen. Thus, it is recommended to encrypt the parts of your filesystem that hold confidential data.

There are several ways to protect your data by means of encryption:

Encrypting a Hard Disk Partition

You can create an encrypted partition with YaST during installation or in an already installed system. Refer to Section 10.1.1, “Creating an Encrypted Partition during Installation” and Section 10.1.2, “Creating an Encrypted Partition on a Running System” for details. This option can also be used for removable media, such as external hard disks, as described in Section 10.1.4, “Encrypting the Content of Removable Media”.

Creating an Encrypted File as Container

You can create an encrypted file on your hard disk or on a removable medium with YaST at any time. The encrypted file can then be used to store other files or folders. For more information, refer to Section 10.1.3, “Creating an Encrypted File as a Container”.

Encrypting Home Directories

With openSUSE, you can also create encrypted user home directories. When the user logs in to the system, the encrypted home directory is mounted and the contents are made available to the user. Refer to Section 10.2, “Using Encrypted Home Directories” for more information.

Encrypting Single ASCII Text Files

If you only have a small number of ASCII text files that hold sensitive or confidential data, you can encrypt them individually and protect them with a password using Kgpg or the vi editor. Refer to and Section 10.3, “Using vi to Encrypt Single ASCII Text Files” for more information.

[Warning]Encrypted Media Offers Limited Protection

The methods described in this chapter offer only a limited protection. You cannot protect your running system from being compromised. After the encrypted medium is successfully mounted, everybody with appropriate permissions has access to it. However, encrypted media are useful in case of loss or theft of your computer, or to prevent unauthorized individuals from reading your confidential data.

10.1. Setting Up an Encrypted File System with YaST

Use YaST to encrypt partitions or parts of your file system during installation or in an already installed system. However, encrypting a partition in an already-installed system is more difficult, because you have to resize and change existing partitions. In such cases, it may be more convenient to create an encrypted file of a defined size, in which to store other files or parts of your file system. To encrypt an entire partition, dedicate a partition for encryption in the partition layout. The standard partitioning proposal as suggested by YaST, does not include an encrypted partition by default. Add it manually in the partitioning dialog.

10.1.1. Creating an Encrypted Partition during Installation

[Warning]Password Input

Make sure to memorize the password for your encrypted partitions well. Without that password, you cannot access or restore the encrypted data.

The YaST expert dialog for partitioning offers the options needed for creating an encrypted partition. To create a new encrypted partition proceed as follows:

  1. Run the YaST Expert Partitioner with System+Partitioner.

  2. Select a harddisk, click Add, and select a primary or an extended partition.

  3. Select the partition size or the region to use on the disk.

  4. Select the file system, and mount point of this partition.

  5. Activate the Encrypt device check box.

    [Note]Additional Software Required

    After checking Encrypt device, a popup window asking for installing additional software may appear. Confirm to install all the required packages to ensure that the encrypted partition works well.

  6. Click Next and enter a password which is used to encrypt this partition. This password is not displayed. To prevent typing errors, you need to enter the password twice.

  7. Complete the process by clicking Finish. The newly-encrypted partition is now created.

During the boot process, the operating system asks for the password before mounting any encrypted partition which is set to be auto-mounted in /etc/fstab. Such a partition is then available to all users once it has been mounted.

To skip mounting the encrypted partition during start-up, click Enter when prompted for the password. Then decline the offer to enter the password again. In this case, the encrypted file system is not mounted and the operating system continues booting, blocking access to your data.

When you need to mount an encrypted partition which is not mounted during the boot process, open your favorite file manager and click on the partition entry in the pane listing common places on your filesystem. You will be prompted for a password and the partition will be mounted.

When you are installing your system on a machine where partitions already exist, you can also decide to encrypt an existing partition during installation. In this case follow the description in Section 10.1.2, “Creating an Encrypted Partition on a Running System” and be aware that this action destroys all data on the existing partition.

10.1.2. Creating an Encrypted Partition on a Running System

[Warning]Activating Encryption on a Running System

It is also possible to create encrypted partitions on a running system. However, encrypting an existing partition destroys all data on it, and requires resizing and restructuring of existing partitions.

On a running system, select System+Partitioner in the YaST Control Center. Click Yes to proceed. In the Expert Partitioner, select the partition to encrypt and click Edit. The rest of the procedure is the same as described in Section 10.1.1, “Creating an Encrypted Partition during Installation”.

10.1.3. Creating an Encrypted File as a Container

Instead of using a partition, it is possible to create an encrypted file, which can hold other files or folders containing confidential data. Such container files are created from the YaST Expert Partitioner dialog. Select Crypt Files+Add Crypt File and enter the full path to the file and its size. If YaST should create the container file, activate the checkbox Create Loop File. Accept or change the proposed formatting settings and the file system type. Specify the mount point and make sure that Encrypt Device is checked.

Click Next, enter your password for decrypting the file, and confirm with Finish.

The advantage of encrypted container files over encrypted partitions is that they can be added without repartitioning the hard disk. They are mounted with the help of a loop device and behave just like normal partitions.

10.1.4. Encrypting the Content of Removable Media

YaST treats removable media (like external hard disks or USB flash drives) the same as any other hard disk. Container files or partitions on such media can be encrypted as described above. Do not, however, enable mounting at boot time, because removable media are usually only connected while the system is running.

If you encrypted your removable device with YaST, the KDE and GNOME desktops automatically recognize the encrypted partition and prompt for the password when the device is detected. If you plug in a FAT formatted removable device while running KDE or GNOME, the desktop user entering the password automatically becomes the owner of the device and can read and write files. For devices with a file system other than FAT, change the ownership explicitly for users other than root to enable these users to read or write files on the device.

10.2. Using Encrypted Home Directories

To protect data in home directories from being stolen and consequent unauthorized access, use the YaST user management module to enable encryption of home directories. You can create encrypted home directories for new or existing users. To encrypt or decrypt home directories of already existing users, you need to know their login password. See Section “Managing Encrypted Home Directories” (Chapter 10, Managing Users with YaST, ↑Start-Up) for instructions.

Encrypted home partitions are created within a file container as described in Section 10.1.3, “Creating an Encrypted File as a Container”. Two files are created under /home for each encrypted home directory:


The image holding the directory


The image key, protected with the user's login password.

On login, the home directory automatically gets decrypted. Internally, it works through the pam module called pam_mount. If you need to add an additional login method that provides encrypted home directories, you have to add this module to the respective configuration file in /etc/pam.d/. For more information see also Chapter 2, Authentication with PAM and the man page of pam_mount.

[Warning]Security Restrictions

Encrypting a user's home directory does not provide strong security from other users. If strong security is required, the system should not be shared physically.

To enhance security, also encrypt the swap partition and the /tmp and /var/tmp directories, because these may contain temporary images of critical data. You can encrypt swap, /tmp, and /var/tmp with the YaST partitioner as described in Section 10.1.1, “Creating an Encrypted Partition during Installation” or Section 10.1.3, “Creating an Encrypted File as a Container”.

10.3. Using vi to Encrypt Single ASCII Text Files

The disadvantage of using encrypted partitions is obvious: While the partition is mounted, at least root can access the data. To prevent this, vi can be used in encrypted mode.

Use vi -x filename to edit a new file. vi prompts you to set a password, after which it encrypts the content of the file. Whenever you access this file, vi requests the correct password.

For even more security, you can place the encrypted text file in an encrypted partition. This is recommended because the encryption used in vi is not very strong.

Chapter 11. Intrusion Detection with AIDE

Securing your systems is a mandatory task for any mission-critical system administrator. Because it is impossible to always guarantee that the system is not compromised, it is very important to do extra checks regularly (for example with cron) to ensure that the system is still under your control. This is where AIDE, the Advanced Intrusion Detection Environment, comes into play.

11.1. Why Using AIDE?

An easy check that often can reveal unwanted changes can be done by means of RPM. The package manager has a built-in verify function that checks all the managed files in the system for changes. To verify of all files, run the command rpm -Va. However, this command will also display changes in configuration files and you will have to do some filtering to detect important changes.

An additional problem to the method with RPM is that an intelligent attacker will modify rpm itself to hide any changes that might have been done by some kind of rootkit which allows the attacker to mask its intrusion and gain root privilege. To solve this, you should implement a secondary check that can also be run completely independent of the installed system.

11.2. Setting Up an AIDE Database

[Important]Initialize AIDE Database After Installation

Before you install your system, verify the checksum of your medium (see Section “Checking Media” (Appendix A, Help and Troubleshooting, ↑Start-Up)) to make sure you do not use a compromised source. After you have installed the system, initialize the AIDE database. To be really sure that all went well during and after the installation, do an installation directly on the console, without any network attached to the computer. Do not leave the computer unattended or connected to any network before AIDE creates its database.

AIDE is not installed by default on openSUSE. To install it, either use Computer+Install Software, or enter zypper install aide on the command line as root.

To tell AIDE which attributes of which files should be checked, use the /etc/aide.conf configuration file. It must be modified to become the actual configuration. The first section handles general parameters like the location of the AIDE database file. More relevant for local configurations are the Custom Rules and the Directories and Files sections. A typical rule looks like the following:

Binlib     = p+i+n+u+g+s+b+m+c+md5+sha1

After defining the variable Binlib, the respective checking options are used in the files section. Important options include the following:

Table 11.1. Important AIDE Checking Options




Check for the file permissions of the selected files or directories.


Check for the inode number. Every filename has a unique inode number that should not change.


Check for the number of links pointing to the relevant file.


Check if the owner of the file has changed.


Check if the group of the file has changed.


Check if the file size has changed.


Check if the block count used by the file has changed.


Check if the modification time of the file has changed.


Check if the files access time has changed.


Check if the md5 checksum of the file has changed.


Check if the sha1 (160 Bit) checksum of the file has changed.

This is a configuration that checks for all files in /sbin with the options defined in Binlib but omits the /sbin/conf.d/ directory:

/sbin  Binlib

To create the AIDE database, proceed as follows:

  1. Open /etc/aide.conf.

  2. Define which files should be checked with which checking options. For a complete list of available checking options, see /usr/share/doc/packages/aide/manual.html. The definition of the file selection needs some knowledge about regular expressions. Save your modifications.

  3. To check whether the configuration file is valid, run:

    aide --config-check

    Any output of this command is a hint that the configuration is not valid. For example, if you get the following output:

    aide --config-check
    35:syntax error:!
    35:Error while reading configuration:!
    Configuration error

    The error is to be expected in line 36 of /etc/aide.conf. Note that the error message contains the last successfully read line of the configuration file.

  4. Initialize the AIDE database. Run the command:

    aide -i
  5. Copy the generated database to a save location like a CD-R or DVD-R, a remote server or a USB disk for later use.


    This step is essential as it avoids compromising your database. It is recommended to use a medium which can be written only once to prevent the database being modified. Never leave the database on the computer which you want to monitor.

11.3. Local AIDE Checks

To perform a filesystem check, proceed as follows:

  1. Rename the database:

    mv /var/lib/aide/ /var/lib/aide/aide.db
  2. After any configuration change, you always have to reinitialize the AIDE database and subsequently move the newly generated database. It is also a good idea to make a backup of this database. See Section 11.2, “Setting Up an AIDE Database” for more information.

  3. Perform the check with the following command:

    aide --check

If the output is empty, everything is fine. If AIDE found changes, it displays a summary of changes, for example:

aide --check
AIDE found differences between database and filesystem!!

  Total number of files:        1992
  Added files:                  0
  Removed files:                0
  Changed files:                1 

To learn about the actual changes, increase the verbose level of the check with the parameter -V. For the previous example, this could look like the following:

aide --check -V
AIDE found differences between database and filesystem!!
Start timestamp: 2009-02-18 15:14:10

  Total number of files:        1992
  Added files:                  0
  Removed files:                0
  Changed files:                1

Changed files:

changed: /etc/passwd

Detailed information about changes:

File: /etc/passwd
  Mtime    : 2009-02-18 15:11:02              , 2009-02-18 15:11:47
  Ctime    : 2009-02-18 15:11:02              , 2009-02-18 15:11:47

In this example, the file /etc/passwd was touched to demonstrate the effect.

11.4. System Independent Checking

For the risk-averse administrator (and of course this is all about risk-aversion) it is advisable to also run the AIDE binary from a trusted source. This excludes the risk that some attacker also modified the aide binary to hide his traces.

To accomplish this task, AIDE must be run from a rescue system that is independent of the installed system. With openSUSE it is relatively easy to extend the rescue system with arbitrary programs, and thus add the needed functionality.

Before you can start using the rescue system, you need to provide two packages to the system. These are included with the same syntax as you would add a driver update disk to the system. For a detailed description about the possibilities of linuxrc that are used for this purpose, see In the following, one possible way to accomplish this task is discussed.

Procedure 11.1. Starting a Rescue System with AIDE

  1. Provide an FTP server as a second machine.

  2. Copy the packages aide and mhash to the FTP server directory, in our case /srv/ftp/. Replace the placeholders ARCH and VERSION with the corresponding values:

    cp DVD1/suse/ARCH/aideVERSION.ARCH.rpm /srv/ftp
    cp DVD1/suse/ARCH/mhashVERSION.ARCH.rpm /srv/ftp
  3. Create an info file /srv/ftp/info.txt that provides the needed boot parameters for the rescue system:


    Replace your FTP domain name, VERSION and ARCH with the values used on your system.

  4. Restart the server that needs to go through an AIDE check with the Rescue system from your DVD. Add the following string to the boot parameters:


    This parameter tells linuxrc to also read in all information from the info.txt file.

After the rescue system has booted, the AIDE program is ready for use.

11.5. For More Information

Information about AIDE is available at the following places:

Part III. Network Security

Chapter 12. SSH: Secure Network Operations

In networked environments, it is often necessary to access hosts from a remote location. If a user sends login and password strings for authentication purposes as plain text, they could be intercepted and misused to gain access to that user account without the authorized user knowing about it. This would open all the user's files to an attacker and the illegal account could be used to obtain administrator or root access, or to penetrate other systems. In the past, remote connections were established with telnet, rsh or rlogin, which offered no guards against eavesdropping in the form of encryption or other security mechanisms. There are other unprotected communication channels, like the traditional FTP protocol and some remote copying programs like rcp.

The SSH suite provides the necessary protection by encrypting the authentication strings (usually a login name and a password) and all the other data exchanged between the hosts. With SSH, the data flow could still be recorded by a third party, but the contents are encrypted and cannot be reverted to plain text unless the encryption key is known. So SSH enables secure communication over insecure networks, such as the Internet. The SSH implementation coming with openSUSE is OpenSSH.

openSUSE installs the OpenSSH package by default providing the commands ssh, scp, and sftp. In the default configuration, remote access of a openSUSE system is only possible with the OpenSSH utilities, and only if the sshd is running and the firewall permits access.

12.1. ssh—Secure Shell

By using the ssh program, it is possible to log in to remote systems and to work interactively. To log in to the host sun as user tux user one of the following commands:

ssh tux@sun
ssh -l tux sun

If the username is the same on both machines, you may omit it: ssh sun. The remote host prompts for the remote user's password. After a successful authentication, you can work on the remote command line or use interactive applications, such as YaST in text mode.

Furthermore, ssh offers the possibility to just run non-interactive commands on remote systems by running ssh HOST COMMAND. COMMAND needs to be properly quoted. Multiple commands can be concatenated as on a regular shell.

ssh root@sun "dmesg | tail -n 25"
ssh root@sun "cat /etc/issue && uptime"

12.1.1. Starting X Applications on a Remote Host

SSH also simplifies the use of remote X applications. If you run ssh with the -X option, the DISPLAY variable is automatically set on the remote machine and all X output is exported to the remote machine over the existing SSH connection. At the same time, X applications started remotely cannot be intercepted by unauthorized individuals.

12.1.2. Agent Forwarding

By adding the -A option, the ssh-agent authentication mechanism is carried over to the next machine. This way, you can work from different machines without having to enter a password, but only if you have distributed your public key to the destination hosts and properly saved it there.

This mechanism is deactivated in the default settings, but can be permanently activated at any time in the systemwide configuration file /etc/ssh/sshd_config by setting AllowAgentForwarding yes.

12.2. scp—Secure Copy

scp copies files to or from a remote machine. If the username on jupiter is different than the username on sun, specify the latter using the username@host format. If the file should be copied into a directory other then the remote user's home directory, specify it as sun:DIRECTORY. The following examples show how to copy a file from a local to a remote machine and vice versa.

# local -> remote
scp ~/MyLetter.tex tux@sun:/tmp
# remote -> local
scp tux@sun:/tmp/MyLetter.tex ~
[Tip]The -l Option

With the ssh command, the option -l can be used to specify a remote user (as an alternative to the username@host format). With scp the option -l is used to limit the bandwidth consumed by scp.

After the correct password is entered, scp starts the data transfer. It displays a progress bar and the time remaining for each file that is copied. Suppress all output with the -q option.

scp also provides a recursive copying feature for entire directories. The command

scp -r src/ sun:backup/

copies the entire contents of the directory src including all subdirectories to the ~/backup directory on the host sun. If this subdirectory does not exist it is created automatically.

The -p option tells scp to leave the time stamp of files unchanged. -C compresses the data transfer. This minimizes the data volume to transfer, but creates a heavier burden on the processors of both machines.

12.3. sftp—Secure File Transfer

If you want to copy several files from and/or to different locations, sftp is a convenient alternative to scp. It opens a shell with a set of commands similar to a regular ftp shell. Type help at the sftp-prompt to get a list of available commands. More details are available from the sftp (1) man page.

sftp sun
Enter passphrase for key '/home/tux/.ssh/id_rsa': 
Connected to sun.
sftp> help
Available commands:
bye                                Quit sftp
cd path                            Change remote directory to 'path'

12.4. The SSH Daemon (sshd)

To work with the SSH client programs ssh and scp, a server (the SSH daemon) must be running in the background, listening for connections on TCP/IP port 22. The daemon generates three key pairs when starting for the first time. Each key pair consists of a private and a public key. Therefore, this procedure is referred to as public key-based. To guarantee the security of the communication via SSH, access to the private key files must be restricted to the system administrator. The file permissions are set accordingly by the default installation. The private keys are only required locally by the SSH daemon and must not be given to anyone else. The public key components (recognizable by the name extension .pub) are sent to the client requesting the connection. They are readable for all users.

A connection is initiated by the SSH client. The waiting SSH daemon and the requesting SSH client exchange identification data to compare the protocol and software versions, and to prevent connections through the wrong port. Because a child process of the original SSH daemon replies to the request, several SSH connections can be made simultaneously.

For the communication between SSH server and SSH client, OpenSSH supports versions 1 and 2 of the SSH protocol. Version 2 of the SSH protocol is used by default. Override this to use version 1 of protocol with the -1 option.

When using version 1 of SSH, the server sends its public host key and a server key, which is regenerated by the SSH daemon every hour. Both allow the SSH client to encrypt a freely chosen session key, which is sent to the SSH server. The SSH client also tells the server which encryption method (cipher) to use. Version 2 of the SSH protocol does not require a server key. Both sides use an algorithm according to Diffie-Hellman to exchange their keys.

The private host and server keys are absolutely required to decrypt the session key and cannot be derived from the public parts. Only the contacted SSH daemon can decrypt the session key using its private keys. This initial connection phase can be watched closely by turning on verbose debugging using the -v option of the SSH client.

It is recommended to back up the private and public keys stored in /etc/ssh/ in a secure, external location. In this way, key modifications can be detected or the old ones can be used again after having installed a new system.

[Tip]Existing SSH Host Keys

If you install openSUSE on a machine with existing Linux installations, the installation routine automatically imports the SSH host key with the most recent access time from an existing installation.

When establishing a secure connection with a remote host for the first time, the client stores all public host keys in ~/.ssh/known_hosts. This prevents any man-in-the-middle attacks—attempts by foreign SSH servers to use spoofed names and IP addresses. Such attacks are detected either by a host key that is not included in ~/.ssh/known_hosts, or by the server's inability to decrypt the session key in the absence of an appropriate private counterpart.

In case the public keys of a host have really changed (something that needs to be verified before attempting to connect to such a server), the offending keys can be removed with the command ssh-keygen -r HOSTNAME

12.5. SSH Authentication Mechanisms

In it's simplest form, authentication is done by entering the user's password just as if logging in locally. However, having to memorize passwords of several users on remote machines, is inefficient. Whats more, these passwords may change. On the other hand—when granting root access—an administrator needs to be able to quickly revoke such a permission without having to change the root password.

In order to accomplish a login that does not require to enter the remote user's password, SSH uses another key pair, which needs to be generated by the user. It consists of a public ( or and a private key (id_rsa or id_dsa).

In order to be able to log in without having to specify the remote user's password, the public key of the SSH user must be present in ~/.ssh/authorized_keys. This approach also ensures that the remote user has got full control: adding the key requires the remote user's password and removing the key revokes the permission to log in from remote.

For maximum security such a key should be protected by a passphrase which needs to be entered every time you use ssh, scp, or sftp. Contrary to the simple authentication, this passphrase is independent from the remote user and therefore always the same.

An alternative to the key-based authentication described above, SSH also offers a host-based authentication. With host-based authentication, users on a trusted host can log into another host on which this feature is enabled using the same username. openSUSE is set up for using key-based authentication, covering setting up host-based authentication on openSUSE is beyond the scope of this manual.

[Note]File Permissions for Host-Based Authentication

If the host-based authentication is to be used, the file /usr/lib/ssh/ssh-keysign (32-bit systems) or /usr/lib64/ssh/ssh-keysign (64-bit systems) should have the setuid bit set, which is not the default setting in openSUSE. In such case, set the file permissions manually. You should use /etc/permissions.local for this purpose, to make sure that the setuid bit is preserved after security updates of openssh.

12.5.1. Generating an SSH Key

  1. To generate a key with default parameters (RSA, 2048 bits), enter the command ssh-keygen.

  2. Accept the default location to store the key (~/.ssh/id_rsa) be pressing Enter (strongly recommended) or enter an alternative location.

  3. Enter a passphrase consisting of 10 to 30 characters. The same rules as for creating safe passwords apply. It is strongly advised to refrain from specifying no passphrase.

You should make absolutely sure that the private key is not accessible by anyone other than yourself (always set its permissions to 0600). The private key must never fall into the hands of another person.

In order to change the password of an existing key pair, use the command ssh-keygen -p.

12.5.2. Copying an SSH Key

To copy a public SSH key to ~/.ssh/authorized_keys of a user on a remote machine, use the command ssh-copy-id. In order to copy your personal key stored under ~/.ssh/ you may use the short form. In order to copy DSA keys or keys of other users, you need to specify the path:

# ~/.ssh/
ssh-copy-id -i tux@sun

# ~/.ssh/
ssh-copy-id -i ~/.ssh/  tux@sun

# ~notme/.ssh/
ssh-copy-id -i ~notme/.ssh/  tux@sun

In order to successfully copy the key, you need to enter the remote user's password. To remove an existing key, manually edit ~/.ssh/authorized_keys.

12.5.3. Using the ssh-agent

When doing lots of secure shell operations it is cumbersome to type the SSH passphrase for each such operation. Therefore, the SSH package provides another tool, ssh-agent, which retains the private keys for the duration of an X or terminal session. All other windows or programs are started as clients to the ssh-agent. By starting the agent, a set of environment variables is set, which will be used by ssh, scp, or sftp to locate the agent for automatic login. See man 1 ssh-agent for details.

Once the ssh-agent is started, you need to add your keys by using ssh-add. It will prompt for the passphrase. Once entered, you can use the secure shell commands within the running session without having to provide your password. Using ssh-agent in an X Session

On openSUSE the ssh-agent is automatically started by the GNOME or KDE display managers. In order to also invoke ssh-add to add your keys to the agent at the beginning of an X session, do the following:

  1. Log in as the desired user and check whether the file ~/.xinitrc exists.

  2. If it does not exist, use an existing template or copy it from /etc/skel:

    if [ -f ~/.xinitrc.template ]; then mv ~/.xinitrc.template ~/.xinitrc; \
    else cp /etc/skel/.xinitrc.template ~/.xinitrc; fi
  3. If you have copied the template, search for the following lines and uncomment them. If ~/.xinitrc already existed, add the following lines (without comment signs).

    # if test -S "$SSH_AUTH_SOCK" -a -x "$SSH_ASKPASS"; then
    #       ssh-add < /dev/null
    # fi
  4. When starting a new X session, you will be prompted for your SSH passphrase. Using ssh-agent in a Terminal Session

In a terminal session you need to manually start the ssh-agent and then call ssh-add afterwards. There are two ways to start the agent. The first example given below starts a new bash shell on top of your existing shell. The second example starts the agent in the existing shell and modifies the environment as needed.

ssh-agent -s /bin/bash
eval $(ssh-agent)

After the agent has been started, run ssh-add to provide the agent with your keys.

12.5.4. Host-based Authentication

12.6. Port Forwarding

ssh can also be used to redirect TCP/IP connections. This feature, also called SSH tunneling, redirects TCP connections to a certain port to another machine via an encrypted channel.

With the following command, any connection directed to jupiter port 25 (SMTP) is redirected to the SMTP port on sun. This is especially useful for those using SMTP servers without SMTP-AUTH or POP-before-SMTP features. From any arbitrary location connected to a network, e-mail can be transferred to the home mail server for delivery.

ssh -L 25:sun:25 jupiter

Similarly, all POP3 requests (port 110) on jupiter can be forwarded to the POP3 port of sun with this command:

ssh -L 110:sun:110 jupiter

Both commands must be executed as root, because the connection is made to privileged local ports. E-mail is sent and retrieved by normal users in an existing SSH connection. The SMTP and POP3 host must be set to localhost for this to work. Additional information can be found in the manual pages for each of the programs described above and also in the OpenSSH package documentation under /usr/share/doc/packages/openssh.

12.7. Configuring An SSH Daemon with YaST

The YaST SSHD Configuration module is not part of the default installation. To make it available, install the package yast2-sshd.

To configure an sshd server with YaST run YaST and choose Network Services+SSHD Configuration. Then proceed as follows:

  1. On the General tab, select the ports sshd should listen on in the SSHD TCP Ports table. The default port number is 22. Multiple ports are allowed. To add a new port, click Add, enter the port number and click OK. To delete a port, select it in the table, click Delete and confirm.

  2. Select the features the sshd daemon should support. To disable TCP forwarding, uncheck Allow TCP Forwarding. Disabling TCP forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. See Section 12.6, “Port Forwarding” for more information about TCP forwarding.

    To disable X forwarding, uncheck Allow X11 Forwarding. If this option is disabled, any X11 forward requests by the client will return an error. However users can always install their own forwarders. See Section 12.1, “ssh—Secure Shell” for more information about X forwarding.

    In Allow Compression determine, whether the connection between the server and clients should be compressed. After setting these options, click Next.

  3. The Login Settings tab contains general login and authentication settings. In Print Message of the Day After Login determine, whether sshd should print message from /etc/motd when a user logs in interactively. If you want to disable connection of a user root, uncheck Permit Root Login.

    In Maximum Authentication Tries enter the maximum allowed number of authentication attempts per connection. RSA Authentication specifies whether pure RSA authentication is allowed. This option applies to SSH protocol version 1 only. Public Key Authentication specifies whether public key authentication is allowed. This option applies to protocol version 2 only.

  4. On the Protocol and Ciphers tab, determine which versions of the SSH protocol should be supported. You can choose to support version 1 only, version 2 only, or to support both SSH version 2 and 1.

    Under Supported Ciphers, all supported ciphers are listed. You can remove a cipher by selecting it in the list and clicking Delete. To add a cipher to the list, select it from the dropdown menu and click Add.

  5. Click Finish to save the configuration.

12.8. For More Information

The homepage of OpenSSH

The OpenSSH Wikibook

man sshd

Manpage of the OpenSSH daemon

man ssh_config

Manpage of the OpenSSH SSH client configuration files

man scp , man sftp , man slogin , man ssh , man ssh-add , man ssh-agent , man ssh-copy-id , man ssh-keyconvert , man ssh-keygen , man ssh-keyscan

Manpage of several binary files to copy (scp, sftp), login (slogin, ssh), and manage keys.

Chapter 13. Masquerading and Firewalls

Whenever Linux is used in a network environment, you can use the kernel functions that allow the manipulation of network packets to maintain a separation between internal and external network areas. The Linux netfilter framework provides the means to establish an effective firewall that keeps different networks apart. With the help of iptables—a generic table structure for the definition of rule sets—precisely control the packets allowed to pass a network interface. Such a packet filter can be set up quite easily with the help of SuSEfirewall2 and the corresponding YaST module.

13.1. Packet Filtering with iptables

The components netfilter and iptables are responsible for the filtering and manipulation of network packets as well as for network address translation (NAT). The filtering criteria and any actions associated with them are stored in chains, which must be matched one after another by individual network packets as they arrive. The chains to match are stored in tables. The iptables command allows you to alter these tables and rule sets.

The Linux kernel maintains three tables, each for a particular category of functions of the packet filter:


This table holds the bulk of the filter rules, because it implements the packet filtering mechanism in the stricter sense, which determines whether packets are let through (ACCEPT) or discarded (DROP), for example.


This table defines any changes to the source and target addresses of packets. Using these functions also allows you to implement masquerading, which is a special case of NAT used to link a private network with the Internet.


The rules held in this table make it possible to manipulate values stored in IP headers (such as the type of service).

These tables contain several predefined chains to match packets:


This chain is applied to incoming packets.


This chain is applied to packets destined for the system's internal processes.


This chain is applied to packets that are only routed through the system.


This chain is applied to packets originating from the system itself.


This chain is applied to all outgoing packets.

Figure 13.1, “iptables: A Packet's Possible Paths” illustrates the paths along which a network packet may travel on a given system. For the sake of simplicity, the figure lists tables as parts of chains, but in reality these chains are held within the tables themselves.

In the simplest of all possible cases, an incoming packet destined for the system itself arrives at the eth0 interface. The packet is first referred to the PREROUTING chain of the mangle table then to the PREROUTING chain of the nat table. The following step, concerning the routing of the packet, determines that the actual target of the packet is a process of the system itself. After passing the INPUT chains of the mangle and the filter table, the packet finally reaches its target, provided that the rules of the filter table are actually matched.

Figure 13.1. iptables: A Packet's Possible Paths

iptables: A Packet's Possible Paths

13.2. Masquerading Basics

Masquerading is the Linux-specific form of NAT (network address translation). It can be used to connect a small LAN (where hosts use IP addresses from the private range—see Section “Netmasks and Routing” (Chapter 13, Basic Networking, ↑Reference)) with the Internet (where official IP addresses are used). For the LAN hosts to be able to connect to the Internet, their private addresses are translated to an official one. This is done on the router, which acts as the gateway between the LAN and the Internet. The underlying principle is a simple one: The router has more than one network interface, typically a network card and a separate interface connecting with the Internet. While the latter links the router with the outside world, one or several others link it with the LAN hosts. With these hosts in the local network connected to the network card (such as eth0) of the router, they can send any packets not destined for the local network to their default gateway or router.

[Important]Using the Correct Network Mask

When configuring your network, make sure both the broadcast address and the netmask are the same for all local hosts. Failing to do so prevents packets from being routed properly.

As mentioned, whenever one of the LAN hosts sends a packet destined for an Internet address, it goes to the default router. However, the router must be configured before it can forward such packets. For security reasons, this is not enabled in a default installation. To enable it, set the variable IP_FORWARD in the file /etc/sysconfig/sysctl to IP_FORWARD=yes.

The target host of the connection can see your router, but knows nothing about the host in your internal network where the packets originated. This is why the technique is called masquerading. Because of the address translation, the router is the first destination of any reply packets. The router must identify these incoming packets and translate their target addresses, so packets can be forwarded to the correct host in the local network.

With the routing of inbound traffic depending on the masquerading table, there is no way to open a connection to an internal host from the outside. For such a connection, there would be no entry in the table. In addition, any connection already established has a status entry assigned to it in the table, so the entry cannot be used by another connection.

As a consequence of all this, you might experience some problems with a number of application protocols, such as ICQ, cucme, IRC (DCC, CTCP), and FTP (in PORT mode). Web browsers, the standard FTP program, and many other programs use the PASV mode. This passive mode is much less problematic as far as packet filtering and masquerading are concerned.

13.3. Firewalling Basics

Firewall is probably the term most widely used to describe a mechanism that provides and manages a link between networks while also controlling the data flow between them. Strictly speaking, the mechanism described in this section is called a packet filter. A packet filter regulates the data flow according to certain criteria, such as protocols, ports, and IP addresses. This allows you to block packets that, according to their addresses, are not supposed to reach your network. To allow public access to your Web server, for example, explicitly open the corresponding port. However, a packet filter does not scan the contents of packets with legitimate addresses, such as those directed to your Web server. For example, if incoming packets were intended to compromise a CGI program on your Web server, the packet filter would still let them through.

A more effective but more complex mechanism is the combination of several types of systems, such as a packet filter interacting with an application gateway or proxy. In this case, the packet filter rejects any packets destined for disabled ports. Only packets directed to the application gateway are accepted. This gateway or proxy pretends to be the actual client of the server. In a sense, such a proxy could be considered a masquerading host on the protocol level used by the application. One example for such a proxy is Squid, an HTTP and FTP proxy server. To use Squid, the browser must be configured to communicate via the proxy. Any HTTP pages or FTP files requested are served from the proxy cache and objects not found in the cache are fetched from the Internet by the proxy.

The following section focuses on the packet filter that comes with openSUSE. For further information about packet filtering and firewalling, read the Firewall HOWTO included in the howto package. If this package is installed, read the HOWTO with

less /usr/share/doc/howto/en/txt/Firewall-HOWTO.gz

13.4. SuSEfirewall2

SuSEfirewall2 is a script that reads the variables set in /etc/sysconfig/SuSEfirewall2 to generate a set of iptables rules. It defines three security zones, although only the first and the second one are considered in the following sample configuration:

External Zone

Given that there is no way to control what is happening on the external network, the host needs to be protected from it. In most cases, the external network is the Internet, but it could be another insecure network, such as a WLAN.

Internal Zone

This refers to the private network, in most cases the LAN. If the hosts on this network use IP addresses from the private range (see Section “Netmasks and Routing” (Chapter 13, Basic Networking, ↑Reference)), enable network address translation (NAT), so hosts on the internal network can access the external one. All ports are open in the internal zone. The main benefit of putting interfaces into the internal zone (rather than stopping the firewall) is that the firewall still runs, so when you add new interfaces, they will be put into the external zone by default. That way an interface is not accidentally open by default.

Demilitarized Zone (DMZ)

While hosts located in this zone can be reached both from the external and the internal network, they cannot access the internal network themselves. This setup can be used to put an additional line of defense in front of the internal network, because the DMZ systems are isolated from the internal network.

Any kind of network traffic not explicitly allowed by the filtering rule set is suppressed by iptables. Therefore, each of the interfaces with incoming traffic must be placed into one of the three zones. For each of the zones, define the services or protocols allowed. The rule set is only applied to packets originating from remote hosts. Locally generated packets are not captured by the firewall.

The configuration can be performed with YaST (see Section 13.4.1, “Configuring the Firewall with YaST”). It can also be made manually in the file /etc/sysconfig/SuSEfirewall2, which is well commented. Additionally, a number of example scenarios are available in /usr/share/doc/packages/SuSEfirewall2/EXAMPLES.

13.4.1. Configuring the Firewall with YaST

[Important]Automatic Firewall Configuration

After the installation, YaST automatically starts a firewall on all configured interfaces. If a server is configured and activated on the system, YaST can modify the automatically-generated firewall configuration with the options Open Ports on Selected Interface in Firewall or Open Ports on Firewall in the server configuration modules. Some server module dialogs include a Firewall Details button for activating additional services and ports. The YaST firewall configuration module can be used to activate, deactivate, or reconfigure the firewall.

The YaST dialogs for the graphical configuration can be accessed from the YaST Control Center. Select Security and Users+Firewall. The configuration is divided into seven sections that can be accessed directly from the tree structure on the left side.


Set the start-up behavior in this dialog. In a default installation, SuSEfirewall2 is started automatically. You can also start and stop the firewall here. To implement your new settings in a running firewall, use Save Settings and Restart Firewall Now.

Figure 13.2. The YaST Firewall Configuration

The YaST Firewall Configuration


All known network interfaces are listed here. To remove an interface from a zone, select the interface, press Change, and choose No Zone Assigned. To add an interface to a zone, select the interface, press Change and choose any of the available zones. You may also create a special interface with your own settings by using Custom.

Allowed Services

You need this option to offer services from your system to a zone from which it is protected. By default, the system is only protected from external zones. Explicitly allow the services that should be available to external hosts. After selecting the desired zone in Allowed Services for Selected Zone, activate the services from the list.


Masquerading hides your internal network from external networks (such as the Internet) while enabling hosts in the internal network to access the external network transparently. Requests from the external network to the internal one are blocked and requests from the internal network seem to be issued by the masquerading server when seen externally. If special services of an internal machine need to be available to the external network, add special redirect rules for the service.


In this dialog, configure the UDP ports that allow broadcasts. Add the required port numbers or services to the appropriate zone, separated by spaces. See also the file /etc/services.

The logging of broadcasts that are not accepted can be enabled here. This may be problematic, because Windows hosts use broadcasts to know about each other and so generate many packets that are not accepted.

IPsec Support

Configure whether the IPsec service should be available to the external network in this dialog. Configure which packets are trusted under Details.

There is another functionality under Details: IPsec packets are packed in an encrypted format, so they have to be decrypted and you can configure the way the firewall will handle the decrypted packets. If you select Internal Zone, the decrypted IPsec packets will be trusted as if they came from the Internal Zone - although they could possibly come from the external one. Choose Same Zone as Original Source Network to avoid this situation.

Logging Level

There are two rules for logging: accepted and not accepted packets. Packets that are not accepted are DROPPED or REJECTED. Select from Log All, Log Only Critical, or Do Not Log Any.

Custom Rules

Here, set special firewall rules that allow connections, matching specified criteria such as source network, protocol, destination port, and source port. Configure such rules for external, internal, and demilitarized zones.

When finished with the firewall configuration, exit this dialog with Next. A zone-oriented summary of your firewall configuration then opens. In it, check all settings. All services, ports, and protocols that have been allowed and all custom rules are listed in this summary. To modify the configuration, use Back. Press Finish to save your configuration.

13.4.2. Configuring Manually

The following paragraphs provide step-by-step instructions for a successful configuration. Each configuration item is marked as to whether it is relevant to firewalling or masquerading. Use port range (for example, 500:510) whenever appropriate. Aspects related to the DMZ (demilitarized zone) as mentioned in the configuration file are not covered here. They are applicable only to a more complex network infrastructure found in larger organizations (corporate networks), which require extensive configuration and in-depth knowledge about the subject.

First, use the YaST module System Services (Runlevel) to enable SuSEfirewall2 in your runlevel (3 or 5 most likely). It sets the symlinks for the SuSEfirewall2_* scripts in the /etc/init.d/rc?.d/ directories.

FW_DEV_EXT (firewall, masquerading)

The device linked to the Internet. For a modem connection, enter ppp0. For an ISDN link, use ippp0. DSL connections use dsl0. Specify auto to use the interface that corresponds to the default route.

FW_DEV_INT (firewall, masquerading)

The device linked to the internal, private network (such as eth0). Leave this blank if there is no internal network and the firewall protects only the host on which it runs.

FW_ROUTE (firewall, masquerading)

If you need the masquerading function, set this to yes. Your internal hosts will not be visible to the outside, because their private network addresses (e.g., 192.168.x.x) are ignored by Internet routers.

For a firewall without masquerading, set this to yes if you want to allow access to the internal network. Your internal hosts need to use officially registered IP addresses in this case. Normally, however, you should not allow access to your internal network from the outside.

FW_MASQUERADE (masquerading)

Set this to yes if you need the masquerading function. This provides a virtually direct connection to the Internet for the internal hosts. It is more secure to have a proxy server between the hosts of the internal network and the Internet. Masquerading is not needed for services that a proxy server provides.

FW_MASQ_NETS (masquerading)

Specify the hosts or networks to masquerade, leaving a space between the individual entries. For example:


Set this to yes to protect your firewall host from attacks originating in your internal network. Services are only available to the internal network if explicitly enabled. Also see FW_SERVICES_INT_TCP and FW_SERVICES_INT_UDP.


Enter the TCP ports that should be made available. Leave this blank for a normal workstation at home that should not offer any services.


Leave this blank unless you run a UDP service and want to make it available to the outside. The services that use UDP include include DNS servers, IPsec, TFTP, DHCP and others. In that case, enter the UDP ports to use.


List services to allow from the Internet. This is a more generic form of the FW_SERVICES_EXT_TCP and FW_SERVICES_EXT_UDP settings, and more specific than FW_TRUSTED_NETS. The notation is a space-separated list of net,protocol[,dport][,sport], for example 0/0,tcp,22 or 0/0,tcp,22,,hitcount=3,blockseconds=60,recentname=ssh, which means: allow a maximum of three SSH connects per minute from one IP address.


With this variable, define the services available for the internal network. The notation is the same as for FW_SERVICES_EXT_TCP, but the settings are applied to the internal network. The variable only needs to be set if FW_PROTECT_FROM_INT is set to yes.




List services to allow from internal hosts. See FW_SERVICES_ACCEPT_EXT.


This is how the SuSEfirewall2 implementation considers packets RELATED by netfilter.

For example, to allow finer grained filtering of Samba broadcast packets, RELATED packets are not accepted unconditionally. Variables starting with FW_SERVICES_ACCEPT_RELATED_ allow restricting RELATED packets handling to certain networks, protocols and ports.

This means that adding connection tracking modules (conntrack modules) to FW_LOAD_MODULES does not automatically result in accepting the packets tagged by those modules. Additionally, you must set variables starting with FW_SERVICES_ACCEPT_RELATED_ to a suitable value.


Uncomment this variable to install custom rules. Find examples in /etc/sysconfig/scripts/SuSEfirewall2-custom.

After configuring the firewall, test your setup. The firewall rule sets are created by entering rcSuSEfirewall2 start as root. Then use telnet, for example, from an external host to see whether the connection is actually denied. After that, review /var/log/messages, where you should see something like this:

Mar 15 13:21:38 linux kernel: SFW2-INext-DROP-DEFLT IN=eth0 
OUT= MAC=00:80:c8:94:c3:e7:00:a0:c9:4d:27:56:08:00 SRC= 
DST= LEN=60 TOS=0x10 PREC=0x00 TTL=64 ID=15330 DF PROTO=TCP 
SPT=48091 DPT=23 WINDOW=5840 RES=0x00 SYN URGP=0 
OPT (020405B40402080A061AFEBC0000000001030300)

Other packages to test your firewall setup are Nmap (portscanner) or OpenVAS (Open Vulnerability Assessment System). The documentation of Nmap is found at /usr/share/doc/packages/nmap after installing the package and the documentation of openVAS resides at

13.5. For More Information

The most up-to-date information and other documentation about the SuSEfirewall2 package is found in /usr/share/doc/packages/SuSEfirewall2. The home page of the netfilter and iptables project,, provides a large collection of documents in many languages.

Chapter 14. Configuring VPN Server

Nowadays, the Internet connection is cheap and available almost everywhere. It is important that the connection is as secure as possible. Virtual Private Network (VPN), is a secure network within a second, insecure network such as the Internet or WLAN. It can be implemented in different ways and serves several purposes. In this chapter, we focus on VPNs to link branch offices via secure wide area networks (WANs).

14.1. Conceptual Overview

This section defines some term regarding to VPN and introduces a brief overview of some scenarios.

14.1.1. Terminology


The two ends of a tunnel, the source or destination client

Tap Device

A tap device simulates an Ethernet device (layer 2 packets in the OSI model such as IP packets). A tap device is used for creating a network bridge. It works with Ethernet frames.

Tun Device

A tun device simulates a point-to-point network (layer 3 packets in the OSI model such as Ethernet frames). A tun device is used with routing and works with IP frames.


Linking two locations through a primarily public network. From a more technical viewpoint, it is a connection between the client's device and the server's device. Usually a tunnel is encrypted, but it does need to be by definition.

14.1.2. VPN Scenarios

Whenever you setup a VPN connection your IP packets are transferred over your secured tunnel. A tunnel can use a so-called tun or tap device. They are virtual network kernel drivers which implement the transmission of ethernet frames or ip frames/packets.

Any userspace program OpenVPN can attach itself to a tun or tap device to receive packets sent by your OS. The program is also able to write packets to the device.

There are many solutions to set up and build a VPN connection. This section focuses on the OpenVPN package. Compared to other VPN software, OpenVPN can be operated in two modes:

Routed VPN

Routing is an easy solution to set up. It is more efficient and scales better than bridged VPN. Furthermore, it allows the user to tune MTU (Maximum Transfer Unit) to raise efficiency. However, in a heterogeneous environment NetBIOS broadcasts do not work if you do not have a Samba server on the gateway. If you need IPv6, each tun drivers on both ends must support this protocol explicitly. This scenario is depicted in Figure 14.1, “Routed VPN”

Figure 14.1. Routed VPN

Routed VPN

Bridged VPN

Bridging is a more complex solution. It is recommended when you need to browse Windows file shares across the VPN without setting up a Samba or WINS server. Bridged VPN is also needed if you want to use non-IP protocols (such as IPX) or applications relying on network broadcasts. However, it is less efficient than routed VPN. Another disadvantage is that it does not scale well. This scenarios is depicted in the following figures.

Figure 14.2. Bridged VPN - Scenario 1

Bridged VPN - Scenario 1

Figure 14.3. Bridged VPN - Scenario 2

Bridged VPN - Scenario 2

Figure 14.4. Bridged VPN - Scenario 3

Bridged VPN - Scenario 3

The major difference between bridging and routing is that a routed VPN cannot IP-broadcast while a bridged VPN can.

14.2. Creating the Simplest VPN Example

The following example creates a point-to-point VPN tunnel. It demonstrates how to create a VPN tunnel between one client and a server. It is assumed that your VPN server will use private IP addresses like and your client the IP address You can modify these private IP addresses to your needs but make sure you select addresses which do not conflict with other IP addresses.

[Warning]Use It Only For Testing

This scenario is only useful for testing and is considered as an example to get familiar with VPN. Do not use this as a real world scenario to connect as it can compromise your security and the safety of your IT infrastructure!

14.2.1. Configuring the VPN Server

To configure a VPN server, proceed as follows:

Procedure 14.1. VPN Server Configuration

  1. Install the package openvpn on the machine that will later become your VPN server.

  2. Open a shell, become root and create the VPN secret key:

    openvpn --genkey --secret /etc/openvpn/secret.key
  3. Copy the secret key to your client:

    scp /etc/openvpn/secret.key root@
  4. Create the file /etc/openvpn/server.conf with the following content:

    dev tun
    secret secret.key
  5. If you use a firewall, start YaST and open UDP port 1194 (Security and Users+Firewall+Allowed Services.

  6. Start the OpenVPN service as root:

    rcopenvpn start

14.2.2. Configuring the VPN Client

To configure the VPN client, do the following:

Procedure 14.2. VPN Client Configuration

  1. Install the package openvpn on your client VPN machine.

  2. Create /etc/openvpn/client.conf with the following content:

    remote IP_OF_SERVER
    dev tun
    secret secret.key

    Replace the placeholder IP_OF_SERVER in the first line with either the domain name, or the public IP address of your server.

  3. If you use a firewall, start YaST and open UDP port 1194 as described in Step 5 of Procedure 14.1, “VPN Server Configuration”.

  4. Start the OpenVPN service as root:

    rcopenvpn start

14.2.3. Testing the VPN Example

After the OpenVPN is successfully started, test if the tun device is available with the following command:

ifconfig tun0

To verify the VPN connection, use ping on both client and server to see if you can reach each other. Ping server from client:

ping -I tun0

Ping client from server:

ping -I tun0

14.3. Setting Up Your VPN Server Using Certificate Authority

The example shown in Section 14.2 is useful for testing, but not for daily work. This section explains how to build a VPN server that allows more than one connection at the same time. This is done with a public key infrastructure (PKI). A PKI consists of a pair of public and private keys for the server and each client and a master certificate authority (CA), which is used to sign every server and client certificate.

The general overview of this process involves the following steps explained in these sections:

14.3.1. Creating Certificates

Before a VPN connection gets established, the client must authenticate the server certificate. Conversely, the server must also authenticate the client certificate. This is called mutual authentication.

You can use two methods to create the respective certificates and keys: Generating Certificates with easy-rsa

The easy-rsa utilities use the openssl.cnf file stored under /usr/share/openvpn/easy-rsa/VER. In most cases you can leave this file as it is.

Procedure 14.3. Generate the Master CA And Key

  1. Open a shell and become root.

  2. Change the directory to /usr/share/openvpn/easy-rsa/VER/. Replace the placeholder VER with the current version— either 1.0 or 2.0.

  3. Copy the file vars to /etc/openvpn and set export EASY_RSA to /usr/share/openvpn/easy-rsa:

    export EASY_RSA="/usr/share/openvpn/easy-rsa/VER"
  4. In the vars file change the KEY_COUNTRY, KEY_PROVINCE, KEY_CITY, KEY_ORG, and KEY_EMAIL variables according to your needs.

  5. Initialize the PKI:

    source /etc/openvpn/vars && ./clean-all && ./build-ca
  6. Enter the data required by the build-ca script. Usually you can take the defaults that you have set in Step 4. Additionally set Organizational Unit Name and Common Name that were not set previously.

Once done, the master certificate and key are saved as /usr/share/openvpn/easy-rsa/VER/keys/ca.*.

Procedure 14.4. Generate The Private Server Key

  1. Change to the /usr/share/openvpn/easy-rsa/VER/. directory.

  2. Run the following script:

    ./build-key-server server

    The argument (here: server) is used for the private key filename.

  3. Accept the default parameters, but fill server for the Common Name option.

  4. Answer the next two questions (Sign the certificate? [y/n] and 1 out of 1 certificate requests certified, commit? [y/n]) with y (yes).

Once done, the private server key is saved as /usr/share/openvpn/easy-rsa/VER/keys/server.*.

Procedure 14.5. Generate Certificates and Keys for a Client

  1. Change to the /usr/share/openvpn/easy-rsa/VER/ directory. Replace the placeholder VER with either 1.0 or 2.0.

  2. Create the key as in Step 2 of Procedure 14.4, “Generate The Private Server Key”:

    ./build-key client
  3. Repeat the previous step for each client that is allowed to connect to the VPN server. Make sure you use a different name (other than client) and an appropriate Common Name, because this parameter has to be unique for each client.

Once done, the client certificate keys are saved as /usr/share/openvpn/easy-rsa/keys/client.* (depending on the name that you have given for the build-key command).

Procedure 14.6. Final Configuration Steps

  1. Make sure your current working directory is /usr/share/openvpn/easy-rsa/VER/.

  2. Create the Diffie-Hellman parameter:

  3. Create the /etc/openvpn/ssl directory.

  4. Copy the following files to /etc/openvpn/ssl:

    cp keys/ca.{crt,key} keys/dh1024.pem keys/server.{crt,key} /etc/openvpn/ssl
  5. Copy the client keys to the relevant client machine. You should have the files client.crt and client.key in the /etc/openvpn/ssl directory. Configuring Certificates with YaST CA

You can skip this section if you have already configured the certificates with the easy-rsa utilties.

14.3.2. Configuring the Server

The configuration file is mostly a summary of /usr/share/doc/packages/openvpn/sample-config-files/server.conf without the comments and with some small changes concerning some paths.

Example 14.1. VPN Server Configuration File

# /etc/openvpn/server.conf
port 1194 1
proto udp 2
dev tun0 3

# Security 4
ca   /etc/openvpn/ssl/ca.crt
cert /etc/openvpn/ssl/server.crt
key  /etc/openvpn/ssl/server.key
dh   /etc/openvpn/ssl/dh1024.pem

server 5
ifconfig-pool-persist /var/run/openvpn/ipp.txt 6

# Privileges 7
user nobody
group nobody

# Other configuration 8
keepalive 10 120
status      /var/log/openvpn-status.log
log-append  /var/log/openvpn.log
verb 4


The TCP/UDP port to which OpenVPN listens. You have to open up the port in the Firewall, see Chapter 13, Masquerading and Firewalls. The standard port for VPN is 1194, so in most cases you can leave that as it is.


The protocol, either UDP or TCP.


The tun or tap device, see Section 14.1.1, “Terminology” for the differences.


The following lines contain the relative or absolute path to the root server CA certificate (ca), the root CA key (cert), the private server key (key) and the Diffie-Hellman parameters (dh). These were generated in Section 14.3.1, “Creating Certificates”.


Supplies a VPN subnet. The server can be reached by


Records a mapping of clients and its virtual IP address in the given file. Useful when the server goes down and (after the restart) the clients get their previously assigned IP address.


For security reasons it is a good idea to run the OpenVPN daemon with reduced privileges. For this reason the group and user nobody is used.


Several other configurations, see comment in the original configuration from /usr/share/doc/packages/openvpn/sample-config-files.

After this configuration, you can see log messages from your OpenVPN server under /var/log/openvpn.log. When you have started it for the first time, it should finish it with:

... Initialization Sequence Completed

If you do not see this message, check the log carefully. Usually OpenVPN gives you some hints what is wrong in your configuration file.

14.3.3. Configuring the Clients

The configuration file is mostly a summary from /usr/share/doc/packages/openvpn/sample-config-files/client.conf without the comments and with some small changes concerning some paths.

Example 14.2. VPN Client Configuration File

# /etc/openvpn/client.conf
client 1
dev tun 2
proto udp 3
remote IP_OR_HOSTNAME 1194 4
resolv-retry infinite

# Privileges 5
user nobody
group nobody

# Try to preserve some state across restarts.

# Security 6
ca   /etc/openvpn/ssl/ca.crt
cert /etc/openvpn/ssl/client.crt
key  /etc/openvpn/ssl/client.key

comp-lzo 7


We must specify that this machine is a client.


The network device. Both clients and server must use the same device.


The protocol. Use the same settings as on the server.


Replace the placeholder IP_OR_HOSTNAME with the respective hostname or IP address of your VPN server. After the hostname the port of the server is given. You can have multiple lines of remote entries pointing to different VPN servers. This is useful for load balancing between different VPN servers.


For security reasons it is a good idea to run the OpenVPN daemon with reduced privileges. For this reason the group and user nobody is used.


Contains the client files. For security reasons, it is better to have a separate file pair for each client.


Turns compression on. Use it only when the server has this parameter switched on as well.

14.4. Changing Nameservers in VPN

If you need to change nameservers before or during your VPN session, use netconfig.

Use the following procedure to change a nameserver:

Procedure 14.7. Changing Nameservers

  1. Open a shell and log in as root.

  2. Create the file /etc/openvpn/client.up with the following contents:

    /sbin/netconfig modify -i "${1}" -s openvpn <<EOT
  3. Start your VPN connection with rcopenvpn start.

  4. Create the file /etc/openvpn/client.down with the following contents:

    /sbin/netconfig remove -i "${1}" -s openvpn
  5. Run netconfig and replace the line DNSSERVERS with your respective entry:

    netconfig modify -i tun0 -s openvpn <<EOT

    To check, if the entry has been successfully inserted into /etc/resolv.conf, execute:

    grep -v ^# /etc/resolv.conf 
    nameserver ...
    nameserver ...
  6. To remove the DNS entry, execute:

    netconfig remove -i tun0 -s openvpn

Find another example in /usr/share/doc/packages/openvpn/contrib/pull-resolv-conf/.

If you need to specify a ranking list of fallback services, use the NETCONFIG_DNS_RANKING variable in /etc/sysconfig/network/config. The default value is auto which resolves to:

+strongswan +openswan +racoon +openvpn -avahi

Preferred service names have the + prefix, fallback services the - prefix.

14.5. KDE- and GNOME Applets For Clients

The following sections describe the setup of OpenVPN connections with the GNOME and KDE desktop tools.

14.5.1. KDE

To setup an OpenVPN connection in KDE4 that can be easily turned on or off, proceed as follows:

  1. Make sure you have installed the NetworkManager-openvpn-kde4 package with all dependencies resolved.

  2. Right-click on a widget of your panel and select Panel Options+Add Widgets....

  3. Select Networks.

  4. Right-click on the icon and choose Manage Connections.

  5. Add a new VPN connection with Add+OpenVPN. A new window opens.

  6. Choose the Connection Type between X.509 Certificates or X.509 With Password depending on what you have setup with your OpenVPN server.

  7. Insert the necessary files into the respective text fields. From our example configuration these are:

    CA file







    The user


    The password for the user

  8. If you have not used the KDE Wallet System, you are asked if you want to configure it. Follow the steps in the wizard. After you have finished this step, you are reverted back to the Network Settings dialog.

  9. Finish with Ok.

  10. Enable the connection with your Network manager applet.

14.5.2. GNOME

To setup a OpenVPN connection in GNOME that can be easily turned on or off, proceed as follows:

  1. Make sure you have installed the package NetworkManager-openvpn-gnome and have resolved all dependencies.

  2. Start the Network Connection Editor with Alt+F2 and insert nm-connection-editor into the text field. A new window appears.

  3. Select the VPN tab and click Add.

  4. Choose the VPN connection type, in this case OpenVPN.

  5. Choose the Authentication type. Select between Certificates (TLS) or Password with Certificates (TLS) depending on the setup of your OpenVPN server.

  6. Insert the necessary files into the respective text fields. According to the example configuration, these are:


    The user (only available when you have selected Password with Certificates (TLS))


    The password for the user (only available when you have selected Password with Certificates (TLS))

    User Certificate


    CA Certificate


    Private Key


  7. Finish with Apply and Close.

  8. Enable the connection with your Network Manager applet.

14.6. For More Information

For more information about VPN, visit:

  • Homepage of VPN

  • /usr/share/doc/packages/openvpn/sample-config-files/: Examples of configuration files for different scenarios

  • /usr/src/linux/Documentation/networking/tuntap.txt, install the kernel-source package

Chapter 15. Managing X.509 Certification

An increasing number of authentication mechanisms are based on cryptographic procedures. Digital certificates that assign cryptographic keys to their owners play an important role in this context. These certificates are used for communication and can also be found, for example, on company ID cards. The generation and administration of certificates is mostly handled by official institutions that offer this as a commercial service. In some cases, however, it may make sense to carry out these tasks yourself. For example, if a company does not wish to pass personal data to third parties.

YaST provides two modules for certification, which offer basic management functions for digital X.509 certificates. The following sections explain the basics of digital certification and how to use YaST to create and administer certificates of this type.

15.1. The Principles of Digital Certification

Digital certification uses cryptographic processes to encrypt and protect data from access by unauthorized people. The user data is encrypted using a second data record, or key. The key is applied to the user data in a mathematical process, producing an altered data record in which the original content can no longer be identified. Asymmetrical encryption is now in general use (public key method). Keys always occur in pairs:

Private Key

The private key must be kept safely by the key owner. Accidental publication of the private key compromises the key pair and renders it useless.

Public Key

The key owner circulates the public key for use by third parties.

15.1.1. Key Authenticity

Because the public key process is in widespread use, there are many public keys in circulation. Successful use of this system requires that every user be sure that a public key actually belongs to the assumed owner. The assignment of users to public keys is confirmed by trustworthy organizations with public key certificates. Such certificates contain the name of the key owner, the corresponding public key, and the electronic signature of the person issuing the certificate.

Trustworthy organizations that issue and sign public key certificates are usually part of a certification infrastructure that is also responsible for the other aspects of certificate management, such as publication, withdrawal, and renewal of certificates. An infrastructure of this kind is generally referred to as a public key infrastructure or PKI. One familiar PKI is the OpenPGP standard in which users publish their certificates themselves without central authorization points. These certificates become trustworthy when signed by other parties in the web of trust.

The X.509 Public Key Infrastructure (PKIX) is an alternative model defined by the IETF (Internet Engineering Task Force) that serves as a model for almost all publicly-used PKIs today. In this model, authentication is made by certificate authorities (CA) in a hierarchical tree structure. The root of the tree is the root CA, which certifies all sub-CAs. The lowest level of sub-CAs issue user certificates. The user certificates are trustworthy by certification that can be traced to the root CA.

The security of such a PKI depends on the trustworthiness of the CA certificates. To make certification practices clear to PKI customers, the PKI operator defines a certification practice statement (CPS) that defines the procedures for certificate management. This should ensure that the PKI only issues trustworthy certificates.

15.1.2. X.509 Certificates

An X.509 certificate is a data structure with several fixed fields and, optionally, additional extensions. The fixed fields mainly contain the name of the key owner, the public key, and the data relating to the issuing CA (name and signature). For security reasons, a certificate should only have a limited period of validity, so a field is also provided for this date. The CA guarantees the validity of the certificate in the specified period. The CPS usually requires the PKI (the issuing CA) to create and distribute a new certificate before expiration.

The extensions can contain any additional information. An application is only required to be able to evaluate an extension if it is identified as critical. If an application does not recognize a critical extension, it must reject the certificate. Some extensions are only useful for a specific application, such as signature or encryption.

Table 15.1 shows the fields of a basic X.509 certificate in version 3.

Table 15.1. X.509v3 Certificate




The version of the certificate, for example, v3

Serial Number

Unique certificate ID (an integer)


The ID of the algorithm used to sign the certificate


Unique name (DN) of the issuing authority (CA)


Period of validity


Unique name (DN) of the owner

Subject Public Key Info

Public key of the owner and the ID of the algorithm

Issuer Unique ID

Unique ID of the issuing CA (optional)

Subject Unique ID

Unique ID of the owner (optional)


Optional additional information, such as KeyUsage or BasicConstraints

15.1.3. Blocking X.509 Certificates

If a certificate becomes untrustworthy before it has expired, it must be blocked immediately. This can become necessary if, for example, the private key has accidentally been made public. Blocking certificates is especially important if the private key belongs to a CA rather than a user certificate. In this case, all user certificates issued by the relevant CA must be blocked immediately. If a certificate is blocked, the PKI (the responsible CA) must make this information available to all those involved using a certificate revocation list (CRL).

These lists are supplied by the CA to public CRL distribution points (CDPs) at regular intervals. The CDP can optionally be named as an extension in the certificate, so a checker can fetch a current CRL for validation purposes. One way to do this is the online certificate status protocol (OCSP). The authenticity of the CRLs is ensured with the signature of the issuing CA. Table 15.2 shows the basic parts of a X.509 CRL.

Table 15.2. X.509 Certificate Revocation List (CRL)




The version of the CRL, such as v2


The ID of the algorithm used to sign the CRL


Unique name (DN) of the publisher of the CRL (usually the issuing CA)

This Update

Time of publication (date, time) of this CRL

Next Update

Time of publication (date, time) of the next CRL

List of revoked certificates

Every entry contains the serial number of the certificate, the time of revocation, and optional extensions (CRL entry extensions)


Optional CRL extensions

15.1.4. Repository for Certificates and CRLs

The certificates and CRLs for a CA must be made publicly accessible using a repository. Because the signature protects the certificates and CRLs from being forged, the repository itself does not need to be secured in a special way. Instead, it tries to grant the simplest and fastest access possible. For this reason, certificates are often provided on an LDAP or HTTP server. Find explanations about LDAP in Chapter 4, LDAP—A Directory Service. Chapter 20, The Apache HTTP Server (↑Reference) contains information about the HTTP server.

15.1.5. Proprietary PKI

YaST contains modules for the basic management of X.509 certificates. This mainly involves the creation of CAs, sub-CAs, and their certificates. The services of a PKI go far beyond simply creating and distributing certificates and CRLs. The operation of a PKI requires a well-conceived administrative infrastructure allowing continuous update of certificates and CRLs. This infrastructure is provided by commercial PKI products and can also be partly automated. YaST provides tools for creating and distributing CAs and certificates, but cannot currently offer this background infrastructure. To set up a small PKI, you can use the available YaST modules. However, you should use commercial products to set up an official or commercial PKI.

15.2. YaST Modules for CA Management

YaST provides two modules for basic CA management. The primary management tasks with these modules are explained here.

15.2.1. Creating a Root CA

The first step when setting up a PKI is to create a root CA. Do the following:

  1. Start YaST and go to Security and Users+CA Management.

  2. Click Create Root CA.

  3. Enter the basic data for the CA in the first dialog, shown in Figure 15.1. The text fields have the following meanings:

    Figure 15.1. YaST CA Module—Basic Data for a Root CA

    YaST CA Module—Basic Data for a Root CA

    CA Name

    Enter the technical name of the CA. Directory names, among other things, are derived from this name, which is why only the characters listed in the help can be used. The technical name is also displayed in the overview when the module is started.

    Common Name

    Enter the name for use in referring to the CA.

    E-Mail Addresses

    Several e-mail addresses can be entered that can be seen by the CA user. This can be helpful for inquiries.


    Select the country where the CA is operated.

    Organisation, Organisational Unit, Locality, State

    Optional values

    Proceed with Next.

  4. Enter a password in the second dialog. This password is always required when using the CA—when creating a sub-CA or generating certificates. The text fields have the following meaning:

    Key Length

    Key Length contains a meaningful default and does not generally need to be changed unless an application cannot deal with this key length. The higher the number the more secure your password is.

    Valid Period (days)

    The Valid Period in the case of a CA defaults to 3650 days (roughly ten years). This long period makes sense because the replacement of a deleted CA involves an enormous administrative effort.

    Clicking Advanced Options opens a dialog for setting different attributes from the X.509 extensions (Figure 15.4, “YaST CA Module—Extended Settings”). These values have rational default settings and should only be changed if you are really sure of what you are doing. Proceed with Next.

  5. Review the summary. YaST displays the current settings for confirmation. Click Create. The root CA is created then appears in the overview.


In general, it is best not to allow user certificates to be issued by the root CA. It is better to create at least one sub-CA and create the user certificates from there. This has the advantage that the root CA can be kept isolated and secure, for example, on an isolated computer on secure premises. This makes it very difficult to attack the root CA.

15.2.2. Changing Password

If you need to change your password for your CA, proceed as follows:

  1. Start YaST and open the CA module.

  2. Select the required root CA and click Enter CA.

  3. Enter the password if you entered a CA the first time. YaST displays the CA key information in the Description tab (see Figure 15.2).

  4. Click Advanced and select Change CA Password. A dialog box opens.

  5. Enter the old and the new password.

  6. Finish with OK

15.2.3. Creating or Revoking a Sub-CA

A sub-CA is created in exactly the same way as a root CA.


The validity period for a sub-CA must be fully within the validity period of the parent CA. A sub-CA is always created after the parent CA, therefore, the default value leads to an error message. To avoid this, enter a permissible value for the period of validity.

Do the following:

  1. Start YaST and open the CA module.

  2. Select the required root CA and click Enter CA.

  3. Enter the password if you are entering a CA for the first time. YaST displays the CA key information in the tab Description (see Figure 15.2).

    Figure 15.2. YaST CA Module—Using a CA

    YaST CA Module—Using a CA

  4. Click Advanced and select Create SubCA. This opens the same dialog as for creating a root CA.

  5. Proceed as described in Section 15.2.1, “Creating a Root CA”.

    It is possible to use one password for all your CAs. Enable Use CA Password as Certificate Password to give your sub-CAs the same password as your root CA. This helps to reduce the amount of passwords for your CAs.

    [Note]Check your Valid Period

    Take into account that the valid period must be lower than the valid period in the root CA.

  6. Select the Certificates tab. Reset compromised or otherwise unwanted sub-CAs here, using Revoke. Revocation alone is not enough to deactivate a sub-CA. You must also publish revoked sub-CAs in a CRL. The creation of CRLs is described in Section 15.2.6, “Creating Certificate Revocation Lists (CRLs)”.

  7. Finish with OK

15.2.4. Creating or Revoking User Certificates

Creating client and server certificates is very similar to creating CAs in Section 15.2.1, “Creating a Root CA”. The same principles apply here. In certificates intended for e-mail signature, the e-mail address of the sender (the private key owner) should be contained in the certificate to enable the e-mail program to assign the correct certificate. For certificate assignment during encryption, it is necessary for the e-mail address of the recipient (the public key owner) to be included in the certificate. In the case of server and client certificates, the hostname of the server must be entered in the Common Name field. The default validity period for certificates is 365 days.

To create client and server certificates, do the following:

  1. Start YaST and open the CA module.

  2. Select the required root CA and click Enter CA.

  3. Enter the password if you are entering a CA for the first time. YaST displays the CA key information in the Description tab.

  4. Click Certificates (see Figure 15.3).

    Figure 15.3. Certificates of a CA

    Certificates of a CA

  5. Click Add+Add Server Certificate and create a server certificate.

  6. Click Add+Add Client Certificate and create a client certificate. Do not forget to enter an e-mail address.

  7. Finish with OK

To revoke compromised or otherwise unwanted certificates, do the following:

  1. Start YaST and open the CA module.

  2. Select the required root CA and click Enter CA.

  3. Enter the password if you are entering a CA for the first time. YaST displays the CA key information in the Description tab.

  4. Click Certificates (see Section 15.2.3, “Creating or Revoking a Sub-CA”.)

  5. Select the certificate to revoke and click Revoke.

  6. Choose a reason to revoke this certificate

  7. Finish with OK.


Revocation alone is not enough to deactivate a certificate. Also publish revoked certificates in a CRL. Section 15.2.6, “Creating Certificate Revocation Lists (CRLs)” explains how to create CRLs. Revoked certificates can be completely removed after publication in a CRL with Delete.

15.2.5. Changing Default Values

The previous sections explained how to create sub-CAs, client certificates, and server certificates. Special settings are used in the extensions of the X.509 certificate. These settings have been given rational defaults for every certificate type and do not normally need to be changed. However, it may be that you have special requirements for these extensions. In this case, it may make sense to adjust the defaults. Otherwise, start from scratch every time you create a certificate.

  1. Start YaST and open the CA module.

  2. Enter the required root CA, as described in Section 15.2.3, “Creating or Revoking a Sub-CA”.

  3. Click Advanced+Edit Defaults.

  4. Choose the type the settings to change. The dialog for changing the defaults, shown in Figure 15.4, “YaST CA Module—Extended Settings”, then opens.

    Figure 15.4. YaST CA Module—Extended Settings

    YaST CA Module—Extended Settings

  5. Change the associated value on the right side and set or delete the critical setting with critical.

  6. Click Next to see a short summary.

  7. Finish your changes with Save.


All changes to the defaults only affect objects created after this point. Already-existing CAs and certificates remain unchanged.

15.2.6. Creating Certificate Revocation Lists (CRLs)

If compromised or otherwise unwanted certificates need to be excluded from further use, they must first be revoked. The procedure for this is explained in Section 15.2.3, “Creating or Revoking a Sub-CA” (for sub-CAs) and Section 15.2.4, “Creating or Revoking User Certificates” (for user certificates). After this, a CRL must be created and published with this information.

The system maintains only one CRL for each CA. To create or update this CRL, do the following:

  1. Start YaST and open the CA module.

  2. Enter the required CA, as described in Section 15.2.3, “Creating or Revoking a Sub-CA”.

  3. Click CRL. The dialog that opens displays a summary of the last CRL of this CA.

  4. Create a new CRL with Generate CRL if you have revoked new sub-CAs or certificates since its creation.

  5. Specify the period of validity for the new CRL (default: 30 days).

  6. Click OK to create and display the CRL. Afterwards, you must publish this CRL.


Applications that evaluate CRLs reject every certificate if the CRL is not available or has expired. As a PKI provider, it is your duty always to create and publish a new CRL before the current CRL expires (period of validity). YaST does not provide a function for automating this procedure.

15.2.7. Exporting CA Objects to LDAP

The executing computer should be configured with the YaST LDAP client for LDAP export. This provides LDAP server information at runtime that can be used when completing dialog fields. Otherwise (although export may be possible), all LDAP data must be entered manually. You must always enter several passwords (see Table 15.3, “Passwords during LDAP Export”).

Table 15.3. Passwords during LDAP Export



LDAP Password

Authorizes the user to make entries in the LDAP tree.

Certificate Password

Authorizes the user to export the certificate.

New Certificate Password

The PKCS12 format is used during LDAP export. This format forces the assignment of a new password for the exported certificate.

Certificates, CAs, and CRLs can be exported to LDAP.

Exporting a CA to LDAP

To export a CA, enter the CA as described in Section 15.2.3, “Creating or Revoking a Sub-CA”. Select Extended+Export to LDAP in the subsequent dialog, which opens the dialog for entering LDAP data. If your system has been configured with the YaST LDAP client, the fields are already partly completed. Otherwise, enter all the data manually. Entries are made in LDAP in a separate tree with the attribute caCertificate.

Exporting a Certificate to LDAP

Enter the CA containing the certificate to export then select Certificates. Select the required certificate from the certificate list in the upper part of the dialog and select Export+Export to LDAP. The LDAP data is entered here in the same way as for CAs. The certificate is saved with the corresponding user object in the LDAP tree with the attributes userCertificate (PEM format) and userPKCS12 (PKCS12 format).

Exporting a CRL to LDAP

Enter the CA containing the CRL to export and select CRL. If desired, create a new CRL and click Export. The dialog that opens displays the export parameters. You can export the CRL for this CA either once or in periodical time intervals. Activate the export by selecting Export to LDAP and enter the respective LDAP data. To do this at regular intervals, select the Repeated Recreation and Export radio button and change the interval, if appropriate.

15.2.8. Exporting CA Objects as a File

If you have set up a repository on the computer for administering CAs, you can use this option to create the CA objects directly as a file at the correct location. Different output formats are available, such as PEM, DER, and PKCS12. In the case of PEM, it is also possible to choose whether a certificate should be exported with or without key and whether the key should be encrypted. In the case of PKCS12, it is also possible to export the certification path.

Export a file in the same way for certificates, CAs as with LDAP, described in Section 15.2.7, “Exporting CA Objects to LDAP”, except you should select Export as File instead of Export to LDAP. This then takes you to a dialog for selecting the required output format and entering the password and filename. The certificate is stored at the required location after clicking OK.

For CRLs click Export, select Export to file, choose the export format (PEM or DER) and enter the path. Proceed with OK to save it to the respective location.


You can select any storage location in the file system. This option can also be used to save CA objects on a transport medium, such as a USB stick. The /media directory generally holds any type of drive except the hard drive of your system.

15.2.9. Importing Common Server Certificates

If you have exported a server certificate with YaST to your media on an isolated CA management computer, you can import this certificate on a server as a common server certificate. Do this during installation or at a later point with YaST.


You need one of the PKCS12 formats to import your certificate successfully.

The general server certificate is stored in /etc/ssl/servercerts and can be used there by any CA-supported service. When this certificate expires, it can easily be replaced using the same mechanisms. To get things functioning with the replaced certificate, restart the participating services.


If you select Import here, you can select the source in the file system. This option can also be used to import certificates from a transport medium, such as a USB stick.

To import a common server certificate, do the following:

  1. Start YaST and open Common Server Certificate under Security and Users

  2. View the data for the current certificate in the description field after YaST has been started.

  3. Select Import and the certificate file.

  4. Enter the password and click Next. The certificate is imported then displayed in the description field.

  5. Close YaST with Finish.

15.3. For More Information

Detailed information about X.509 certificates, refer to

Part IV. Confining Privileges with AppArmor


16. Introducing AppArmor
16.1. Background Information on AppArmor Profiling
17. Getting Started
17.1. Installing AppArmor
17.2. Enabling and Disabling AppArmor
17.3. Choosing the Applications to Profile
17.4. Building and Modifying Profiles
17.5. Updating Your Profiles
18. Immunizing Programs
18.1. Introducing the AppArmor Framework
18.2. Determining Programs to Immunize
18.3. Immunizing cron Jobs
18.4. Immunizing Network Applications
19. Profile Components and Syntax
19.1. Breaking a AppArmor Profile into Its Parts
19.2. Profile Types
19.3. #include Statements
19.4. Capability Entries (POSIX.1e)
19.5. Network Access Control
19.6. Paths and Globbing
19.7. File Permission Access Modes
19.8. Execute Modes
19.9. Resource Limit Control
19.10. Auditing Rules
20. AppArmor Profile Repositories
20.1. Using the Local Repository
21. Building and Managing Profiles with YaST
21.1. Adding a Profile Using the Wizard
21.2. Manually Adding a Profile
21.3. Editing Profiles
21.4. Deleting a Profile
21.5. Updating Profiles from Log Entries
21.6. Managing AppArmor
22. Building Profiles from the Command Line
22.1. Checking the AppArmor Module Status
22.2. Building AppArmor Profiles
22.3. Adding or Creating an AppArmor Profile
22.4. Editing an AppArmor Profile
22.5. Deleting an AppArmor Profile
22.6. Two Methods of Profiling
22.7. Important Filenames and Directories
23. Profiling Your Web Applications Using ChangeHat
23.1. Apache ChangeHat
23.2. Configuring Apache for mod_apparmor
24. Confining Users with pam_apparmor
25. Managing Profiled Applications
25.1. Reacting to Security Event Rejections
25.2. Maintaining Your Security Profiles
26. Support
26.1. Updating AppArmor Online
26.2. Using the Man Pages
26.3. For More Information
26.4. Troubleshooting
26.5. Reporting Bugs for AppArmor
27. AppArmor Glossary

Chapter 16. Introducing AppArmor

Many security vulnerabilities result from bugs in trusted programs. A trusted program runs with privileges that attackers would like to have. The program fails to keep that trust if there is a bug in the program that allows the attacker to acquire said privilege.

AppArmor® is an application security solution designed specifically to apply privilege confinement to suspect programs. AppArmor allows the administrator to specify the domain of activities the program can perform by developing a security profile for that application (a listing of files that the program may access and the operations the program may perform). AppArmor secures applications by enforcing good application behavior without relying on attack signatures, so it can prevent attacks even if previously unknown vulnerabilities are being exploited.

AppArmor consists of:

  • A library of AppArmor profiles for common Linux* applications, describing what files the program needs to access.

  • A library of AppArmor profile foundation classes (profile building blocks) needed for common application activities, such as DNS lookup and user authentication.

  • A tool suite for developing and enhancing AppArmor profiles, so that you can change the existing profiles to suit your needs and create new profiles for your own local and custom applications.

  • Several specially modified applications that are AppArmor enabled to provide enhanced security in the form of unique subprocess confinement (including Apache and Tomcat).

  • The AppArmor–loadable kernel module and associated control scripts to enforce AppArmor policies on your openSUSE® system.

16.1. Background Information on AppArmor Profiling

For more information about the science and security of AppArmor, refer to the following papers:

SubDomain: Parsimonious Server Security by Crispin Cowan, Steve Beattie, Greg Kroah-Hartman, Calton Pu, Perry Wagle, and Virgil Gligor

Describes the initial design and implementation of AppArmor. Published in the proceedings of the USENIX LISA Conference, December 2000, New Orleans, LA. This paper is now out of date, describing syntax and features that are different from the current AppArmor product. This paper should be used only for background, and not for technical documentation.

Defcon Capture the Flag: Defending Vulnerable Code from Intense Attack by Crispin Cowan, Seth Arnold, Steve Beattie, Chris Wright, and John Viega

A good guide to strategic and tactical use of AppArmor to solve severe security problems in a very short period of time. Published in the Proceedings of the DARPA Information Survivability Conference and Expo (DISCEX III), April 2003, Washington, DC.

AppArmor for Geeks by Seth Arnold

This document tries to convey a better understanding of the technical details of AppArmor. It is available at

Chapter 17. Getting Started

Prepare a successful deployment of AppArmor on your system by carefully considering the following items:

  1. Determine the applications to profile. Read more on this in Section 17.3, “Choosing the Applications to Profile”.

  2. Build the needed profiles as roughly outlined in Section 17.4, “Building and Modifying Profiles”. Check the results and adjust the profiles when necessary.

  3. Update your profiles whenever your environment changes or you need to react to security events logged by AppArmor's reporting tool. Refer to Section 17.5, “Updating Your Profiles”.

17.1. Installing AppArmor

AppArmor is not installed by default on openSUSE®. Install the pattern apparmor for a complete AppArmor installation. Either use the YaST Software Management module for installation, or the following zypper command:

zypper in -t pattern apparmor
  • apparmor-docs

  • apparmor-parser

  • apparmor-profiles

  • apparmor-utils

  • audit

  • libapparmor1

  • perl-libapparmor

  • yast2-apparmor

17.2. Enabling and Disabling AppArmor

AppArmor is configured to run by default on any fresh installation of openSUSE. There are two ways of toggling the status of AppArmor:

Using YaST System Services (Runlevel)

Disable or enable AppArmor by removing or adding its boot script to the sequence of scripts executed on system boot. Status changes are applied on reboot.

Using AppArmor Control Panel

Toggle the status of AppArmor in a running system by switching it off or on using the YaST AppArmor Control Panel. Changes made here are applied instantaneously. The Control Panel triggers a stop or start event for AppArmor and removes or adds its boot script in the system's boot sequence.

To disable AppArmor permanently (by removing it from the sequence of scripts executed on system boot) proceed as follows:

  1. Start YaST.

  2. Select System+System Services (Runlevel).

  3. Select Expert Mode.

  4. Select boot.apparmor and click Set/Reset+Disable the service.

  5. Exit the YaST Runlevel tool with Finish.

AppArmor will not be initialized on reboot, and stays inactive until you reenable it. Reenabling a service using the YaST Runlevel tool is similar to disabling it.

Toggle the status of AppArmor in a running system by using the AppArmor Control Panel. These changes take effect as soon as you apply them and survive a reboot of the system. To toggle AppArmor's status, proceed as follows:

  1. Start YaST.

  2. Select AppArmor+AppArmor Control Panel.

  3. Select Enable AppArmor. To disable AppArmor, uncheck this option.

  4. Exit the AppArmor Control Panel with Done.

17.3. Choosing the Applications to Profile

You only need to protect the programs that are exposed to attacks in your particular setup, so only use profiles for those applications you actually run. Use the following list to determine the most likely candidates:

Network Agents
Web Applications
Cron Jobs

To find out which processes are currently running with open network ports and might need a profile to confine them, run aa-unconfined as root.

Example 17.1. Output of aa-unconfined

19848 /usr/sbin/cupsd not confined
19887 /usr/sbin/sshd not confined
19947 /usr/lib/postfix/master not confined
29205 /usr/sbin/sshd confined by '/usr/sbin/sshd (enforce)'

Each of the processes in the above example labeled not confined might need a custom profile to confine it. Those labeled confined by are already protected by AppArmor.

[Tip]For More Information

For more information about choosing the the right applications to profile, refer to Section 18.2, “Determining Programs to Immunize”.

17.4. Building and Modifying Profiles

AppArmor on openSUSE ships with a preconfigured set of profiles for the most important applications. In addition, you can use AppArmor to create your own profiles for any application you want.

There are two ways of managing profiles. One is to use the graphical front-end provided by the YaST AppArmor modules and the other is to use the command line tools provided by the AppArmor suite itself. Both methods basically work the same way.

For each application, perform the following steps to create a profile:

  1. As root, let AppArmor create a rough outline of the application's profile by running aa-genprof programname


    Outline the basic profile by running YaST+AppArmor+Add Profile Wizard and specifying the complete path to the application you want to profile.

    A basic profile is outlined and AppArmor is put into learning mode, which means that it logs any activity of the program you are executing, but does not yet restrict it.

  2. Run the full range of the application's actions to let AppArmor get a very specific picture of its activities.

  3. Let AppArmor analyze the log files generated in Step 2 by typing S in aa-genprof.


    Analyze the logs by clicking Scan System Log for AppArmor Events in the Add Profile Wizard and following the instructions given in the wizard until the profile is completed.

    AppArmor scans the logs it recorded during the application's run and asks you to set the access rights for each event that was logged. Either set them for each file or use globbing.

  4. Depending on the complexity of your application, it might be necessary to repeat Step 2 and Step 3. Confine the application, exercise it under the confined conditions, and process any new log events. To properly confine the full range of an application's capabilities, you might be required to repeat this procedure often.

  5. Once all access permissions are set, your profile is set to enforce mode. The profile is applied and AppArmor restricts the application according to the profile just created.

    If you started aa-genprof on an application that had an existing profile that was in complain mode, this profile remains in learning mode upon exit of this learning cycle. For more information about changing the mode of a profile, refer to Section, “aa-complain—Entering Complain or Learning Mode” and Section, “aa-enforce—Entering Enforce Mode”.

Test your profile settings by performing every task you need with the application you just confined. Normally, the confined program runs smoothly and you do not notice AppArmor activities at all. However, if you notice certain misbehavior with your application, check the system logs and see if AppArmor is too tightly confining your application. Depending on the log mechanism used on your system, there are several places to look for AppArmor log entries:


To adjust the profile, analyze the log messages relating to this application again as described in Step 3. Determine the access rights or restrictions when prompted.

[Tip]For More Information

For more information about profile building and modification, refer to Chapter 19, Profile Components and Syntax, Chapter 21, Building and Managing Profiles with YaST, and Chapter 22, Building Profiles from the Command Line.

17.5. Updating Your Profiles

Software and system configurations change over time. As a result, your profile setup for AppArmor might need some fine-tuning from time to time. AppArmor checks your system log for policy violations or other AppArmor events and lets you adjust your profile set accordingly. Any application behavior that is outside of any profile definition can also be addressed using the Update Profile Wizard.

To update your profile set, proceed as follows:

  1. Start YaST and choose AppArmor+Update Profile Wizard.

  2. Adjust access or execute rights to any resource or for any executable that has been logged when prompted.

  3. Leave YaST after you have answered all questions. Your changes are applied to the respective profiles.

[Tip]For More Information

For more information about updating your profiles from the system logs, refer to Section 21.5, “Updating Profiles from Log Entries”.

Chapter 18. Immunizing Programs

Effective hardening of a computer system requires minimizing the number of programs that mediate privilege, then securing the programs as much as possible. With AppArmor, you only need to profile the programs that are exposed to attack in your environment, which drastically reduces the amount of work required to harden your computer. AppArmor profiles enforce policies to make sure that programs do what they are supposed to do, but nothing else.

AppArmor® provides immunization technologies that protect applications from the inherent vulnerabilities they possess. After installing AppArmor, setting up AppArmor profiles, and rebooting the computer, your system becomes immunized because it begins to enforce the AppArmor security policies. Protecting programs with AppArmor is referred to as immunizing.

Administrators need only concern themselves with the applications that are vulnerable to attacks, and generate profiles for these. Hardening a system thus comes down to building and maintaining the AppArmor profile set and monitoring any policy violations or exceptions logged by AppArmor's reporting facility.

Users should not notice AppArmor at all. It runs behind the scenes and does not require any user interaction. Performance is not noticeably affected by AppArmor. If some activity of the application is not covered by an AppArmor profile or if some activity of the application is prevented by AppArmor, the administrator needs to adjust the profile of this application to cover this kind of behavior.

AppArmor sets up a collection of default application profiles to protect standard Linux services. To protect other applications, use the AppArmor tools to create profiles for the applications that you want protected. This chapter introduces the philosophy of immunizing programs. Proceed to Chapter 19, Profile Components and Syntax, Chapter 21, Building and Managing Profiles with YaST, or Chapter 22, Building Profiles from the Command Line if you are ready to build and manage AppArmor profiles.

AppArmor provides streamlined access control for network services by specifying which files each program is allowed to read, write, and execute, and which type of network it is allowed to access. This ensures that each program does what it is supposed to do, and nothing else. AppArmor quarantines programs to protect the rest of the system from being damaged by a compromised process.

AppArmor is a host intrusion prevention or mandatory access control scheme. Previously, access control schemes were centered around users because they were built for large timeshare systems. Alternatively, modern network servers largely do not permit users to log in, but instead provide a variety of network services for users (such as Web, mail, file, and print servers). AppArmor controls the access given to network services and other programs to prevent weaknesses from being exploited.

[Tip]Background Information for AppArmor

To get a more in-depth overview of AppArmor and the overall concept behind it, refer to Section 16.1, “Background Information on AppArmor Profiling”.

18.1. Introducing the AppArmor Framework

This section provides a very basic understanding of what is happening behind the scenes (and under the hood of the YaST interface) when you run AppArmor.

An AppArmor profile is a plain text file containing path entries and access permissions. See Section 19.1, “Breaking a AppArmor Profile into Its Parts” for a detailed reference profile. The directives contained in this text file are then enforced by the AppArmor routines to quarantine the process or program.

The following tools interact in the building and enforcement of AppArmor profiles and policies:

aa-unconfined / unconfined

aa-unconfined detects any application running on your system that listens for network connections and is not protected by an AppArmor profile. Refer to Section, “aa-unconfined—Identifying Unprotected Processes” for detailed information about this tool.

aa-autodep / autodep

aa-autodep creates a basic framework of a profile that needs to be fleshed out before it is put to use in production. The resulting profile is loaded and put into complain mode, reporting any behavior of the application that is not (yet) covered by AppArmor rules. Refer to Section, “aa-autodep—Creating Approximate Profiles” for detailed information about this tool.

aa-genprof / genprof

aa-genprof generates a basic profile and asks you to refine this profile by executing the application and generating log events that need to be taken care of by AppArmor policies. You are guided through a series of questions to deal with the log events that have been triggered during the application's execution. After the profile has been generated, it is loaded and put into enforce mode. Refer to Section, “aa-genprof—Generating Profiles” for detailed information about this tool.

aa-logprof / logprof

aa-logprof interactively scans and reviews the log entries generated by an application that is confined by an AppArmor profile in complain mode. It assists you in generating new entries in the profile concerned. Refer to Section, “aa-logprof—Scanning the System Log” for detailed information about this tool.

aa-complain / complain

aa-complain toggles the mode of an AppArmor profile from enforce to complain. Exceptions to rules set in a profile are logged, but the profile is not enforced. Refer to Section, “aa-complain—Entering Complain or Learning Mode” for detailed information about this tool.

aa-enforce / enforce

aa-enforce toggles the mode of an AppArmor profile from complain to enforce. Exceptions to rules set in a profile are logged, but not permitted—the profile is enforced. Refer to Section, “aa-enforce—Entering Enforce Mode” for detailed information about this tool.

Once a profile has been built and is loaded, there are two ways in which it can get processed:

aa-complain / complain

In complain mode, violations of AppArmor profile rules, such as the profiled program accessing files not permitted by the profile, are detected. The violations are permitted, but also logged. To improve the profile, turn complain mode on, run the program through a suite of tests to generate log events that characterize the program's access needs, then postprocess the log with the AppArmor tools (YaST or aa-logprof) to transform log events into improved profiles.

aa-enforce / enforce

In enforce mode, violations of AppArmor profile rules, such as the profiled program accessing files not permitted by the profile, are detected. The violations are logged and not permitted. The default is for enforce mode to be enabled. To log the violations only, but still permit them, use complain mode. Enforce toggles with complain mode.

18.2. Determining Programs to Immunize

Now that you have familiarized yourself with AppArmor, start selecting the applications for which to build profiles. Programs that need profiling are those that mediate privilege. The following programs have access to resources that the person using the program does not have, so they grant the privilege to the user when used:

cron Jobs

Programs that are run periodically by cron. Such programs read input from a variety of sources and can run with special privileges, sometimes with as much as root privilege. For example, cron can run /usr/sbin/logrotate daily to rotate, compress, or even mail system logs. For instructions for finding these types of programs, refer to Section 18.3, “Immunizing cron Jobs”.

Web Applications

Programs that can be invoked through a Web browser, including CGI Perl scripts, PHP pages, and more complex Web applications. For instructions for finding these types of programs, refer to Section 18.4.1, “Immunizing Web Applications”.

Network Agents

Programs (servers and clients) that have open network ports. User clients, such as mail clients and Web browsers mediate privilege. These programs run with the privilege to write to the user's home directory and they process input from potentially hostile remote sources, such as hostile Web sites and e-mailed malicious code. For instructions for finding these types of programs, refer to Section 18.4.2, “Immunizing Network Agents”.

Conversely, unprivileged programs do not need to be profiled. For instance, a shell script might invoke the cp program to copy a file. Because cp does not have its own profile, it inherits the profile of the parent shell script, so can copy any files that the parent shell script's profile can read and write.

18.3. Immunizing cron Jobs

To find programs that are run by cron, inspect your local cron configuration. Unfortunately, cron configuration is rather complex, so there are numerous files to inspect. Periodic cron jobs are run from these files:


For root's cron jobs, edit the tasks with crontab -e and list root's cron tasks with crontab -l. You must be root for these to work.

Once you find these programs, you can use the Add Profile Wizard to create profiles for them. Refer to Section 21.1, “Adding a Profile Using the Wizard”.

18.4. Immunizing Network Applications

An automated method for finding network server daemons that should be profiled is to use the aa-unconfined tool.

The aa-unconfined tool uses the command netstat -nlp to inspect your open ports from inside your computer, detect the programs associated with those ports, and inspect the set of AppArmor profiles that you have loaded. aa-unconfined then reports these programs along with the AppArmor profile associated with each program, or reports none (if the program is not confined).


If you create a new profile, you must restart the program that has been profiled to have it be effectively confined by AppArmor.

Below is a sample aa-unconfined output:

2325 /sbin/portmap not confined 
37021 /usr/sbin/sshd2 confined
   by '/usr/sbin/sshd3 (enforce)' 
4040 /usr/sbin/ntpd confined by '/usr/sbin/ntpd (enforce)' 
4373 /usr/lib/postfix/master confined by '/usr/lib/postfix/master (enforce)' 
4505 /usr/sbin/httpd2-prefork confined by '/usr/sbin/httpd2-prefork (enforce)'
5274 /sbin/dhcpcd not confined 
5592 /usr/bin/ssh not confined 
7146 /usr/sbin/cupsd confined by '/usr/sbin/cupsd (complain)'


The first portion is a number. This number is the process ID number (PID) of the listening program.


The second portion is a string that represents the absolute path of the listening program


The final portion indicates the profile confining the program, if any.


aa-unconfined requires root privileges and should not be run from a shell that is confined by an AppArmor profile.

aa-unconfined does not distinguish between one network interface and another, so it reports all unconfined processes, even those that might be listening to an internal LAN interface.

Finding user network client applications is dependent on your user preferences. The aa-unconfined tool detects and reports network ports opened by client applications, but only those client applications that are running at the time the aa-unconfined analysis is performed. This is a problem because network services tend to be running all the time, while network client applications tend only to be running when the user is interested in them.

Applying AppArmor profiles to user network client applications is also dependent on user preferences. Therefore, we leave the profiling of user network client applications as an exercise for the user.

To aggressively confine desktop applications, the aa-unconfined command supports a paranoid option, which reports all processes running and the corresponding AppArmor profiles that might or might not be associated with each process. The user can then decide whether each of these programs needs an AppArmor profile.

If you have new or modified profiles, you can submit them to the mailing list along with a use case for the application behavior that you exercised. The AppArmor team reviews and may submit the work into openSUSE. We cannot guarantee that every profile will be included, but we make a sincere effort to include as much as possible so that end users can contribute to the security profiles that ship in openSUSE.

Alternatively, use the AppArmor profile repository to make your profiles available to other users and to download profiles created by other AppArmor users and the AppArmor developers. Refer to Chapter 20, AppArmor Profile Repositories for more information on how to use the AppArmor profile repository.

18.4.1. Immunizing Web Applications

To find Web applications, investigate your Web server configuration. The Apache Web server is highly configurable and Web applications can be stored in many directories, depending on your local configuration. openSUSE, by default, stores Web applications in /srv/www/cgi-bin/. To the maximum extent possible, each Web application should have an AppArmor profile.

Once you find these programs, you can use the AppArmor Add Profile Wizard to create profiles for them. Refer to Section 21.1, “Adding a Profile Using the Wizard”.

Because CGI programs are executed by the Apache Web server, the profile for Apache itself, usr.sbin.httpd2-prefork for Apache2 on openSUSE, must be modified to add execute permissions to each of these programs. For instance, adding the line /srv/www/cgi-bin/ rpx grants Apache permission to execute the Perl script and requires that there be a dedicated profile for If does not have a dedicated profile associated with it, the rule should say /srv/www/cgi-bin/ rix to cause to inherit the usr.sbin.httpd2-prefork profile.

Some users might find it inconvenient to specify execute permission for every CGI script that Apache might invoke. Instead, the administrator can grant controlled access to collections of CGI scripts. For instance, adding the line /srv/www/cgi-bin/*.{pl,py,pyc} rix allows Apache to execute all files in /srv/www/cgi-bin/ ending in .pl (Perl scripts) and .py or .pyc (Python scripts). As above, the ix part of the rule causes Python scripts to inherit the Apache profile, which is appropriate if you do not want to write individual profiles for each Python script.


If you want the subprocess confinement module (apache2-mod-apparmor) functionality when Web applications handle Apache modules (mod_perl and mod_php), use the ChangeHat features when you add a profile in YaST or at the command line. To take advantage of the subprocess confinement, refer to Section 23.1, “Apache ChangeHat”.

Profiling Web applications that use mod_perl and mod_php requires slightly different handling. In this case, the program is a script interpreted directly by the module within the Apache process, so no exec happens. Instead, the AppArmor version of Apache calls change_hat() using a subprofile (a hat) corresponding to the name of the URI requested.


The name presented for the script to execute might not be the URI, depending on how Apache has been configured for where to look for module scripts. If you have configured your Apache to place scripts in a different place, the different names appear in log file when AppArmor complains about access violations. See Chapter 25, Managing Profiled Applications.

For mod_perl and mod_php scripts, this is the name of the Perl script or the PHP page requested. For example, adding this subprofile allows the localtime.php page to execute and access the local system time:

/usr/bin/httpd2-prefork {
  # ...
  ^/cgi-bin/localtime.php {
    /etc/localtime                  r,
    /srv/www/cgi-bin/localtime.php  r,
    /usr/lib/locale/**              r,

If no subprofile has been defined, the AppArmor version of Apache applies the DEFAULT_URI hat. This subprofile is basically sufficient to display an HTML Web page. The DEFAULT_URI hat that AppArmor provides by default is the following:

    /usr/sbin/suexec2                  mixr,
    /var/log/apache2/**                rwl,
    @{HOME}/public_html                r,
    @{HOME}/public_html/**             r,
    /srv/www/htdocs                    r,
    /srv/www/htdocs/**                 r,
    /srv/www/icons/*.{gif,jpg,png}     r,
    /srv/www/vhosts                    r,
    /srv/www/vhosts/**                 r,
    /usr/share/apache2/**              r,
    /var/lib/php/sess_*                rwl }

To use a single AppArmor profile for all Web pages and CGI scripts served by Apache, a good approach is to edit the DEFAULT_URI subprofile.

18.4.2. Immunizing Network Agents

To find network server daemons and network clients (such as fetchmail, Firefox, Amarok or Banshee) that need to be profiled, you should inspect the open ports on your machine, consider the programs that are answering on those ports, and provide profiles for as many of those programs as possible. If you provide profiles for all programs with open network ports, an attacker cannot get to the file system on your machine without passing through a AppArmor profile policy.

Scan your server for open network ports manually from outside the machine using a scanner (such as nmap), or from inside the machine using the netstat --inet -n -p command. Then, inspect the machine to determine which programs are answering on the discovered open ports.


Refer to the man page of the netstat command for a detailed reference of all possible options.

Chapter 19. Profile Components and Syntax

Building AppArmor profiles to confine an application is very straightforward and intuitive. AppArmor ships with several tools that assist in profile creation. It does not require you to do any programming or script handling. The only task that is required of the administrator is to determine a policy of strictest access and execute permissions for each application that needs to be hardened.

Updates or modifications to the application profiles are only required if the software configuration or the desired range of activities changes. AppArmor offers intuitive tools to handle profile updates and modifications.

You are ready to build AppArmor profiles after you select the programs to profile. To do so, it is important to understand the components and syntax of profiles. AppArmor profiles contain several building blocks that help build simple and reusable profile code:

#include Files

#include statements are used to pull in parts of other AppArmor profiles to simplify the structure of new profiles.


Abstractions are #include statements grouped by common application tasks.

Program Chunks

Program chunks are #include statements that contain chunks of profiles that are specific to program suites.

Capability Entries

Capability entries are profile entries for any of the POSIX.1e Linux capabilities allowing a fine-grained control over what a confined process is allowed to do through system calls that require privileges.

Network Access Control Entries

Network Access Control Entries mediate network access based on the address type and family.

Local Variable Definitions

Local variables define shortcuts for paths.

File Access Control Entries

File Access Control Entries specify the set of files an application can access.

rlimit Entries

rlimit entries set and control an application's resource limits.

For help determining the programs to profile, refer to Section 18.2, “Determining Programs to Immunize”. To start building AppArmor profiles with YaST, proceed to Chapter 21, Building and Managing Profiles with YaST. To build profiles using the AppArmor command line interface, proceed to Chapter 22, Building Profiles from the Command Line.

19.1. Breaking a AppArmor Profile into Its Parts

The easiest way of explaining what a profile consists of and how to create one is to show the details of a sample profile, in this case for a hypothetical application called /usr/bin/foo:

#include <tunables/global>1

# a comment naming the application to confine
   #include <abstractions/base>4

   capability setgid5,
   network inet tcp6,

   link /etc/sysconfig/foo -> /etc/foo.conf,7
   /bin/mount            ux,
   /dev/{,u}8random     r,
   /etc/      r,
   /etc/foo/*            r,
   /lib/ld-*.so*         mr,
   /lib/lib*.so*         mr,
   /proc/[0-9]**         r,
   /usr/lib/**           mr,
   /tmp/9                r,
   /tmp/          wr,
   /tmp/foo.*            lrw,
   /@{HOME}10/.foo_file   rw,
   /@{HOME}/.foo_lock    kw,
   owner11 /shared/foo/** rw,
   /usr/bin/foobar       cx,12
   /bin/**               px -> bin_generic,13

   # a comment about foo's local (children)profile for /usr/bin/foobar.

   profile /usr/bin/foobar14 {
      /bin/bash          rmix,
      /bin/cat           rmix,
      /bin/more          rmix,
      /var/log/foobar*   rwl,
      /etc/foobar        r,

  # foo's hat, bar.
   ^bar15 {
    /lib/ld-*.so*         mr,
    /usr/bin/bar          px,
    /var/spool/*          rwl,


This loads a file containing variable definitions.


The normalized path to the program that is confined.


The curly braces ({}) serve as a container for include statements, subprofiles, path entries, capability entries, and network entries.


This directive pulls in components of AppArmor profiles to simplify profiles.


Capability entry statements enable each of the 29 POSIX.1e draft capabilities.


A directive determining the kind of network access allowed to the application. For details, refer to Section 19.5, “Network Access Control”.


A link pair rule specifying the source and the target of a link. See Section 19.7.6, “Link Pair” for more information.


The curly braces ({}) make this rule apply to the path both with and without the content enclosed by the braces.


A path entry specifying what areas of the file system the program can access. The first part of a path entry specifies the absolute path of a file (including regular expression globbing) and the second part indicates permissible access modes (for example r for read, w for write, and x for execute). A whitespace of any kind (spaces or tabs) can precede pathnames or separate the pathname from the access modes. Spaces between the access mode and the trailing comma are optional. Find a comprehensive overview of the available access modes in Section 19.7, “File Permission Access Modes”.


This variable expands to a value that can be changed without changing the entire profile.


An owner conditional rule, granting read and write permission on files owned by the user. Refer to Section 19.7.7, “Owner Conditional Rules” for more information.


This entry defines a transition to the local profile /usr/bin/foobar. Find a comprehensive overview of the available execute modes in Section 19.8, “Execute Modes”.


A named profile transition to the profile bin_generic located in the global scope. See Section 19.8.7, “Named Profile Transitions” for details.


The local profile /usr/bin/foobar is defined in this section.


This section references a hat subprofile of the application. For more details on AppArmor's ChangeHat feature, refer to Chapter 23, Profiling Your Web Applications Using ChangeHat.

When a profile is created for a program, the program can access only the files, modes, and POSIX capabilities specified in the profile. These restrictions are in addition to the native Linux access controls.

Example:  To gain the capability CAP_CHOWN, the program must have both access to CAP_CHOWN under conventional Linux access controls (typically, be a root-owned process) and have the capability chown in its profile. Similarly, to be able to write to the file /foo/bar the program must have both the correct user ID and mode bits set in the files attributes (see the chmod and chown man pages) and have /foo/bar w in its profile.

Attempts to violate AppArmor rules are recorded in /var/log/audit/audit.log if the audit package is installed or otherwise in /var/log/messages. In many cases, AppArmor rules prevent an attack from working because necessary files are not accessible and, in all cases, AppArmor confinement restricts the damage that the attacker can do to the set of files permitted by AppArmor.

19.2. Profile Types

AppArmor knows four different types of profiles: standard profiles, unattached profiles, local profiles and hats. Standard and unattached profiles are stand-alone profiles, each stored in a file under /etc/apparmor.d/. Local profiles and hats are children profiles embedded inside of a parent profile used to provide tighter or alternate confinement for a subtask of an application.

19.2.1. Standard Profiles

The default AppArmor profile is attached to a program by its name, so a profile name must match the path to the application it is to confine.

/usr/bin/foo {

This profile will be automatically used whenever an unconfined process executes /usr/bin/foo.

19.2.2. Unattached Profiles

Unattached profiles do not reside in the file system namespace and therefore are not automatically attached to an application. The name of an unattached profile is preceded by the keyword profile. You can freely choose a profile name, except for the following limitations: the name must not begin with a : or . character. If it contains a whitespace, it must be quoted. If the name begins with a /, the profile is considered to be a standard profile, so the following two profiles are identical:

profile /usr/bin/foo {
/usr/bin/foo {

Unattached profiles are never used automatically, nor can they be transitioned to through a px rule. They need to be attached to a program by either using a named profile transition (see Section 19.8.7, “Named Profile Transitions”) or with the change_profile rule (see Section 19.2.5, “Change rules”).

Unattached profiles are useful for specialized profiles for system utilities that generally should not be confined by a system wide profile (for example, /bin/bash). They can also be used to set up roles or to confine a user.

19.2.3. Local Profiles

Local profiles provide a convenient way to provide specialized confinement for utility programs launched by a confined application. They are specified just like standard profiles except they are embedded in a parent profile and begin with the profile keyword:

/parent/profile {
   profile local/profile {

To transition to a local profile, either use a cx rule (see Section 19.8.2, “Discrete Local Profile Execute Mode (cx)”) or a named profile transition (see Section 19.8.7, “Named Profile Transitions”).

19.2.4. Hats

AppArmor "hats" are a local profiles with some additional restrictions and an implicit rule allowing for change_hat to be used to transition to them. Refer to Chapter 23, Profiling Your Web Applications Using ChangeHat for a detailed description.

19.2.5. Change rules

AppArmor provides change_hat and change_profile rules that control domain transitioning. change_hat are specified by defining hats in a profile, while change_profile rules refer to another profile and start with the keyword change_profile:

change_profile /usr/bin/foobar,

Both change_hat and change_profile provide for an application directed profile transition, without having to launch a separate application. change_profile provides a generic one way transition between any of the loaded profiles. change_hat provides for a returnable parent child transition where an application can switch from the parent profile to the hat profile and if it provides the correct secret key return to the parent profile at a later time.

change_profile is best used in situations where an application goes through a trusted setup phase and then can lower its privilege level. Any resources mapped or opened during the start-up phase may still be accessible after the profile change, but the new profile will restrict the opening of new resources, and will even limit some of the resources opened before the switch. Specifically, memory resources will still be available while capability and file resources (as long as they are not memory mapped) can be limited.

change_hat is best used in situations where an application runs a virtual machine or an interpreter that does not provide direct access to the applications resources (e.g. Apache's mod_php). Since change_hat stores the return secret key in the application's memory the phase of reduced privilege should not have direct access to memory. It is also important that file access is properly separated, since the hat can restrict accesses to a file handle but does not close it. If an application does buffering and provides access to the open files with buffering, the accesses to these files may not be seen by the kernel and hence not restricted by the new profile.

[Warning]Safety of Domain Transitions

The change_hat and change_profile domain transitions are less secure than a domain transition done through an exec because they do not affect a processes memory mappings, nor do they close resources that have already been opened.

19.3. #include Statements

#include statements are directives that pull in components of other AppArmor profiles to simplify profiles. Include files retrieve access permissions for programs. By using an include, you can give the program access to directory paths or files that are also required by other programs. Using includes can reduce the size of a profile.

By default, AppArmor adds /etc/apparmor.d to the path in the #include statement. AppArmor expects the include files to be located in /etc/apparmor.d. Unlike other profile statements (but similar to C programs), #include lines do not end with a comma.

To assist you in profiling your applications, AppArmor provides three classes of #includes: abstractions, program chunks and tunables.

19.3.1. Abstractions

Abstractions are #includes that are grouped by common application tasks. These tasks include access to authentication mechanisms, access to name service routines, common graphics requirements, and system accounting. Files listed in these abstractions are specific to the named task. Programs that require one of these files usually require some of the other files listed in the abstraction file (depending on the local configuration as well as the specific requirements of the program). Find abstractions in /etc/apparmor.d/abstractions.

19.3.2. Program Chunks

The program-chunks directory (/etc/apparmor.d/program-chunks) contains some chunks of profiles that are specific to program suites and not generally useful outside of the suite, thus are never suggested for use in profiles by the profile wizards (aa-logprof and aa-genprof). Currently, program chunks are only available for the postfix program suite.

19.3.3. Tunables

The tunables directory (/etc/apparmor.d/tunables) contains global variable definitions. When used in a profile, these variables expand to a value that can be changed without changing the entire profile. Add all the tunables definitions that should be available to every profile to /etc/apparmor.d/tunables/global.

19.4. Capability Entries (POSIX.1e)

Capability statements are simply the word capability followed by the name of the POSIX.1e capability as defined in the capabilities(7) man page.

19.5. Network Access Control

AppArmor allows mediation of network access based on the address type and family. The following illustrates the network access rule syntax:

network [[<domain>1][<type2>][<protocol3>]]


Supported domains: inet, ax25, ipx, appletalk, netrom, bridge, x25, inet6, rose, netbeui, security, key, packet, ash, econet, atmsvc, sna, irda, pppox, wanpipe, bluetooth


Supported types: stream, dgram, seqpacket, rdm, raw, packet


Supported protocols: tcp, udp, icmp

The AppArmor tools support only family and type specification. The AppArmor module emits only network domain type in access denied messages. And only these are output by the profile generation tools, both YaST and command line.

The following examples illustrate possible network-related rules to be used in AppArmor profiles. Note that the syntax of the last two are not currently supported by the AppArmor tools.

network inet2,
network inet63,
network inet stream4,
network inet tcp5,
network tcp6, 


Allow all networking. No restrictions applied with regards to domain, type, or protocol.


Allow general use of IPv4 networking.


Allow general use of IPv6 networking.


Allow the use of IPv4 TCP networking.


Allow the use of IPv4 TCP networking, paraphrasing the rule above.


Allow the use of both IPv4 and IPv6 TCP networking.

19.6. Paths and Globbing

AppArmor explicitly distinguishes directory path names from file path names. Use a trailing / for any directory path that needs to be explicitly distinguished:

/some/random/example/* r

Allow read access to files in the /some/random/example directory.

/some/random/example/ r

Allow read access to the directory only.

/some/**/ r

Give read access to any directories below /some.

/some/random/example/** r

Give read access to files and directories under /some/random/example.

/some/random/example/**[^/] r

Give read access to files under /some/random/example. Explicitly exclude directories ([^/]).

Globbing (or regular expression matching) is when you modify the directory path using wild cards to include a group of files or subdirectories. File resources can be specified with a globbing syntax similar to that used by popular shells, such as csh, Bash, and zsh.


Substitutes for any number of any characters, except /.

Example: An arbitrary number of file path elements.


Substitutes for any number of characters, including /.

Example: An arbitrary number of path elements, including entire directories.


Substitutes for any single character, except /.


Substitutes for the single character a, b, or c.

Example: a rule that matches /home[01]/*/.plan allows a program to access .plan files for users in both /home0 and /home1.


Substitutes for the single character a, b, or c.


Expands to one rule to match ab and one rule to match cd.

Example: a rule that matches /{usr,www}/pages/** grants access to Web pages in both /usr/pages and /www/pages.

[ ^a ]

Substitutes for any character except a.

19.6.1. Using Variables in Profiles

AppArmor allows to use variables holding paths in profiles. Use global variables to make your profiles portable and local variables to create shortcuts for paths.

A typical example of when global variables come in handy are network scenarios in which user home directories are mounted in different locations. Instead of rewriting paths to home directories in all affected profiles, you only need to change the value of a variable. Global variables are defined under /etc/apparmor.d/tunables and have to be made available via an #include statement. Find the variable definitions for this use case (@{HOME} and @{HOMEDIRS}) in the /etc/apparmor.d/tunables/home file.

Local variables are defined at the head of a profile. This is useful to provide the base of for a chrooted path, for example:

/sbin/syslog-ng {
# chrooted applications
@{CHROOT_BASE}/var/lib/*/dev/log w,
@{CHROOT_BASE}/var/log/** w,

With the current AppArmor tools, variables can only be used when manually editing and maintaining a profile.

19.6.2. Alias rules

Alias rules provide an alternative way to manipulate profile path mappings to site specific layouts. They are an alternative form of path rewriting to using variables, and are done post variable resolution:

alias /home/ -> /mnt/users/

With the current AppArmor tools, alias rules can only be used when manually editing and maintaining a profile. Whats more, they are deactivated by disabled. Enable alias rules by editing /etc/apparmor.d/tunables/alias

19.7. File Permission Access Modes

File permission access modes consist of combinations of the following modes:


Read mode


Write mode (mutually exclusive to a)


Append mode (mutually exclusive to w)


File locking mode


Link mode

link file -> target

Link pair rule (cannot be combined with other access modes)

19.7.1. Read Mode (r)

Allows the program to have read access to the resource. Read access is required for shell scripts and other interpreted content and determines if an executing process can core dump.

19.7.2. Write Mode (w)

Allows the program to have write access to the resource. Files must have this permission if they are to be unlinked (removed).

19.7.3. Append Mode (a)

Allows a program to write to the end of a file. In contrast to the w mode, the append mode does not include the ability to overwrite data, to rename, or to remove a file. The append permission is typically used with applications who need to be able to write to log files, but which should not be able to manipulate any existing data in the log files. As the append permission is just a subset of the permissions associated with the write mode, the w and a permission flags cannot be used together and are mutually exclusive.

19.7.4. File Locking Mode (k)

The application can take file locks. Former versions of AppArmor allowed files to be locked if an application had access to them. By using a separate file locking mode, AppArmor makes sure locking is restricted only to those files which need file locking and tightens security as locking can be used in several denial of service attack scenarios.

19.7.5. Link Mode (l)

The link mode mediates access to hard links. When a link is created, the target file must have the same access permissions as the link created (with the exception that the destination does not need link access).

19.7.6. Link Pair

The link mode grants permission to create links to arbitrary files, provided the link has a subset of the permissions granted by the target (subset permission test). By specifying origin and destination, the link pair rule provides greater control over how hard links are created. Link pair rules by default do not enforce the link subset permission test that the standard rules link permission requires. To force the rule to require the test the subset keyword is used. The following rules are equivalent:

/link    l,
link subset /link -> /**,

Currently link pair rules are not supported by YaST and the command line tools. Manually edit your profiles to use them. Updating such profiles using the tools is safe, because the link pair entries will not be touched.

19.7.7. Owner Conditional Rules

The file rules can be extended so that they can be conditional upon the the user being the owner of the file (the fsuid has to match the file's uid). For this purpose the owner keyword is prepended to the rule. Owner conditional rules accumulate just as regular file rules.

owner /home/*/** rw

When using file ownership conditions with link rules the ownership test is done against the target file so the user must own the file to be able to link to it.

[Note]Precedence of Regular File Rules

Owner conditional rules are considered a subset of regular file rules. If a regular file rule overlaps with an owner conditional file rule, the resultant permissions will be that of the regular file rule.

19.7.8. Deny Rules

Deny rules can be used to annotate or quiet known rejects. The profile generating tools will not ask about a known reject treated with a deny rule. Such a reject will also not show up in the audit logs when denied, keeping the log files lean. If this is not desired, prepend the deny entry with the keyword audit.

It is also possible to use deny rules in combination with allow rules. This allows you to specify a broad allow rule, and then subtract a few known files that should not be allowed. Deny rules can also be combined with owner rules, to deny files owned by the user. The following example allows read/write access to everything in a users directory except write access to the .ssh/ files:

deny /home/*/.ssh/** w,
/home/*/** rw,

The extensive use of deny rules is generally not encouraged, because it makes it much harder to understand what a profile does. However a judicious use of deny rules can simplify profiles. Therefore the tools only generate profiles denying specific files and will not make use of globbing in deny rules. Manually edit your profiles to add deny rules using globbing. Updating such profiles using the tools is safe, because the deny entries will not be touched.

19.8. Execute Modes

Execute modes, also named profile transitions, consist of the following modes:


Discrete profile execute mode


Discrete local profile execute mode


Unconstrained execute mode


Inherit execute mode


Allow PROT_EXEC with mmap(2) calls

19.8.1. Discrete Profile Execute Mode (px)

This mode requires that a discrete security profile is defined for a resource executed at an AppArmor domain transition. If there is no profile defined, the access is denied.

[Warning]Using the Discrete Profile Execute Mode

px does not scrub the environment of variables such as LD_PRELOAD. As a result, the calling domain may have an undue amount of influence over the called item.

Incompatible with Ux, ux, Px, and ix.

19.8.2. Discrete Local Profile Execute Mode (cx)

As px, but instead of searching the global profile set, cx only searches the local profiles of the current profile. This profile transition provides a way for an application to have alternate profiles for helper applications.

[Note]Limitations of the Discrete Local Profile Execute Mode (cx)

Currently, cx transitions are limited to top level profiles and can not be used in hats and children profiles. This restriction will be removed in the future.

Incompatible with Ux, ux, Px, px, Cx, and ix.

19.8.3. Unconstrained Execute Mode (ux)

Allows the program to execute the resource without any AppArmor profile applied to the executed resource. This mode is useful when a confined program needs to be able to perform a privileged operation, such as rebooting the machine. By placing the privileged section in another executable and granting unconstrained execution rights, it is possible to bypass the mandatory constraints imposed on all confined processes. For more information about what is constrained, see the apparmor(7) man page.

[Warning]Using Unconstrained Execute Mode (ux)

Use ux only in very special cases. It enables the designated child processes to be run without any AppArmor protection. ux does not scrub the environment of variables such as LD_PRELOAD. As a result, the calling domain may have an undue amount of influence over the called resource. Use this mode only if the child absolutely must be run unconfined and LD_PRELOAD must be used. Any profile using this mode provides negligible security. Use at your own risk.

This mode is incompatible with Ux, px, Px, and ix.

19.8.4. Clean Exec modes

The clean exec modes allow the named program to run in px, cx and ux mode, but AppArmor invokes the Linux kernel's unsafe_exec routines to scrub the environment, similar to setuid programs. The clean exec modes are specified with an uppercase letter: Px, Cx and Ux. See the man page of for some information about setuid and setgid environment scrubbing.

19.8.5. Inherit Execute Mode (ix)

ix prevents the normal AppArmor domain transition on execve(2) when the profiled program executes the named program. Instead, the executed resource inherits the current profile.

This mode is useful when a confined program needs to call another confined program without gaining the permissions of the target's profile or losing the permissions of the current profile. There is no version to scrub the environment because ix executions do not change privileges.

Incompatible with cx, ux, and px. Implies m.

19.8.6. Allow Executable Mapping (m)

This mode allows a file to be mapped into memory using mmap(2)'s PROT_EXEC flag. This flag marks the pages executable. It is used on some architectures to provide non executable data pages, which can complicate exploit attempts. AppArmor uses this mode to limit which files a well-behaved program (or all programs on architectures that enforce non executable memory access controls) may use as libraries, to limit the effect of invalid -L flags given to ld(1) and LD_PRELOAD, LD_LIBRARY_PATH, given to

19.8.7. Named Profile Transitions

By default, the px and cx (and their clean exec variants, too) transition to a profile who's name matches the executable name. With named profile transitions, you can specify a profile to be transitioned to. This is useful if multiple binaries need to share a single profile, or if they need to use a different profile than their name would specify. Named profile transitions can be used in conjunction with cx, Cx, px and Px. Currently there is a limit of twelve named profile transitions per profile.

Named profile transitions use -> to indicate the name of the profile that needs to be transitioned to:

  /bin/** px -> shared_profile,
  /usr/*bash cx -> local_profile,
  profile local_profile 
[Note]Difference Between Normal and Named Transitions

When used with globbing, normal transitions provide a one to many relationship—/bin/** px will transition to /bin/ping, /bin/cat, etc, depending on the program being run.

Named transitions provide a many to one relationship—all programs that match the rule regardless of their name will transition to the specified profile.

Named profile transitions show up in the log as having the mode Nx. The name of the profile to be changed to is listed in the name2 field.

19.8.8. Inheritance Fallback for Profile Transitions

The px and cx transitions specify a hard dependency (if the specified profile does not exist, the exec will fail). With the inheritance fallback, the execution will succeed but inherit the current profile. To specify inheritance fallback, ix is combined with cx, Cx, px and Px into the modes cix, Cix, pix and Pix. The fallback modes can be used with named profile transitions, too.

19.8.9. Variable Settings in Execution Modes

When choosing one of the Px, Cx or Ux execution modes, take into account that the following environment variables are removed from the environment before the child process inherits it. As a consequence, applications or processes relying on any of these variables do not work anymore if the profile applied to them carries Px, Cx or Ux flags:






















19.9. Resource Limit Control

AppArmor provides the ability to set and control an application's resource limits (rlimits, also known as ulimits). By default AppArmor does not control applications rlimits, and it will only control those limits specified in the confining profile. For more information about resource limits, refer to the setrlimit(2), ulimit(1), or ulimit(3) man pages.

AppArmor leverages the system's rlimits and as such does not provide an additional auditing that would normally occur. It also cannot raise rlimits set by the system, AppArmor rlimits can only reduce an application's current resource limits.

The values will be inherited by the children of a process and will remain even if a new profile is transitioned to or the application becomes unconfined. So when an application transitions to a new profile, that profile has the ability to further reduce the applications rlimits.

AppArmor's rlimit rules will also provide mediation of setting an application's hard limits, should it try to raise them. The application will not be able to raise its hard limits any further than specified in the profile. The mediation of raising hard limits is not inherited as the set value is, so that once the application transitions to a new profile it is free to raise its limits as specified in the profile.

AppArmor's rlimit control does not affect an application's soft limits beyond ensuring that they are less than or equal to the application's hard limits.

AppArmor's hard limit rules have the general form of:

set rlimit resource <= value,

where resource and value are to be replaced with the following values:


currently not supported

fsize, data, stack, core, rss, as, memlock, msgqueue

a number in bytes, or a number with a suffix where the suffix can be K (kilobytes), M (megabytes), G (gigabytes), for example

rlimit data <= 100M,
fsize, nofile, locks, sigpending, nproc*, rtprio

a number greater or equal to 0


a value between -20 and 19

*The nproc rlimit is handled different than all the other rlimits. Instead of indicating the standard process rlimit it controls the maximum number of processes that can be running under the profile at any given time. Once the limit is exceeded the creation of new processes under the profile will fail until the number of currently running processes is reduced.


Currently the tools can not be used to add rlimit rules to profiles. The only way to add rlimit controls to a profile is to manually edit the profile with a text editor. The tools will still work with profiles containing rlimit rules and will not remove them, so it is safe to use the tools to update profiles containing them.

19.10. Auditing Rules

AppArmor provides the ability to audit given rules so that when they are matched an audit message will appear in the audit log. To enable audit messages for a given rule, the audit keyword is prepended to the rule:

audit /etc/foo/*        rw,

If it is desirable to audit only a given permission the rule can be split into two rules. The following example will result in audit messages when files are opened for writing, but not when they are opened for just reading:

audit /etc/foo/*  w,
/etc/foo/*        r,

Audit messages are not generated for every read or write of a file but only when a file is opened for read or write.

Audit control can be combined with owner conditional file rules to provide auditing when users access files they own (at the moment it is not possible to audit files they don't own):

audit owner /home/*/.ssh/**       rw,

Chapter 20. AppArmor Profile Repositories

AppArmor ships a set of profiles enabled by default and created by the AppArmor developers, and kept under the /etc/apparmor.d. In addition to these profiles, openSUSE ships profiles for individual applications together with the relevant application. These profiles are not enabled by default, and reside under another directory than the standard AppArmor profiles, /etc/apparmor/profiles/extras.

20.1. Using the Local Repository

The AppArmor tools (YaST and aa-genprof and aa-logprof) support the use of a local repository. Whenever you start to create a new profile from scratch, and there already is one inactive profile in your local repository, you are asked whether you would like to use the existing inactive one from /etc/apparmor/profiles/extras and whether you want to base your efforts on it. If you decide to use this profile, it gets copied over to the directory of profiles enabled by default (/etc/apparmor.d) and loaded whenever AppArmor is started. Any further further adjustments will be done to the active profile under /etc/apparmor.d.

Chapter 21. Building and Managing Profiles with YaST

YaST provides an easy way to build profiles and manage AppArmor®. It provides two interfaces: a graphical one and a text-based one. The text-based interface consumes less resources and bandwidth, making it a better choice for remote administration, or for times when a local graphical environment is inconvenient. Although the interfaces have differing appearances, they offer the same functionality in similar ways. Another alternative is to use AppArmor commands, which can control AppArmor from a terminal window or through remote connections. The command line tools are described in Chapter 22, Building Profiles from the Command Line.

Start YaST from the main menu and enter your root password when prompted for it. Alternatively, start YaST by opening a terminal window, logging in as root, and entering yast2 for the graphical mode or yast for the text-based mode.

Figure 21.1. YaST Controls for AppArmor

YaST's main controls for

The right frame shows the AppArmor options:

Add Profile Wizard

For detailed steps, refer to Section 21.1, “Adding a Profile Using the Wizard”.

Manually Add Profile

Add a AppArmor profile for an application on your system without the help of the wizard. For detailed steps, refer to Section 21.2, “Manually Adding a Profile”.

Edit Profile

Edits an existing AppArmor profile on your system. For detailed steps, refer to Section 21.3, “Editing Profiles”.

Delete Profile

Deletes an existing AppArmor profile from your system. For detailed steps, refer to Section 21.4, “Deleting a Profile”.

Update Profile Wizard

For detailed steps, refer to Section 21.5, “Updating Profiles from Log Entries”.

AppArmor Control Panel

For detailed steps, refer to Section 21.6, “Managing AppArmor”.

21.1. Adding a Profile Using the Wizard

Add Profile Wizard is designed to set up AppArmor profiles using the AppArmor profiling tools, aa-genprof (generate profile) and aa-logprof (update profiles from learning mode log file). For more information about these tools, refer to Section 22.6.3, “Summary of Profiling Tools”.

  1. Stop the application before profiling it to ensure that application start-up is included in the profile. To do this, make sure that the application or daemon is not running.

    For example, enter rcPROGRAM stop (or /etc/init.d/PROGRAM stop) in a terminal window while logged in as root, replacing PROGRAM with the name of the program to profile.

  2. Start YaST and select AppArmor+Add Profile Wizard.

    Choose the application to
  3. Enter the name of the application or browse to the location of the program.

  4. Click Create. This runs an AppArmor tool named aa-autodep, which performs a static analysis of the program to profile and loads an approximate profile into the AppArmor module. For more information about aa-autodep, refer to Section, “aa-autodep—Creating Approximate Profiles”.

    Depending on whether the profile you are about to create already exists either in the local profile repository (see Section 20.1, “Using the Local Repository”) or in the external profile repository (see Chapter 20, AppArmor Profile Repositories) or whether it does not exist yet, proceed with one of the following options:

    • Determine whether you want to use or fine-tune an already existing profile from your local profile repository, as outlined in Step 5.

    • Determine whether you want to use or fine-tune an already existing profile from the external profile repository, as outlined in Step 6.

    • Create the profile from scratch and proceed with Step 7 and beyond.

  5. If the profile already exists in the local profile repository under /etc/apparmor/profiles/extra, YaST informs you that there is an inactive profile which you can either use as a base for your own efforts or which you can just accept as is.

    Alternatively, you can choose not to use the local version at all and start creating the profile from scratch. In any case, proceed with Step 7.

  6. If the profile already exists in the external profile repository and this is the first time you tried to create a profile that already exists in the repository, configure your access to the server and determine how to use it:

    1. Determine whether you want to enable access to the external repository or postpone this decision. In case you have selected Enable Repository, determine the access mode (download/upload) in a next step. In case you want to postpone the decision, select Ask Me Later and proceed directly to Step 7.

    2. Provide username and password for your account on the profile repository server and register at the server.

    3. Select the profile to use and proceed to Step 7.

  7. Run the application to profile.

  8. Perform as many of the application functions as possible, so that learning mode can log the files and directories to which the program requires access to function properly. Be sure to include restarting and stopping the program in the exercised functions. AppArmor needs to handle these events, as well as any other program function.

  9. Click Scan system log for AppArmor events to parse the learning mode log files. This generates a series of questions that you must answer to guide the wizard in generating the security profile.

    If requests to add hats appear, proceed to Chapter 23, Profiling Your Web Applications Using ChangeHat.

    The questions fall into two categories:

    Each of these cases results in a series of questions that you must answer to add the resource to the profile or to add the program to the profile. For an example of each case, see Figure 21.2, “Learning Mode Exception: Controlling Access to Specific Resources” and Figure 21.3, “Learning Mode Exception: Defining Execute Permissions for an Entry”. Subsequent steps describe your options in answering these questions.

    [Note]Varying Processing Options

    Depending on the type of entry processed, the available options vary.

    Figure 21.2. Learning Mode Exception: Controlling Access to Specific Resources

    Learning Mode Exception: Controlling Access to Specific Resources

    Figure 21.3. Learning Mode Exception: Defining Execute Permissions for an Entry

    Learning Mode Exception: Defining Execute Permissions for an Entry

  10. The Add Profile Wizard begins suggesting directory path entries that have been accessed by the application profiled (as seen in Figure 21.2, “Learning Mode Exception: Controlling Access to Specific Resources”) or requires you to define execute permissions for entries (as seen in Figure 21.3, “Learning Mode Exception: Defining Execute Permissions for an Entry”).

    • For Figure 21.2: Learning Mode Exception: Controlling Access to Specific Resources: Select the option that satisfies the request for access, which could be a suggested include, a particular globbed version of the path, or the actual pathname. Depending on the situation, these options are available:


      The section of a AppArmor profile that refers to an include file. Include files give access permissions for programs. By using an include, you can give the program access to directory paths or files that are also required by other programs. Using includes can reduce the size of a profile. It is good practice to select includes when suggested.

      Globbed Version

      Accessed by clicking Glob. For information about globbing syntax, refer to Section 19.6, “Paths and Globbing”.

      Actual Pathname

      Literal path that the program needs to access to run properly.

      After selecting a directory path, process it as an entry to the AppArmor profile by clicking Allow or Deny. If you are not satisfied with the directory path entry as it is displayed, you can also Glob or Edit it.

      The following options are available to process the learning mode entries and build the profile:


      Grant the program access to the specified directory path entries. The Add Profile Wizard suggests file permission access. For more information about this, refer to Section 19.7, “File Permission Access Modes”.


      Click Deny to prevent the program from accessing the specified paths.


      Clicking this modifies the directory path (using wild cards) to include all files in the suggested directory. Double-clicking it grants access to all files and subdirectories beneath the one shown. For more information about globbing syntax, refer to Section 19.6, “Paths and Globbing”.

      Glob w/Ext

      Modify the original directory path while retaining the filename extension. A single click causes /etc/apache2/file.ext to become /etc/apache2/*.ext, adding the wild card (asterisk) in place of the filename. This allows the program to access all files in the suggested directories that end with the .ext extension. When you double-click it, access is granted to all files with the particular extension and subdirectories beneath the one shown.


      Edit the highlighted line. The new edited line appears at the bottom of the list.


      Abort aa-logprof, losing all rule changes entered so far and leaving all profiles unmodified.


      Close aa-logprof, saving all rule changes entered so far and modifying all profiles.

      Click Allow or Deny for each learning mode entry. These help build the AppArmor profile.


      The number of learning mode entries corresponds to the complexity of the application.

    • For Figure 21.3: Learning Mode Exception: Defining Execute Permissions for an Entry: From the following options, select the one that satisfies the request for access. For detailed information about the options available, refer to Section 19.7, “File Permission Access Modes”.


      Stay in the same security profile (parent's profile).


      Require a separate profile to exist for the executed program. When selecting this option, also select whether AppArmor should sanitize the environment when switching profiles by removing certain environment variables that can modify the execution behavior of the child process. Unless these variables are absolutely required to properly execute the child process, always choose the more secure, sanitized option.


      Execute the program without a security profile. When prompted, have AppArmor sanitize the environment to avoid adding security risks by inheriting certain environmental variables from the parent process.

      [Warning]Risks of Running Unconfined

      Unless absolutely necessary, do not run unconfined. Choosing the Unconfined option executes the new program without any protection from AppArmor.


      Click Deny to prevent the program from accessing the specified paths.


      Abort aa-logprof, losing all rule changes entered so far, and leaving all profiles unmodified.


      Close aa-logprof, saving all rule changes entered so far, and modifying all profiles.

  11. Repeat the previous steps if you need to execute more functionality of the application.

    When you are done, click Finish. Choose to apply your changes to the local profile set. If you have previously chosen to upload your profile to the external profile repository, provide a brief change log entry describing your work and upload the profile. If you had postponed the decision on whether to upload the profile or not, YaST asks you again and you can create an account the upload the profile now or not upload it at all.

    As soon as you exit the Profile Creation Wizard, the profile is saved both locally and on the repository server, if you have chosen to upload it. The profile is then loaded into the AppArmor module.

21.2. Manually Adding a Profile

AppArmor enables you to create a AppArmor profile by manually adding entries into the profile. Select the application for which to create a profile then add entries.

  1. Start YaST and select AppArmor+Manually Add Profile.

  2. Browse your system to find the application for which to create a profile.

  3. When you find the application, select it and click Open. A basic, empty profile appears in the AppArmor Profile Dialog window.

  4. In AppArmor Profile Dialog, add, edit, or delete AppArmor profile entries by clicking the corresponding buttons and referring to Section 21.3.1, “Adding an Entry”, Section 21.3.2, “Editing an Entry”, or Section 21.3.3, “Deleting an Entry”.

  5. When finished, click Done.

21.3. Editing Profiles

AppArmor enables you to edit AppArmor profiles manually by adding, editing, or deleting entries. To edit a profile, proceed as follows:

  1. Start YaST and select AppArmor+Edit Profile.

    Choose the profile to edit
  2. From the list of profiled applications, select the profile to edit.

  3. Click Next. The AppArmor Profile Dialog window displays the profile.

    AppArmor profile dialog
  4. In the AppArmor Profile Dialog window, add, edit, or delete AppArmor profile entries by clicking the corresponding buttons and referring to Section 21.3.1, “Adding an Entry”, Section 21.3.2, “Editing an Entry”, or Section 21.3.3, “Deleting an Entry”.

  5. When you are finished, click Done.

  6. In the pop-up that appears, click Yes to confirm your changes to the profile and reload the AppArmor profile set.

[Tip]Syntax Checking in AppArmor

AppArmor contains a syntax check that notifies you of any syntax errors in profiles you are trying to process with the YaST AppArmor tools. If an error occurs, edit the profile manually as root and reload the profile set with rcapparmor reload.

21.3.1. Adding an Entry

The Add Entry option can be found in Section 21.2, “Manually Adding a Profile” or Section 21.3, “Editing Profiles”. When you select Add Entry, a list shows the types of entries you can add to the AppArmor profile.

From the list, select one of the following:


In the pop-up window, specify the absolute path of a file, including the type of access permitted. When finished, click OK.

You can use globbing if necessary. For globbing information, refer to Section 19.6, “Paths and Globbing”. For file access permission information, refer to Section 19.7, “File Permission Access Modes”.

Select a file to add

In the pop-up window, specify the absolute path of a directory, including the type of access permitted. You can use globbing if necessary. When finished, click OK.

For globbing information, refer to Section 19.6, “Paths and Globbing”. For file access permission information, refer to Section 19.7, “File Permission Access Modes”.

Select a directory to
Network Rule

In the pop-up window, select the appropriate network family and the socket type. For more information, refer to Section 19.5, “Network Access Control”.

Select capabilities

In the pop-up window, select the appropriate capabilities. These are statements that enable each of the 32 POSIX.1e capabilities. Refer to Section 19.4, “Capability Entries (POSIX.1e)” for more information about capabilities. When finished making your selections, click OK.

Select capabilities

In the pop-up window, browse to the files to use as includes. Includes are directives that pull in components of other AppArmor profiles to simplify profiles. For more information, refer to Section 19.3, “#include Statements”.

Select includes

In the pop-up window, specify the name of the subprofile (hat) to add to your current profile and click Create Hat. For more information, refer to Chapter 23, Profiling Your Web Applications Using ChangeHat.

21.3.2. Editing an Entry

When you select Edit Entry, the file browser pop-up window opens. From here, edit the selected entry.

In the pop-up window, specify the absolute path of a file, including the type of access permitted. You can use globbing if necessary. When finished, click OK.

For globbing information, refer to Section 19.6, “Paths and Globbing”. For file access permission information, refer to Section 19.7, “File Permission Access Modes”.

21.3.3. Deleting an Entry

To delete an entry in a given profile, select Delete Entry. AppArmor removes the selected profile entry.

21.4. Deleting a Profile

AppArmor enables you to delete an AppArmor profile manually. Simply select the application for which to delete a profile then delete it as follows:

  1. Start YaST and select AppArmor+Delete Profile.

  2. Select the profile to delete.

  3. Click Next.

  4. In the pop-up that opens, click Yes to delete the profile and reload the AppArmor profile set.

21.5. Updating Profiles from Log Entries

The AppArmor profile wizard uses aa-logprof, the tool that scans log files and enables you to update profiles. aa-logprof tracks messages from the AppArmor module that represent exceptions for all profiles running on your system. These exceptions represent the behavior of the profiled application that is outside of the profile definition for the program. You can add the new behavior to the relevant profile by selecting the suggested profile entry.

  1. Start YaST and select AppArmor+Update Profile Wizard.

    Running Update Profile Wizard (aa-logprof) parses the learning mode log files. This generates a series of questions that you must answer to guide aa-logprof to generate the security profile. The exact procedure is the same as with creating a new profile. Refer to Step 9 in Section 21.1, “Adding a Profile Using the Wizard” for details.

  2. When you are done, click Finish. In the following pop-up, click Yes to exit the Add Profile Wizard. The profile is saved and loaded into the AppArmor module.

21.6. Managing AppArmor

You can change the status of AppArmor by enabling or disabling it. Enabling AppArmor protects your system from potential program exploitation. Disabling AppArmor, even if your profiles have been set up, removes protection from your system. To change the status of AppArmor, start YaST and select AppArmor+AppArmor Control Panel.

The AppArmor control

To change the status of AppArmor, continue as described in Section 21.6.1, “Changing AppArmor Status”. To change the mode of individual profiles, continue as described in Section 21.6.2, “Changing the Mode of Individual Profiles”.

21.6.1. Changing AppArmor Status

When you change the status of AppArmor, set it to enabled or disabled. When AppArmor is enabled, it is installed, running, and enforcing the AppArmor security policies.

  1. Start YaST and select AppArmor+AppArmor Control Panel.

  2. Enable AppArmor by checking Enable AppArmor or disable AppArmor by deselecting it.

  3. Click Done in the AppArmor Configuration window.

  4. Click File+Quit in the YaST Control Center.

21.6.2. Changing the Mode of Individual Profiles

AppArmor can apply profiles in two different modes. In complain or learning mode, violations of AppArmor profile rules, such as the profiled program accessing files not permitted by the profile, are detected. The violations are permitted, but also logged. This mode is convenient for developing profiles and is used by the AppArmor tools for generating profiles. Loading a profile in enforce mode enforces the policy defined in the profile and reports policy violation attempts to syslogd.

The Profile Modes dialog allows you to view and edit the mode of currently loaded AppArmor profiles. This feature is useful for determining the status of your system during profile development. During the course of systemic profiling (see Section 22.6.2, “Systemic Profiling”), you can use this tool to adjust and monitor the scope of the profiles for which you are learning behavior.

To edit an application's profile mode, proceed as follows:

  1. Start YaST and select AppArmor+AppArmor Control Panel.

  2. In the Configure Profile Modes section, select Configure.

  3. Select the profile for which to change the mode.

  4. Select Toggle Mode to set this profile to complain mode or to enforce mode.

  5. Apply your settings and leave YaST with Done.

To change the mode of all profiles, use Set All to Enforce or Set All to Complain.

[Tip]Listing the Profiles Available

By default, only active profiles are listed (any profile that has a matching application installed on your system). To set up a profile before installing the respective application, click Show All Profiles and select the profile to configure from the list that appears.

Chapter 22. Building Profiles from the Command Line

AppArmor® provides the user the ability to use a command line interface rather than a graphical interface to manage and configure the system security. Track the status of AppArmor and create, delete, or modify AppArmor profiles using the AppArmor command line tools.

[Tip]Background Information

Before starting to manage your profiles using the AppArmor command line tools, check out the general introduction to AppArmor given in Chapter 18, Immunizing Programs and Chapter 19, Profile Components and Syntax.

22.1. Checking the AppArmor Module Status

An AppArmor module can be in any one of three states:


The AppArmor module is not loaded into the kernel.


The AppArmor module is loaded into the kernel and is enforcing AppArmor program policies.


The AppArmor module is loaded into the kernel, but no policies are enforced.

Detect the state of the AppArmor module by inspecting /sys/kernel/security/apparmor/profiles. If cat /sys/kernel/security/apparmor/profiles reports a list of profiles, AppArmor is running. If it is empty and returns nothing, AppArmor is stopped. If the file does not exist, AppArmor is unloaded.

Manage AppArmor through the script rcapparmor, which can perform the following operations:

rcapparmor start

Behavior depends on the AppArmor module state. If it is unloaded, start loads the module and starts it, putting it in the running state. If it is stopped, start causes the module to rescan the AppArmor profiles usually found in /etc/apparmor.d and puts the module in the running state. If the module is already running, start reports a warning and takes no action.

rcapparmor stop

Stops the AppArmor module if it is running by removing all profiles from kernel memory, effectively disabling all access controls, and putting the module into the stopped state. If the AppArmor module is unloaded or already stopped, stop tries to unload the profiles again, but nothing happens.

rcapparmor restart

Causes the AppArmor module to rescan the profiles in /etc/apparmor.d without unconfining running processes. Freshly created profiles are enforced and recently deleted ones are removed from the /etc/apparmor.d directory.

rcapparmor kill

Unconditionally removes the AppArmor module from the kernel. However, unloading modules from the Linux kernel is unsafe. This command is provided only for debugging and emergencies (when the module might need to be removed).


AppArmor is a powerful access control system and it is possible to lock yourself out of your own machine to the point where you must boot the machine from a rescue medium (such as the first medium of openSUSE) to regain control.

To prevent such a problem, always ensure that you have a running, unconfined, root login on the machine being configured when you restart the AppArmor module. If you damage your system to the point where logins are no longer possible (for example, by breaking the profile associated with the SSH daemon), you can repair the damage using your running root prompt then restarting the AppArmor module.

22.2. Building AppArmor Profiles

The AppArmor module profile definitions are stored in the /etc/apparmor.d directory as plain text files. For a detailed description of the syntax of these files, refer to Chapter 19, Profile Components and Syntax.

All files in the /etc/apparmor.d directory are interpreted as profiles and are loaded as such. Renaming files in that directory is not an effective way of preventing profiles from being loaded. You must remove profiles from this directory to prevent them from being read and evaluated effectively.

You can use a text editor, such as vim, to access and make changes to these profiles. The following options contain detailed steps for building profiles:

Adding or Creating AppArmor Profiles

Refer to Section 22.3, “Adding or Creating an AppArmor Profile”

Editing AppArmor Profiles

Refer to Section 22.4, “Editing an AppArmor Profile”

Deleting AppArmor Profiles

Refer to Section 22.5, “Deleting an AppArmor Profile”

22.3. Adding or Creating an AppArmor Profile

To add or create an AppArmor profile for an application, you can use a systemic or stand-alone profiling method, depending on your needs. Learn more about these two approaches in Section 22.6, “Two Methods of Profiling”.

22.4. Editing an AppArmor Profile

The following steps describe the procedure for editing an AppArmor profile:

  1. If you are not currently logged in as root, enter su in a terminal window.

  2. Enter the root password when prompted.

  3. Go to the profile directory with cd /etc/apparmor.d/.

  4. Enter ls to view all profiles currently installed.

  5. Open the profile to edit in a text editor, such as vim.

  6. Make the necessary, changes then save the profile.

  7. Restart AppArmor by entering rcapparmor restart in a terminal window.

22.5. Deleting an AppArmor Profile

The following steps describe the procedure for deleting an AppArmor profile.

  1. If you are not currently logged in as root, enter su in a terminal window.

  2. Enter the root password when prompted.

  3. Go to the AppArmor directory with cd /etc/apparmor.d/.

  4. Enter ls to view all the AppArmor profiles that are currently installed.

  5. Delete the profile with rm profilename.

  6. Restart AppArmor by entering rcapparmor restart in a terminal window.

22.6. Two Methods of Profiling

Given the syntax for AppArmor profiles in Chapter 19, Profile Components and Syntax, you could create profiles without using the tools. However, the effort involved would be substantial. To avoid such a hassle, use the AppArmor tools to automate the creation and refinement of profiles.

There are two ways to approach AppArmor profile creation. Tools are available for both methods.

Stand-Alone Profiling

A method suitable for profiling small applications that have a finite run time, such as user client applications like mail clients. For more information, refer to Section 22.6.1, “Stand-Alone Profiling”.

Systemic Profiling

A method suitable for profiling large numbers of programs all at once and for profiling applications that may run for days, weeks, or continuously across reboots, such as network server applications like Web servers and mail servers. For more information, refer to Section 22.6.2, “Systemic Profiling”.

Automated profile development becomes more manageable with the AppArmor tools:

  1. Decide which profiling method suits your needs.

  2. Perform a static analysis. Run either aa-genprof or aa-autodep, depending on the profiling method chosen.

  3. Enable dynamic learning. Activate learning mode for all profiled programs.

22.6.1. Stand-Alone Profiling

Stand-alone profile generation and improvement is managed by a program called aa-genprof. This method is easy because aa-genprof takes care of everything, but is limited because it requires aa-genprof to run for the entire duration of the test run of your program (you cannot reboot the machine while you are still developing your profile).

To use aa-genprof for the stand-alone method of profiling, refer to Section, “aa-genprof—Generating Profiles”.

22.6.2. Systemic Profiling

This method is called systemic profiling because it updates all of the profiles on the system at once, rather than focusing on the one or few targeted by aa-genprof or stand-alone profiling. With systemic profiling, profile construction and improvement are somewhat less automated, but more flexible. This method is suitable for profiling long-running applications whose behavior continues after rebooting, or a large number of programs all at once.

Build an AppArmor profile for a group of applications as follows:

  1. Create profiles for the individual programs that make up your application.

    Although this approach is systemic, AppArmor only monitors those programs with profiles and their children. To get AppArmor to consider a program, you must at least have aa-autodep create an approximate profile for it. To create this approximate profile, refer to Section, “aa-autodep—Creating Approximate Profiles”.

  2. Put relevant profiles into learning or complain mode.

    Activate learning or complain mode for all profiled programs by entering aa-complain /etc/apparmor.d/* in a terminal window while logged in as root. This functionality is also available through the YaST Profile Mode module, described in Section 21.6.2, “Changing the Mode of Individual Profiles”.

    When in learning mode, access requests are not blocked, even if the profile dictates that they should be. This enables you to run through several tests (as shown in Step 3) and learn the access needs of the program so it runs properly. With this information, you can decide how secure to make the profile.

    Refer to Section, “aa-complain—Entering Complain or Learning Mode” for more detailed instructions for using learning or complain mode.

  3. Exercise your application.

    Run your application and exercise its functionality. How much to exercise the program is up to you, but you need the program to access each file representing its access needs. Because the execution is not being supervised by aa-genprof, this step can go on for days or weeks and can span complete system reboots.

  4. Analyze the log.

    In systemic profiling, run aa-logprof directly instead of letting aa-genprof run it (as in stand-alone profiling). The general form of aa-logprof is:

    aa-logprof [ -d /path/to/profiles ] [ -f /path/to/logfile ]

    Refer to Section, “aa-logprof—Scanning the System Log” for more information about using aa-logprof.

  5. Repeat Step 3 and Step 4.

    This generates optimum profiles. An iterative approach captures smaller data sets that can be trained and reloaded into the policy engine. Subsequent iterations generate fewer messages and run faster.

  6. Edit the profiles.

    You might want to review the profiles that have been generated. You can open and edit the profiles in /etc/apparmor.d/ using vim.

  7. Return to enforce mode.

    This is when the system goes back to enforcing the rules of the profiles, not just logging information. This can be done manually by removing the flags=(complain) text from the profiles or automatically by using the aa-enforce command, which works identically to the aa-complain command, except it sets the profiles to enforce mode. This functionality is also available through the YaST Profile Mode module, described in Section 21.6.2, “Changing the Mode of Individual Profiles”.

    To ensure that all profiles are taken out of complain mode and put into enforce mode, enter aa-enforce /etc/apparmor.d/*.

  8. Rescan all profiles.

    To have AppArmor rescan all of the profiles and change the enforcement mode in the kernel, enter rcapparmor restart.

22.6.3. Summary of Profiling Tools

All of the AppArmor profiling utilities are provided by the apparmor-utils RPM package and are stored in /usr/sbin. Each tool has a different purpose. aa-autodep—Creating Approximate Profiles

This creates an approximate profile for the program or application selected. You can generate approximate profiles for binary executables and interpreted script programs. The resulting profile is called approximate because it does not necessarily contain all of the profile entries that the program needs to be properly confined by AppArmor. The minimum aa-autodep approximate profile has, at minimum, a base include directive, which contains basic profile entries needed by most programs. For certain types of programs, aa-autodep generates a more expanded profile. The profile is generated by recursively calling ldd(1) on the executables listed on the command line.

To generate an approximate profile, use the aa-autodep program. The program argument can be either the simple name of the program, which aa-autodep finds by searching your shell's path variable, or it can be a fully qualified path. The program itself can be of any type (ELF binary, shell script, Perl script, etc.). aa-autodep generates an approximate profile to improve through the dynamic profiling that follows.

The resulting approximate profile is written to the /etc/apparmor.d directory using the AppArmor profile naming convention of naming the profile after the absolute path of the program, replacing the forward slash (/) characters in the path with period (.) characters. The general form of aa-autodep is to enter the following in a terminal window when logged in as root:

aa-autodep [ -d /path/to/profiles ] [program1 program2...]

If you do not enter the program name or names, you are prompted for them. /path/to/profiles overrides the default location of /etc/apparmor.d, should you keep profiles in a location other than the default.

To begin profiling, you must create profiles for each main executable service that is part of your application (anything that might start without being a child of another program that already has a profile). Finding all such programs depends on the application in question. Here are several strategies for finding such programs:


If all the programs to profile are in one directory and there are no other programs in that directory, the simple command aa-autodep /path/to/your/programs/* creates basic profiles for all programs in that directory.

ps command

You can run your application and use the standard Linux ps command to find all processes running. Then manually hunt down the location of these programs and run the aa-autodep for each one. If the programs are in your path, aa-autodep finds them for you. If they are not in your path, the standard Linux command find might be helpful in finding your programs. Execute find / -name 'my_application' -print to determine an application's path (my_application being an example application). You may use wild cards if appropriate. aa-complain—Entering Complain or Learning Mode

The complain or learning mode tool (aa-complain) detects violations of AppArmor profile rules, such as the profiled program accessing files not permitted by the profile. The violations are permitted, but also logged. To improve the profile, turn complain mode on, run the program through a suite of tests to generate log events that characterize the program's access needs, then postprocess the log with the AppArmor tools to transform log events into improved profiles.

Manually activating complain mode (using the command line) adds a flag to the top of the profile so that /bin/foo becomes /bin/foo flags=(complain). To use complain mode, open a terminal window and enter one of the following lines as root:

  • If the example program (program1) is in your path, use:

    aa-complain [program1 program2 ...]
  • If the program is not in your path, specify the entire path as follows:

    aa-complain /sbin/program1
  • If the profiles are not in /etc/apparmor.d, use the following to override the default location:

    aa-complain /path/to/profiles/ program1
  • Specify the profile for program1 as follows:

    aa-complain /etc/apparmor.d/sbin.program1

Each of the above commands activates the complain mode for the profiles or programs listed. If the program name does not include its entire path, aa-complain searches $PATH for the program. For instance, aa-complain /usr/sbin/* finds profiles associated with all of the programs in /usr/sbin and puts them into complain mode. aa-complain /etc/apparmor.d/* puts all of the profiles in /etc/apparmor.d into complain mode.

[Tip]Toggling Profile Mode with YaST

YaST offers a graphical front-end for toggling complain and enforce mode. See Section 21.6.2, “Changing the Mode of Individual Profiles” for information. aa-enforce—Entering Enforce Mode

The enforce mode detects violations of AppArmor profile rules, such as the profiled program accessing files not permitted by the profile. The violations are logged and not permitted. The default is for enforce mode to be enabled. To log the violations only, but still permit them, use complain mode. Enforce toggles with complain mode.

Manually activating enforce mode (using the command line) adds a flag to the top of the profile so that /bin/foo becomes /bin/foo flags=(enforce). To use enforce mode, open a terminal window and enter one of the following lines as root.

  • If the example program (program1) is in your path, use:

    aa-enforce [program1 program2 ...]
  • If the program is not in your path, specify the entire path, as follows:

    aa-enforce /sbin/program1
  • If the profiles are not in /etc/apparmor.d, use the following to override the default location:

    aa-enforce /path/to/profiles/program1
  • Specify the profile for program1 as follows:

    aa-enforce /etc/apparmor.d/sbin.program1

Each of the above commands activates the enforce mode for the profiles and programs listed.

If you do not enter the program or profile names, you are prompted to enter one. /path/to/profiles overrides the default location of /etc/apparmor.d.

The argument can be either a list of programs or a list of profiles. If the program name does not include its entire path, aa-enforce searches $PATH for the program.

[Tip]Toggling Profile Mode with YaST

YaST offers a graphical front-end for toggling complain and enforce mode. See Section 21.6.2, “Changing the Mode of Individual Profiles” for information. aa-genprof—Generating Profiles

aa-genprof is AppArmor's profile generating utility. It runs aa-autodep on the specified program, creating an approximate profile (if a profile does not already exist for it), sets it to complain mode, reloads it into AppArmor, marks the log, and prompts the user to execute the program and exercise its functionality. Its syntax is as follows:

aa-genprof [ -d /path/to/profiles ]  program

To create a profile for the the Apache Web server program httpd2-prefork, do the following as root:

  1. Enter rcapache2 stop.

  2. Next, enter aa-genprof httpd2-prefork.

    Now aa-genprof does the following:

    1. Resolves the full path of httpd2-prefork using your shell's path variables. You can also specify a full path. On openSUSE, the default full path is /usr/sbin/httpd2-prefork.

    2. Checks to see if there is an existing profile for httpd2-prefork. If there is one, it updates it. If not, it creates one using the aa-autodep as described in Section 22.6.3, “Summary of Profiling Tools”.

    3. Puts the profile for this program into learning or complain mode so that profile violations are logged, but are permitted to proceed. A log event looks like this (see /var/log/audit/audit.log):

      type=APPARMOR_ALLOWED msg=audit(1189682639.184:20816): operation="file_mmap" requested_mask="::r" denied_mask="::r" fsuid=30 name="/srv/www/htdocs/index.html" pid=27471 profile="null-complain-profile"

      If you are not running the audit daemon, the AppArmor events are logged to /var/log/messages:

      Sep 13 13:20:30 K23 kernel: audit(1189682430.672:20810): operation="file_mmap" requested_mask="::r" denied_mask="::r" fsuid=30 name="/srv/www/htdocs/phpsysinfo/templates/bulix/form.tpl" pid=30405 profile="/usr/sbin/httpd2-prefork///phpsysinfo/"

      They also can be viewed using the dmesg command:

      audit(1189682430.672:20810): operation="file_mmap" requested_mask="::r" denied_mask="::r" fsuid=30 name="/srv/www/htdocs/phpsysinfo/templates/bulix/form.tpl" pid=30405 profile="/usr/sbin/httpd2-prefork///phpsysinfo/"
    4. Marks the log with a beginning marker of log events to consider. For example:

      Sep 13 17:48:52 figwit root: GenProf: e2ff78636296f16d0b5301209a04430d
  3. When prompted by the tool, run the application to profile in another terminal window and perform as many of the application functions as possible. Thus, the learning mode can log the files and directories to which the program requires access in order to function properly. For example, in a new terminal window, enter rcapache2 start.

  4. Select from the following options that are available in the aa-logprof terminal window after you have executed the program function:

    • S runs aa-logprof on the system log from where it was marked when aa-genprof was started and reloads the profile. If system events exist in the log, AppArmor parses the learning mode log files. This generates a series of questions that you must answer to guide aa-genprof in generating the security profile.

    • F exits the tool and returns to the main menu.


    If requests to add hats appear, proceed to Chapter 23, Profiling Your Web Applications Using ChangeHat.

  5. Answer two types of questions:

    Each of these categories results in a series of questions that you must answer to add the resource or program to the profile. Example 22.1, “Learning Mode Exception: Controlling Access to Specific Resources” and Example 22.2, “Learning Mode Exception: Defining Execute Permissions for an Entry” provide examples of each one. Subsequent steps describe your options in answering these questions.

    • Dealing with execute accesses is complex. You must decide how to proceed with this entry regarding which execute permission type to grant to this entry:

      Example 22.1. Learning Mode Exception: Controlling Access to Specific Resources

      Reading log entries from /var/log/audit/audit.log.
      Updating AppArmor profiles in /etc/apparmor.d.
      Profile:  /usr/sbin/xinetd
      Program:  xinetd
      Execute:  /usr/lib/cups/daemon/cups-lpd
      Severity: unknown
      [(I)nherit] / (P)rofile / (U)nconfined / (D)eny / Abo(r)t / (F)inish

      Inherit (ix)

      The child inherits the parent's profile, running with the same access controls as the parent. This mode is useful when a confined program needs to call another confined program without gaining the permissions of the target's profile or losing the permissions of the current profile. This mode is often used when the child program is a helper application, such as the /usr/bin/mail client using less as a pager or the Mozilla* Web browser using Adobe Acrobat* to display PDF files.

      Profile (px)

      The child runs using its own profile, which must be loaded into the kernel. If the profile is not present, attempts to execute the child fail with permission denied. This is most useful if the parent program is invoking a global service, such as DNS lookups or sending mail with your system's MTA.

      Choose the profile with clean exec (Px) option to scrub the environment of environment variables that could modify execution behavior when passed to the child process.

      Unconfined (ux)

      The child runs completely unconfined without any AppArmor profile applied to the executed resource.

      Choose the unconfined with clean exec (Ux) option to scrub the environment of environment variables that could modify execution behavior when passed to the child process. This option introduces a security vulnerability that could be used to exploit AppArmor. Only use it as a last resort.

      mmap (m)

      This permission denotes that the program running under the profile can access the resource using the mmap system call with the flag PROT_EXEC. This means that the data mapped in it can be executed. You are prompted to include this permission if it is requested during a profiling run.


      Prevents the program from accessing the specified directory path entries. AppArmor then continues to the next event.


      Aborts aa-logprof, losing all rule changes entered so far and leaving all profiles unmodified.


      Closes aa-logprof, saving all rule changes entered so far and modifying all profiles.

    • Example 22.2, “Learning Mode Exception: Defining Execute Permissions for an Entry” shows AppArmor suggesting directory path entries that have been accessed by the application being profiled. It might also require you to define execute permissions for entries.

      Example 22.2. Learning Mode Exception: Defining Execute Permissions for an Entry

      Adding /bin/ps ix to profile.
      Profile:  /usr/sbin/xinetd
      Path:     /etc/hosts.allow
      New Mode: r
       [1 - /etc/hosts.allow]
      [(A)llow] / (D)eny / (N)ew / (G)lob / Glob w/(E)xt / Abo(r)t / (F)inish

      AppArmor provides one or more paths or includes. By entering the option number, select the desired options then proceed to the next step.


      All of these options are not always presented in the AppArmor menu.


      This is the section of an AppArmor profile that refers to an include file, which procures access permissions for programs. By using an include, you can give the program access to directory paths or files that are also required by other programs. Using includes can reduce the size of a profile. It is good practice to select includes when suggested.

      Globbed Version

      This is accessed by selecting Glob as described in the next step. For information about globbing syntax, refer to Section 19.6, “Paths and Globbing”.

      Actual Path

      This is the literal path to which the program needs access so that it can run properly.

      After you select the path or include, process it as an entry into the AppArmor profile by selecting Allow or Deny. If you are not satisfied with the directory path entry as it is displayed, you can also Glob it.

      The following options are available to process the learning mode entries and build the profile:

      Select Enter

      Allows access to the selected directory path.


      Allows access to the specified directory path entries. AppArmor suggests file permission access. For more information, refer to Section 19.7, “File Permission Access Modes”.


      Prevents the program from accessing the specified directory path entries. AppArmor then continues to the next event.


      Prompts you to enter your own rule for this event, allowing you to specify a regular expression. If the expression does not actually satisfy the event that prompted the question in the first place, AppArmor asks for confirmation and lets you reenter the expression.


      Select a specific path or create a general rule using wild cards that match a broader set of paths. To select any of the offered paths, enter the number that is printed in front of the path then decide how to proceed with the selected item.

      For more information about globbing syntax, refer to Section 19.6, “Paths and Globbing”.

      Glob w/Ext

      This modifies the original directory path while retaining the filename extension. For example, /etc/apache2/file.ext becomes /etc/apache2/*.ext, adding the wild card (asterisk) in place of the filename. This allows the program to access all files in the suggested directory that end with the .ext extension.


      Aborts aa-logprof, losing all rule changes entered so far and leaving all profiles unmodified.


      Closes aa-logprof, saving all rule changes entered so far and modifying all profiles.

  6. To view and edit your profile using vim, enter vim /etc/apparmor.d/profilename in a terminal window.

  7. Restart AppArmor and reload the profile set including the newly created one using the rcapparmor restart command.

Like the graphical front-end for building AppArmor profiles, the YaST Add Profile Wizard, aa-genprof also supports the use of the local profile repository under /etc/apparmor/profiles/extras and the remote AppArmor profile repository.

To use a profile from the local repository, proceed as follows:

  1. Start aa-genprof as described above.

    If aa-genprof finds an inactive local profile, the following lines appear on your terminal window:

    Profile: /usr/bin/opera
     [1 - Inactive local profile for /usr/bin/opera]
    [(V)iew Profile] / (U)se Profile / (C)reate New Profile / Abo(r)t / (F)inish
  2. If you want to just use this profile, hit U (Use Profile) and follow the profile generation procedure outlined above.

    If you want to examine the profile before activating it, hit V (View Profile).

    If you want to ignore the existing profile, hit C (Create New Profile) and follow the profile generation procedure outlined above to create the profile from scratch.

  3. Leave aa-genprof by hitting F (Finish) when you are done and save your changes.

To use the remote AppArmor profile repository with aa-genprof, proceed as follows:

  1. Start aa-genprof as described above.

    If aa-genprof detects a suitable profile on the repository server, the following lines appear on your terminal window:

    Would you like to enable access to the profile repository?
    (E)nable Repository / (D)isable Repository / Ask Me (L)ater
  2. Hit E (Enable Repository) to enable the repository.

  3. Determine whether you want to aa-genprof to upload any profiles to the repository server:

    Would you like to upload newly created and changed profiles to
          the profile repository?
    (Y)es / (N)o / Ask Me (L)ater

    Hit Y (Yes), if you want to enable profile upload or select N (No), if you want aa-genprof to just pull profiles from the repository, but not to upload any.

  4. Create a new user on the profile repository server to be able to upload profiles. Provide username and password.

  5. Determine whether you want to use the profile downloaded from the server or whether you would just like to review it:

    Profile: /usr/bin/opera
     [1 - novell]
    [(V)iew Profile] / (U)se Profile / (C)reate New Profile / Abo(r)t / (F)inish

    If you want to just use this profile, hit U (Use Profile) and follow the profile generation procedure outlined above.

    If you want to examine the profile before activating it, hit V (View Profile).

    If you want to ignore the existing profile, hit C (Create New Profile) and follow the profile generation procedure outlined above to create the profile from scratch.

  6. Leave aa-genprof by hitting F (Finish) when you are done and save the profile.

    If you opted for uploading your profile, provide a short change log and push it to the repository. aa-logprof—Scanning the System Log

aa-logprof is an interactive tool used to review the learning or complain-mode output found in the log entries in /var/log/audit/audit.log or /var/log/messages (if auditd is not running) and generate new entries in AppArmor security profiles.

When you run aa-logprof, it begins to scan the log files produced in learning or complain mode and, if there are new security events that are not covered by the existing profile set, it gives suggestions for modifying the profile. The learning or complain mode traces program behavior and enters it in the log. aa-logprof uses this information to observe program behavior.

If a confined program forks and executes another program, aa-logprof sees this and asks the user which execution mode should be used when launching the child process. The execution modes ix, px, Px, ux, and Ux are options for starting the child process. If a separate profile exists for the child process, the default selection is px. If one does not exist, the profile defaults to ix. Child processes with separate profiles have aa-autodep run on them and are loaded into AppArmor, if it is running.

When aa-logprof exits, profiles are updated with the changes. If the AppArmor module is running, the updated profiles are reloaded and, if any processes that generated security events are still running in the null-complain-profile, those processes are set to run under their proper profiles.

To run aa-logprof, enter aa-logprof into a terminal window while logged in as root. The following options can be used for aa-logprof:

aa-logprof -d /path/to/profile/directory/

Specifies the full path to the location of the profiles if the profiles are not located in the standard directory, /etc/apparmor.d/.

aa-logprof -f /path/to/logfile/

Specifies the full path to the location of the log file if the log file is not located in the default directory, /var/log/audit/audit.log or /var/log/messages (if auditd is not running).

aa-logprof -m "string marker in logfile"

Marks the starting point for aa-logprof to look in the system log. aa-logprof ignores all events in the system log before the specified mark. If the mark contains spaces, it must be surrounded by quotes to work correctly. For example:

aa-logprof -m"17:04:21"


logprof -m e2ff78636296f16d0b5301209a04430d

aa-logprof scans the log, asking you how to handle each logged event. Each question presents a numbered list of AppArmor rules that can be added by pressing the number of the item on the list.

By default, aa-logprof looks for profiles in /etc/apparmor.d/ and scans the log in /var/log/messages. In many cases, running aa-logprof as root is enough to create the profile.

However, there might be times when you need to search archived log files, such as if the program exercise period exceeds the log rotation window (when the log file is archived and a new log file is started). If this is the case, you can enter zcat -f `ls -1tr /var/log/messages*` | aa-logprof -f -. aa-logprof Example 1

The following is an example of how aa-logprof addresses httpd2-prefork accessing the file /etc/group. [] indicates the default option.

In this example, the access to /etc/group is part of httpd2-prefork accessing name services. The appropriate response is 1, which includes a predefined set of AppArmor rules. Selecting 1 to #include the name service package resolves all of the future questions pertaining to DNS lookups and also makes the profile less brittle in that any changes to DNS configuration and the associated name service profile package can be made just once, rather than needing to revise many profiles.

Profile:  /usr/sbin/httpd2-prefork
Path:     /etc/group
New Mode: r

[1 - #include <abstractions/nameservice>]
 2 - /etc/group
[(A)llow] / (D)eny / (N)ew / (G)lob / Glob w/(E)xt / Abo(r)t / (F)inish

Select one of the following responses:

Select Enter

Triggers the default action, which is, in this example, allowing access to the specified directory path entry.


Allows access to the specified directory path entries. AppArmor suggests file permission access. For more information about this, refer to Section 19.7, “File Permission Access Modes”.


Prevents the program from accessing the specified directory path entries. AppArmor then continues to the next event.


Prompts you to enter your own rule for this event, allowing you to specify whatever form of regular expression you want. If the expression entered does not actually satisfy the event that prompted the question in the first place, AppArmor asks for confirmation and lets you reenter the expression.


Select either a specific path or create a general rule using wild cards that matches on a broader set of paths. To select any of the offered paths, enter the number that is printed in front of the paths then decide how to proceed with the selected item.

For more information about globbing syntax, refer to Section 19.6, “Paths and Globbing”.

Glob w/Ext

This modifies the original directory path while retaining the filename extension. For example, /etc/apache2/file.ext becomes /etc/apache2/*.ext, adding the wild card (asterisk) in place of the filename. This allows the program to access all files in the suggested directory that end with the .ext extension.


Aborts aa-logprof, losing all rule changes entered so far and leaving all profiles unmodified.


Closes aa-logprof, saving all rule changes entered so far and modifying all profiles. aa-logprof Example 2

For example, when profiling vsftpd, see this question:

Profile:  /usr/sbin/vsftpd
Path:     /y2k.jpg
New Mode: r

[1 - /y2k.jpg]

(A)llow / [(D)eny] / (N)ew / (G)lob / Glob w/(E)xt / Abo(r)t / (F)inish

Several items of interest appear in this question. First, note that vsftpd is asking for a path entry at the top of the tree, even though vsftpd on openSUSE serves FTP files from /srv/ftp by default. This is because httpd2-prefork uses chroot and, for the portion of the code inside the chroot jail, AppArmor sees file accesses in terms of the chroot environment rather than the global absolute path.

The second item of interest is that you might want to grant FTP read access to all JPEG files in the directory, so you could use Glob w/Ext and use the suggested path of /*.jpg. Doing so collapses all previous rules granting access to individual .jpg files and forestalls any future questions pertaining to access to .jpg files.

Finally, you might want to grant more general access to FTP files. If you select Glob in the last entry, aa-logprof replaces the suggested path of /y2k.jpg with /*. Alternatively, you might want to grant even more access to the entire directory tree, in which case you could use the New path option and enter /**.jpg (which would grant access to all .jpg files in the entire directory tree) or /** (which would grant access to all files in the directory tree).

These items deal with read accesses. Write accesses are similar, except that it is good policy to be more conservative in your use of regular expressions for write accesses. Dealing with execute accesses is more complex. Find an example in Example 22.1, “Learning Mode Exception: Controlling Access to Specific Resources”.

In the following example, the /usr/bin/mail mail client is being profiled and aa-logprof has discovered that /usr/bin/mail executes /usr/bin/less as a helper application to page long mail messages. Consequently, it presents this prompt:

/usr/bin/nail -> /usr/bin/less
(I)nherit / (P)rofile / (U)nconfined / (D)eny

The actual executable file for /usr/bin/mail turns out to be /usr/bin/nail, which is not a typographical error.

The program /usr/bin/less appears to be a simple one for scrolling through text that is more than one screen long and that is in fact what /usr/bin/mail is using it for. However, less is actually a large and powerful program that makes use of many other helper applications, such as tar and rpm.


Run less on a tar file or an RPM file and it shows you the inventory of these containers.

You do not want to run rpm automatically when reading mail messages (that leads directly to a Microsoft* Outlook–style virus attack, because rpm has the power to install and modify system programs), so, in this case, the best choice is to use Inherit. This results in the less program executed from this context running under the profile for /usr/bin/mail. This has two consequences:

  • You need to add all of the basic file accesses for /usr/bin/less to the profile for /usr/bin/mail.

  • You can avoid adding the helper applications, such as tar and rpm, to the /usr/bin/mail profile so that when /usr/bin/mail runs /usr/bin/less in this context, the less program is far less dangerous than it would be without AppArmor protection.

In other circumstances, you might instead want to use the Profile option. This has two effects on aa-logprof:

  • The rule written into the profile uses px, which forces the transition to the child's own profile.

  • aa-logprof constructs a profile for the child and starts building it, in the same way that it built the parent profile, by assigning events for the child process to the child's profile and asking the aa-logprof user questions.

If a confined program forks and executes another program, aa-logprof sees this and asks the user which execution mode should be used when launching the child process. The execution modes of inherit, profile, unconfined or an option to deny the execution are presented.

If a separate profile exists for the child process, the default selection is profile. If a profile does not exist, the default is inherit. The inherit option, or ix, is described in Section 19.7, “File Permission Access Modes”.

The profile option indicates that the child program should run in its own profile. A secondary question asks whether to sanitize the environment that the child program inherits from the parent. If you choose to sanitize the environment, this places the execution modifier Px in your AppArmor profile. If you select not to sanitize, px is placed in the profile and no environment sanitizing occurs. The default for the execution mode is px if you select profile execution mode.

The unconfined execution mode is not recommended and should only be used in cases where there is no other option to generate a profile for a program reliably. Selecting unconfined opens a warning dialog asking for confirmation of the choice. If you are sure and choose Yes, a second dialog ask whether to sanitize the environment. Choosing Yes uses the execution mode Ux in your profile. Choosing No uses the execution mode ux for your profile. The default value selected is Ux for unconfined execution mode.

[Important]Running Unconfined

Choosing ux is very dangerous and provides no enforcement of policy (from a security perspective) of the resulting execution behavior of the child program. aa-unconfined—Identifying Unprotected Processes

The aa-unconfined command examines open network ports on your system, compares that to the set of profiles loaded on your system, and reports network services that do not have AppArmor profiles. It requires root privileges and that it not be confined by an AppArmor profile.

aa-unconfined must be run as root to retrieve the process executable link from the /proc file system. This program is susceptible to the following race conditions:

  • An unlinked executable is mishandled

  • A process that dies between netstat(8) and further checks is mishandled


This program lists processes using TCP and UDP only. In short, this program is unsuitable for forensics use and is provided only as an aid to profiling all network-accessible processes in the lab.

22.7. Important Filenames and Directories

The following list contains the most important files and directories used by the AppArmor framework. If you intend to manage and troubleshoot your profiles manually, make sure that you know about these files and directories:


Virtualized file representing the currently loaded set of profiles.


Location of AppArmor configuration files.


A local repository of profiles shipped with AppArmor, but not enabled by default.


Location of profiles, named with the convention of replacing the / in paths with . (not for the root /) so profiles are easier to manage. For example, the profile for the program /usr/sbin/ntpd is named usr.sbin.ntpd.


Location of abstractions.


Location of program chunks.


Check this file to review the confinement status of a process and the profile that is used to confine the process. The ps auxZ command retrieves this information automatically.

Chapter 23. Profiling Your Web Applications Using ChangeHat

A AppArmor® profile represents the security policy for an individual program instance or process. It applies to an executable program, but if a portion of the program needs different access permissions than other portions, the program can change hats to use a different security context, distinctive from the access of the main program. This is known as a hat or subprofile.

ChangeHat enables programs to change to or from a hat within a AppArmor profile. It enables you to define security at a finer level than the process. This feature requires that each application be made ChangeHat aware, meaning that it is modified to make a request to the AppArmor module to switch security domains at arbitrary times during the application execution. Two examples for ChangeHat-aware applications are the Apache Web server and Tomcat.

A profile can have an arbitrary number of subprofiles, but there are only two levels: a subprofile cannot have further sub-subprofiles. A subprofile is written as a separate profile and named as the containing profile followed by the subprofile name, separated by a ^. Subprofiles must be stored in the same file as the parent profile.

Note that the security of hats is considerably weaker than that of full profiles. That is to say, if attackers can find just the right kind of bug in a program, they may be able to escape from a hat into the containing profile. This is because the security of hats is determined by a secret key handled by the containing process, and the code running in the hat must not have access to the key. Thus change_hat is most useful in conjunction with application servers, where a language interpreter (such as PERL, PHP, or Java) is isolating pieces of code such that they do not have direct access to the memory of the containing process.

The rest of this chapter describes using change_hat in conjunction with Apache, to contain web server components run using mod_perl and mod_php. Similar approaches can be used with any application server by providing an application module similar to the mod_apparmor described next in Section 23.2.2, “Location and Directory Directives”.

[Note]For More Information

For more information, see the change_hat man page.

23.1. Apache ChangeHat

AppArmor provides a mod_apparmor module (package apache2-mod_apparmor) for the Apache program. This module makes the Apache Web server ChangeHat aware. Install it along with Apache.

When Apache is ChangeHat aware, it checks for the following customized AppArmor security profiles in the order given for every URI request that it receives.

  • URI-specific hat. For example, ^phpsysinfo/templates/classic/images/bar_left.gif



[Note]Apache Configuration

If you install apache2-mod_apparmor, make sure the module gets loaded in Apache by executing the following command:

a2enmod apparmor

23.1.1. Managing ChangeHat-Aware Applications

As with most of the AppArmor tools, you can use two methods for managing ChangeHat, YaST or the command line interface. Managing ChangeHat-aware applications from the command line is much more flexible, but the process is also more complicated. Both methods allow you to manage the hats for your application and populate them with profile entries.

The following steps are a demonstration that adds hats to an Apache profile using YaST. In the Add Profile Wizard, the AppArmor profiling utilities prompt you to create new hats for distinct URI requests. Choosing to create a new hat allows you to create individual profiles for each URI. You can create very tight rules for each request.

If the URI that is processed does not represent significant processing or otherwise does not represent a significant security risk, safely select Use Default Hat to process this URI in the default hat, which is the default security profile.

This example creates a new hat for the URI phpsysinfo and its subsequent accesses. Using the profiling utilities, delegate what to add to this new hat. The resulting hat becomes a tight-security container that encompasses all the processing on the server that occurs when the phpsysinfo URI is passed to the Apache Web server.

The URI runs the application phpsysinfo (refer to for more information). The phpsysinfo package is assumed to be installed in /srv/www/htdocs/phpsysinfo in a clean (new) installation of openSUSE and AppArmor.

  1. Once phpsysinfo is installed, you are ready to add hats to the Apache profile. From the AppArmor GUI, select Add Profile Wizard.

  2. In Application to Profile, enter httpd2-prefork.

  3. Click Create Profile.

    Add Profile Wizard
  4. Restart Apache by entering rcapache2 restart in a terminal window.

    Restart any program you are profiling at this point.

  5. Open http://localhost/phpsysinfo/ in a Web browser window. The browser window should display network usage and system information.

    [Note]Data Caching

    To ensure that this request is processed by the server and you do not review cached data in your browser, refresh the page. To do this, click the browser Refresh button to make sure that Apache processes the request for the phpsysinfo URI.

  6. Click Scan System Log for Entries to Add to Profiles. AppArmor launches the aa-logprof tool, which scans the information learned in the previous step. It begins to prompt you with profile questions.

  7. aa-logprof first prompts with Add Requested Hat or Use Default Hat because it noticed that the phpsysinfo URI was accessed. Select Add Requested Hat.

  8. Click Allow.

    Choosing Add Requested Hat in the previous step creates a new hat in the profile and specifies that the results of subsequent questions about the script's actions are added to the newly created hat rather than the default hat for this application.

    In the next screen, AppArmor displays an external program that the script executed. You can specify that the program should run confined by the phpsysinfo hat (choose Inherit), confined by a separate profile (choose Profile), or that it should run unconfined or without any security profile (choose Unconfined). For the case of the Profile option, a new profile is created for the program if one does not already exist.

    [Note]Security Considerations

    Selecting Unconfined can create a significant security hole and should be done with caution.

    1. Select Inherit for the /bin/bash path. This adds /bin/bash (accessed by Apache) to the phpsysinfo hat profile with the necessary permissions.

    2. Click Allow.

  9. The remaining questions prompt you to generate new hats and add entries to your profile and its hats. The process of adding entries to profiles is covered in detail in the Section 21.1, “Adding a Profile Using the Wizard”.

    When all profiling questions are answered, click Finish to save your changes and exit the wizard.

The following is an example phpsysinfo hat.

Example 23.1. Example phpsysinfo Hat

/usr/sbin/httpd2-prefork {
  ^phpsysinfo {
    #include <abstractions/bash>
    #include <abstractions/nameservice>

    /bin/basename                        ixr,
    /bin/bash                            ixr,
    /bin/df                              ixr,
    /bin/grep                            ixr,
    /bin/mount                           Ux,
    /bin/sed                             ixr,
    /dev/bus/usb/                        r,
    /dev/bus/usb/**                      r,
    /dev/null                            w,
    /dev/tty                             rw,
    /dev/urandom                         r,
    /etc/SuSE-release                    r,
    /etc/                     r,
    /etc/lsb-release                     r,
    /etc/lsb-release.d/                  r,
    /lib/                     ixr,
    /proc/**                             r,
    /sbin/lspci                          ixr,
    /srv/www/htdocs/phpsysinfo/**        r,
    /sys/bus/pci/**                      r,
    /sys/bus/scsi/devices/               r,
    /sys/devices/**                      r,
    /usr/bin/cut                         ixr,
    /usr/bin/getopt                      ixr,
    /usr/bin/head                        ixr,
    /usr/bin/lsb_release                 ixr,
    /usr/bin/lsscsi                      ixr,
    /usr/bin/tr                          ixr,
    /usr/bin/who                         ixr,
    /usr/lib/lib*so*                     mr,
    /usr/lib/locale/**                   r,
    /usr/sbin/lsusb                      ixr,
    /usr/share/locale/**                 r,
    /usr/share/pci.ids                   r,
    /usr/share/usb.ids                   r,
    /var/log/apache2/access_log          w,
    /var/run/utmp                        kr,

[Note]Hat and Parent Profile Relationship

The profile ^phpsysinfo is only valid in the context of a process running under the parent profile httpd2-prefork.

23.1.2. Adding Hats and Entries to Hats

When you use the Edit Profile dialog (for instructions, refer to Section 21.3, “Editing Profiles”) or when you add a new profile using Manually Add Profile (for instructions, refer to Section 21.2, “Manually Adding a Profile”), you are given the option of adding hats (subprofiles) to your AppArmor profiles. Add a ChangeHat subprofile from the AppArmor Profile Dialog window as in the following.

AppArmor profile dialog
  1. From the AppArmor Profile Dialog window, click Add Entry then select Hat. The Enter Hat Name dialog box opens:

    Enter hat name
  2. Enter the name of the hat to add to the AppArmor profile. The name is the URI that, when accessed, receives the permissions set in the hat.

  3. Click Create Hat. You are returned to the AppArmor Profile Dialog screen.

  4. After adding the new hat, click Done.

[Note]For More Information

For an example of an AppArmor profile, refer to Example 23.1, “Example phpsysinfo Hat”.

23.2. Configuring Apache for mod_apparmor

Apache is configured by placing directives in plain text configuration files. The main configuration file is usually httpd.conf. When you compile Apache, you can indicate the location of this file. Directives can be placed in any of these configuration files to alter the way Apache behaves. When you make changes to the main configuration files, you need to start or restart Apache, so the changes are recognized.

23.2.1. Virtual Host Directives

Virtual host directives control whether requests that contain trailing pathname information following an actual filename (or that refer to a nonexistent file in an existing directory) are accepted or rejected. For Apache documentation on virtual host directives, refer to

The ChangeHat-specific configuration keyword is AADefaultHatName. It is used similarly to AAHatName, for example, AADefaultHatName My_Funky_Default_Hat.

The configuration option is actually based on a server directive, which enables you to use the keyword outside of other options, setting it for the default server. Virtual hosts are considered internally within Apache to be separate servers, so you can set a default hat name for the default server as well as one for each virtual host, if desired.

When a request comes in, the following steps reflect the sequence in which mod_apparmor attempts to apply hats.

  1. A location or directory hat as specified by the AAHatName keyword

  2. A hat named by the entire URI path

  3. A default server hat as specified by the AADefaultHatName keyword

  4. DEFAULT_URI (if none of those exist, it goes back to the parent Apache hat)

23.2.2. Location and Directory Directives

Location and directory directives specify hat names in the program configuration file so the program calls the hat regarding its security. For Apache, you can find documentation about the location and directory directives at

The location directive example below specifies that, for a given location, mod_apparmor should use a specific hat:

<Location /foo/> AAHatName MY_HAT_NAME </Location>

This tries to use MY_HAT_NAME for any URI beginning with /foo/ (/foo/, /foo/bar, /foo/cgi/path/blah_blah/blah, etc.).

The directory directive works similarly to the location directive, except it refers to a path in the file system as in the following example:

<Directory "/srv/www/"> 
  # Note lack of trailing slash 

Example:  The program phpsysinfo is used to illustrate a location directive in the following example. The tarball can be downloaded from

  1. After downloading the tarball, install it into /srv/www/htdocs/phpsysinfo.

  2. Create /etc/apache2/conf.d/phpsysinfo.conf and add the following text to it:

    <Location "/phpsysinfo"> 
      AAHatName phpsysinfo

    The following hat should then work for phpsysinfo:

    /usr/sbin/httpd2-prefork {
      ^phpsysinfo {
        #include <abstractions/bash>
        #include <abstractions/nameservice>
        /bin/basename                        ixr,
        /bin/bash                            ixr,
        /bin/df                              ixr,
        /bin/grep                            ixr,
        /bin/mount                           Ux,
        /bin/sed                             ixr,
        /dev/bus/usb/                        r,
        /dev/bus/usb/**                      r,
        /dev/null                            w,
        /dev/tty                             rw,
        /dev/urandom                         r,
        /etc/SuSE-release                    r,
        /etc/                     r,
        /etc/lsb-release                     r,
        /etc/lsb-release.d/                  r,
        /lib/                     ixr,
        /proc/**                             r,
        /sbin/lspci                          ixr,
        /srv/www/htdocs/phpsysinfo/**        r,
        /sys/bus/pci/**                      r,
        /sys/bus/scsi/devices/               r,
        /sys/devices/**                      r,
        /usr/bin/cut                         ixr,
        /usr/bin/getopt                      ixr,
        /usr/bin/head                        ixr,
        /usr/bin/lsb_release                 ixr,
        /usr/bin/lsscsi                      ixr,
        /usr/bin/tr                          ixr,
        /usr/bin/who                         ixr,
        /usr/lib/lib*so*                     mr,
        /usr/lib/locale/**                   r,
        /usr/sbin/lsusb                      ixr,
        /usr/share/locale/**                 r,
        /usr/share/pci.ids                   r,
        /usr/share/usb.ids                   r,
        /var/log/apache2/access_log          w,
        /var/run/utmp                        kr,
  3. Reload AppArmor profiles by entering rcapparmor restart at a terminal window as root.

  4. Restart Apache by entering rcapache2 restart at a terminal window as root.

  5. Enter http://hostname/phpsysinfo/ into a browser to receive the system information that phpsysinfo delivers.

  6. Locate configuration errors by going to /var/log/audit/audit.log or running dmesg and looking for any rejections in the output.

Chapter 24. Confining Users with pam_apparmor

An AppArmor profile applies to an executable program; if a portion of the program needs different access permissions than other portions need, the program can change hats via change_hat to a different role, also known as a subprofile. The pam_apparmor PAM module allows applications to confine authenticated users into subprofiles based on group names, user names, or a default profile. To accomplish this, pam_apparmor needs to be registered as a PAM session module.

The package pam_apparmor may not installed by default, you may need to install it using YaST or zypper. Details about how to set up and configure pam_apparmor can be found in /usr/share/doc/packages/pam_apparmor/README after the package has been installed. For details on PAM, refer to Chapter 2, Authentication with PAM.

pam_apparmor allows you to set up role-based access control (RBAC). A detailed HOWTO on setting up RBAC with AppArmor is available at

Chapter 25. Managing Profiled Applications

After creating profiles and immunizing your applications, openSUSE® becomes more efficient and better protected as long as you perform AppArmor® profile maintenance (which involves analyzing log files, refining your profiles, backing up your set of profiles and keeping it up-to-date). You can deal with these issues before they become a problem by setting up event notification by e-mail, running periodic reports, updating profiles from system log entries by running the aa-logprof tool through YaST, and dealing with maintenance issues.

25.1. Reacting to Security Event Rejections

When you receive a security event rejection, examine the access violation and determine if that event indicated a threat or was part of normal application behavior. Application-specific knowledge is required to make the determination. If the rejected action is part of normal application behavior, run aa-logprof at the command line or the Update Profile Wizard in AppArmor to update your profile.

If the rejected action is not part of normal application behavior, this access should be considered a possible intrusion attempt (that was prevented) and this notification should be passed to the person responsible for security within your organization.

25.2. Maintaining Your Security Profiles

In a production environment, you should plan on maintaining profiles for all of the deployed applications. The security policies are an integral part of your deployment. You should plan on taking steps to back up and restore security policy files, plan for software changes, and allow any needed modification of security policies that your environment dictates.

25.2.1. Backing Up Your Security Profiles

Backing up profiles might save you from having to reprofile all your programs after a disk crash. Also, if profiles are changed, you can easily restore previous settings by using the backed up files.

Back up profiles by copying the profile files to a specified directory.

  1. You should first archive the files into one file. To do this, open a terminal window and enter the following as root:

    tar zclpf profiles.tgz /etc/apparmor.d

    The simplest method to ensure that your security policy files are regularly backed up is to include the directory /etc/apparmor.d in the list of directories that your backup system archives.

  2. You can also use scp or a file manager like Konqueror or Nautilus to store the files on some kind of storage media, the network, or another computer.

25.2.2. Changing Your Security Profiles

Maintenance of security profiles includes changing them if you decide that your system requires more or less security for its applications. To change your profiles in AppArmor, refer to Section 21.3, “Editing Profiles”.

25.2.3. Introducing New Software into Your Environment

When you add a new application version or patch to your system, you should always update the profile to fit your needs. You have several options, depending on your company's software deployment strategy. You can deploy your patches and upgrades into a test or production environment. The following explains how to do this with each method.

If you intend to deploy a patch or upgrade in a test environment, the best method for updating your profiles is one of the following:

If you intend to deploy a patch or upgrade directly into a production environment, the best method for updating your profiles is one of the following:

Chapter 26. Support

This chapter outlines maintenance-related tasks. Learn how to update AppArmor® and get a list of available man pages providing basic help for using the command line tools provided by AppArmor. Use the troubleshooting section to learn about some common problems encountered with AppArmor and their solutions. Report defects or enhancement requests for AppArmor following the instructions in this chapter.

26.1. Updating AppArmor Online

Updates for AppArmor packages are provided in the same way as any other update for openSUSE. Retrieve and apply them exactly like for any other package that ships as part of openSUSE.

26.2. Using the Man Pages

There are man pages available for your use. In a terminal, enter man apparmor to open the apparmor man page. Man pages are distributed in sections numbered 1 through 8. Each section is specific to a category of documentation:

Table 26.1. Man Pages: Sections and Categories




User commands


System calls


Library functions


Device driver information


Configuration file formats




High level concepts


Administrator commands

The section numbers are used to distinguish man pages from each other. For example, exit(2) describes the exit system call, while exit(3) describes the exit C library function.

The AppArmor man pages are:

  • unconfined(8)

  • autodep(1)

  • complain(1)

  • enforce(1)

  • genprof(1)

  • logprof(1)

  • change_hat(2)

  • logprof.conf(5)

  • apparmor.conf(5)

  • apparmor.d(5)

  • apparmor.vim(5)

  • apparmor(7)

  • apparmor_parser(8)

26.3. For More Information

Find more information about the AppArmor product at: Find the product documentation for AppArmor in the installed system at /usr/share/doc/manual.

There is a mailing lists for AppArmor that users can post to or join to communicate with developers. See for details.

26.4. Troubleshooting

This section lists the most common problems and error messages that may occur using AppArmor.

26.4.1. How to React to odd Application Behavior?

If you notice odd application behavior or any other type of application problem, you should first check the reject messages in the log files to see if AppArmor is too closely constricting your application. If you detect reject messages that indicate that your application or service is too closely restricted by AppArmor, update your profile to properly handle your use case of the application. Do this with the Update Profile Profile Wizard in YaST, as described in Section 21.5, “Updating Profiles from Log Entries”.

If you decide to run your application or service without AppArmor protection, remove the application's profile from /etc/apparmor.d or move it to another location.

26.4.2. My Profiles do not Seem to Work Anymore …

If you have been using previous versions of AppArmor and have updated your system (but kept your old set of profiles) you might notice some applications which seemed to work perfectly before you updated behaving strangely, or not working at all .

This version of AppArmor introduces a set of new features to the profile syntax and the AppArmor tools that might cause trouble with older versions of the AppArmor profiles. Those features are:

  • File Locking

  • Network Access Control

  • The SYS_PTRACE Capability

  • Directory Path Access

The current version of AppArmor mediates file locking and introduces a new permission mode (k) for this. Applications requesting file locking permission might misbehave or fail altogether if confined by older profiles which do not explicitly contain permissions to lock files. If you suspect this being the case, check the log file under /var/log/audit/audit.log for entries like the following:

type=APPARMOR_DENIED msg=audit(1188913493.299:9304): operation="file_lock" requested_mask="::k" denied_mask="::k" fsuid=1000 name="/home/tux/.qt/.qtrc.lock" pid=25736 profile="/usr/bin/opera"

Update the profile using the YaST Update Profile Wizard or the aa-logprof command as outlined below.

The new network access control syntax based on the network family and type specification, described in Section 19.5, “Network Access Control”, might cause application misbehavior or even stop applications from working. If you notice a network-related application behaving strangely, check the log file under /var/log/audit/audit.log for entries like the following:

type=APPARMOR_DENIED msg=audit(1188894313.206:9123): operation="socket_create" family="inet" sock_type="raw" protocol=1 pid=23810 profile="/bin/ping"

This log entry means that our example application, /bin/ping in this case, failed to get AppArmor's permission to open a network connection. This permission has to be explicitly stated to make sure that an application has network access. To update the profile to the new syntax, use the YaST Update Profile Wizard or the aa-logprof command as outlined below.

The current kernel requires the SYS_PTRACE capability, if a process tries to access files in /proc/pid/fd/*. New profiles need an entry for the file and the capability, where old profiles only needed the file entry. For example:

/proc/*/fd/**  rw,

in the old syntax would translate to the following rules in the new syntax:

capability SYS_PTRACE,
/proc/*/fd/**  rw,

To update the profile to the new syntax, use the YaST Update Profile Wizard or the aa-logprof command as outlined below.

With this version of AppArmor, a few changes have been made to the profile rule syntax to better distinguish directory from file access. Therefore, some rules matching both file and directory paths in the previous version might now just match a file path. This could lead to AppArmor not being able to access a crucial directory at all, and thus trigger misbehavior of your application and various log messages. The following examples highlight the most important changes to the path syntax.

Using the old syntax, the following rule would allow access to files and directories in /proc/net. It would allow directory access only to read the entries in the directory, but not give access to files or directories under the directory, e.g. /proc/net/dir/foo would be matched by the asterisk (*), but as foo is a file or directory under dir, it cannot be accessed.

/proc/net/*  r, 

To get the same behavior using the new syntax, you need two rules instead of one. The first allows access to the file under /proc/net and the second allows access to directories under /proc/net. Directory access can only be used for listing the contents, not actually accessing files or directories underneath the directory.

/proc/net/*  r,  
/proc/net/*/  r,    

The following rule works similarly both under the old and the new syntax, and allows access to both files and directories under /proc/net:

/proc/net/**  r,   

To distinguish file access from directory access using the above expression in the new syntax, use the following two rules. The first one only allows to recursively access directories under /proc/net while the second one explicitly allows for recursive file access only.

/proc/net/**/  r,  
/proc/net/**[^/]  r,

The following rule works similarly both under the old and the new syntax and allows access to both files and directories beginning with foo under /proc/net:

/proc/net/foo**  r,

To distinguish file access from directory access in the new syntax and use the ** globbing pattern, use the following two rules. The first one would have matched both files and directories in the old syntax, but only matches files in the new syntax due to the missing trailing slash. The second rule matched neither file nor directory in the old syntax, but matches directories only in the new syntax:

/proc/net/**foo  r,
/proc/net/**foo/  r, 

The following rules illustrate how the use of the ? globbing pattern has changed. In the old syntax, the first rule would have matched both files and directories (four characters, last character could be any but a slash). In the new syntax, it matches only files (trailing slash is missing). The second rule would match nothing in the old profile syntax, but matches directories only in the new syntax. The last rule matches explicitly matches a file called bar under /proc/net/foo?. Using the old syntax, this rule would have applied to both files and directories:

/proc/net/foo?  r,    
/proc/net/foo?/  r, 
/proc/net/foo?/bar  r,

To find and resolve issues related to syntax changes, take some time after the update to check the profiles you want to keep and proceed as follows for each application you kept the profile for:

  1. Make sure that AppArmor is running and that the application's profile is loaded.

  2. Start the YaST AppArmor Control Panel and put the application's profile into complain mode. Log entries are made for any actions violating the current profile, but the profile is not enforced and the application's behavior not restricted.

  3. Run the application covering all the tasks you need this application to be able to perform.

  4. Start the YaST Update Profile Wizard to update the application's profile according to the log entries generated while running the application.

  5. Once the profile is updated, put it back into enforce mode via the YaST AppArmor Control Panel.

Using the AppArmor command line tools, you would proceed as follows:

  1. Put the application's profile into complain mode:

    aa-complain /path/to/application
  2. Run the application.

  3. Update the profile according to the log entries made while running the application:

    aa-logprof /path/to/application
  4. Put the resulting profile back into enforce mode:

    aa-enforce /path/to/application

26.4.3. How to Confine KDE Applications with AppArmor?

Currently, it is not possible to confine KDE applications to the same extent as any other application, due to the way KDE manages its processes.

If you want to confine KDE applications, choose one of the following approaches, but note that none of them are really suited for a standard setup:

Create a Single Profile for the Entire KDE Desktop

As all KDE processes are children of one parent process and AppArmor cannot distinguish an individual application's process from the rest, create one huge profile to confine the entire desktop all at once. This approach is only feasible if your setup is a very limited (kiosk-type) one. Maintaining such a profile for a standard KDE desktop (including all of its applications) would be close to impossible.

Modify KDE's process handling

Using KDE_EXEC_SLAVES=1 and KDE_IS_PRELINKED=1 variables force KDE to manage its processes in a way that allows AppArmor to distinguish individual applications from each other and apply profiles to them. This approach might slow down your desktop considerably, as it turns off a crucial optimization for speed. Note that the above mentioned environment variables have to be set before KDM/XDM/GDM or startx are started. One way to achieve this would be to add them to /etc/security/pam_env.conf.

26.4.4. How to Resolve Issues with Apache?

Apache is not starting properly or it is not serving Web pages and you just installed a new module or made a configuration change. When you install additional Apache modules (like apache2-mod_apparmor) or make configuration changes to Apache, you should profile Apache again to catch any additional rules that need to be added to the profile.

26.4.5. How to Exclude Certain Profiles from the List of Profiles Used?

Run aa-disable PROGRAMNAME to disable the profile for PROGRAMNAME. This command creates a symbolic link to the profile in /etc/apparmor.d/disable/. In order to reactivate the profile, just delete that link.

26.4.6. Can I Manage Profiles for Applications not Installed on my System?

Managing profiles with AppArmor requires you to have access to the log of the system on which the application is running. So you do not need to run the application on your profile, build host as long as you have access to the machine that runs the application. You can run the application on one system, transfer the logs (/var/log/audit.log or, if audit is not installed, /var/log/messages) to your profile build host and run aa-logprof -f path_to_logfile.

26.4.7. How to Spot and fix AppArmor Syntax Errors?

Manually editing AppArmor profiles can introduce syntax errors. If you attempt to start or restart AppArmor with syntax errors in your profiles, error results are shown. This example shows the syntax of the entire parser error.

localhost:~ # rcapparmor start
Loading AppArmor profiles AppArmor parser error in /etc/apparmor.d/usr.sbin.squid at line 410: syntax error, unexpected TOK_ID, expecting TOK_MODE
 Profile /etc/apparmor.d/usr.sbin.squid failed to load

Using the AppArmor YaST tools, a graphical error message indicates which profile contained the error and requests you to fix it.

To fix a syntax error, log in to a terminal window as root, open the profile, and correct the syntax. Reload the profile set with rcapparmor reload.

[Tip]AppArmor Syntax Highlighting in vi

The editor vi on openSUSE supports syntax highlighting for AppArmor profiles. Lines containing syntax errors will be displayed with a red background.

26.5. Reporting Bugs for AppArmor

The developers of AppArmor are eager to deliver products of the highest quality. Your feedback and your bug reports help us keep the quality high. Whenever you encounter a bug in AppArmor, file a bug report against this product:

  1. Use your Web browser to go to

  2. Enter the account data of your Novell account and click Login


    Create a new Novell account as follows:

    1. Click Create New Account on the Login to Continue page.

    2. Provide a username and password and additional address data and click Create Login to immediately proceed with the login creation.


      Provide data on which other Novell accounts you maintain to sync all these to one account.

  3. Check whether a problem similar to yours has already been reported by clicking Search Reports. Use a quick search against a given product and keyword or use the Advanced Search.

  4. If your problem has already been reported, check this bug report and add extra information to it, if necessary.

  5. If your problem has not been reported yet, select New from the top navigation bar and proceed to the Enter Bug page.

  6. Select the product against which to file the bug. In your case, this would be your product's release. Click Submit.

  7. Select the product version, component (AppArmor in this case), hardware platform, and severity.

  8. Enter a brief headline describing your problem and add a more elaborate description including log files. You may create attachments to your bug report for screen shots, log files, or test cases.

  9. Click Submit after you have entered all the details to send your report to the developers.

Chapter 27. AppArmor Glossary


Apache is a freely-available UNIX-based Web server. It is currently the most commonly used Web server on the Internet. Find more information about Apache at the Apache Web site at

application firewalling

AppArmor contains applications and limits the actions they are permitted to take. It uses privilege confinement to prevent attackers from using malicious programs on the protected server and even using trusted applications in unintended ways.

attack signature

Pattern in system or network activity that alerts of a possible virus or hacker attack. Intrusion detection systems might use attack signatures to distinguish between legitimate and potentially malicious activity.

By not relying on attack signatures, AppArmor provides "proactive" instead of "reactive" defense from attacks. This is better because there is no window of vulnerability where the attack signature must be defined for AppArmor as it does for products using attack signatures to secure their networks.


Graphical user interface. Refers to a software front-end meant to provide an attractive and easy-to-use interface between a computer user and application. Its elements include such things as windows, icons, buttons, cursors, and scroll bars.


Filename substitution.


Host intrusion prevention. Works with the operating system kernel to block abnormal application behavior in the expectation that the abnormal behavior represents an unknown attack. Blocks malicious packets on the host at the network level before they can hurt the application they target.

mandatory access control

A means of restricting access to objects that is based on fixed security attributes assigned to users, files, and other objects. The controls are mandatory in the sense that they cannot be modified by users or their programs.

profile foundation classes

Profile building blocks needed for common application activities, such as DNS lookup and user authentication.


The RPM Package Manager. An open packaging system available for anyone to use. It works on Red Hat Linux, openSUSE, and other Linux and UNIX systems. It is capable of installing, uninstalling, verifying, querying, and updating computer software packages. See for more information.


Secure Shell. A service that allows you to access your server from a remote computer and issue text commands through a secure connection.

streamlined access control

AppArmor provides streamlined access control for network services by specifying which files each program is allowed to read, write, and execute. This ensures that each program does what it is supposed to do and nothing else.


Universal resource identifier. The generic term for all types of names and addresses that refer to objects on the World Wide Web. A URL is one kind of URI.


Uniform Resource Locator. The global address of documents and other resources on the World Wide Web.

The first part of the address indicates what protocol to use and the second part specifies the IP address or the domain name where the resource is located.

For example, in, http is the protocol to use.


An aspect of a system or network that leaves it open to attack. Characteristics of computer systems that allow an individual to keep it from correctly operating or that allows unauthorized users to take control of the system. Design, administrative, or implementation weaknesses or flaws in hardware, firmware, or software. If exploited, a vulnerability could lead to an unacceptable impact in the form of unauthorized access to information or the disruption of critical processing.

Appendix A. GNU Licenses

This appendix contains the GNU General Public License version 2 and the GNU Free Documentation License version 1.2.

A.1. GNU General Public License

Version 2, June 1991

Copyright (C) 1989, 1991 Free Software Foundation, Inc. 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA

Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

A.1.1. Preamble

The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation’s software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.

When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.

For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.

We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.

Also, for each author’s protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors’ reputations.

Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone’s free use or not licensed at all.

The precise terms and conditions for copying, distribution and modification follow.


0.  This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The Program, below, refers to any such program or work, and a work based on the Program means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language. (Hereinafter, translation is included without limitation in the term modification.) Each licensee is addressed as you.

Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.

1.  You may copy and distribute verbatim copies of the Program’s source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program.

You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.

2.  You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:

a).  You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.

b).  You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.

c).  If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.)

These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program.

In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.

3.  You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:

a).  Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,

b).  Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,

c).  Accompany it with the information you received as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.)

The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.

If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code.

4.  You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

5.  You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it.

6.  Each time you redistribute the Program (or any work based on the Program), the recipient automatically receives a license from the original licensor to copy, distribute or modify the Program subject to these terms and conditions. You may not impose any further restrictions on the recipients’ exercise of the rights granted herein. You are not responsible for enforcing compliance by third parties to this License.

7.  If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Program at all. For example, if a patent license would not permit royalty-free redistribution of the Program by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Program.

If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances.

It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.

This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.

8.  If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Program under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.

9.  The Free Software Foundation may publish revised and/or new versions of the General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and any later version, you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.

10.  If you wish to incorporate parts of the Program into other free programs whose distribution conditions are different, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally.





A.1.3. How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.

To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the copyright line and a pointer to where the full notice is found.

   one line to give the program’s name and an idea of what it does. 
   Copyright (C) yyyy name of author
   This program is free software; you can redistribute it and/or
   modify it under the terms of the GNU General Public License
   as published by the Free Software Foundation; either version 2
   of the License, or (at your option) any later version.
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   GNU General Public License for more details.
   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this when it starts in an interactive mode:

   Gnomovision version 69, Copyright (C) year name of author
   Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
   type `show w’. This is free software, and you are welcome
   to redistribute it under certain conditions; type `show c’ 
   for details.

The hypothetical commands `show w’ and `show c’ should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w’ and `show c’; they could even be mouse-clicks or menu items--whatever suits your program.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a copyright disclaimer for the program, if necessary. Here is a sample; alter the names:

   Yoyodyne, Inc., hereby disclaims all copyright
   interest in the program `Gnomovision’
   (which makes passes at compilers) written 
   by James Hacker.
   signature of Ty Coon, 1 April 1989
   Ty Coon, President of Vice

This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License.

A.2. GNU Free Documentation License

Copyright (C) 2000, 2001, 2002 Free Software Foundation, Inc. 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.


The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.


This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.

The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.

A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not "Transparent" is called "Opaque".

Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.

The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.

A section "Entitled XYZ" means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as "Acknowledgements", "Dedications", "Endorsements", or "History".) To "Preserve the Title" of such a section when you modify the Document means that it remains a section "Entitled XYZ" according to this definition.

The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.


You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and you may publicly display copies.


If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.


You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

  1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.

  2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.

  3. State on the Title page the name of the publisher of the Modified Version, as the publisher.

  4. Preserve all the copyright notices of the Document.

  5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.

  6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.

  7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.

  8. Include an unaltered copy of this License.

  9. Preserve the section Entitled "History", Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.

  10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.

  11. For any section Entitled "Acknowledgements" or "Dedications", Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.

  12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.

  13. Delete any section Entitled "Endorsements". Such a section may not be included in the Modified Version.

  14. Do not retitle any existing section to be Entitled "Endorsements" or to conflict in title with any Invariant Section.

  15. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

You may add a section Entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties--for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.


You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled "History" in the various original documents, forming one section Entitled "History"; likewise combine any sections Entitled "Acknowledgements", and any sections Entitled "Dedications". You must delete all sections Entitled "Endorsements".


You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.


A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an "aggregate" if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.


Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.


You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.


The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See

Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

ADDENDUM: How to use this License for your documents

   Copyright (c) YEAR YOUR NAME.
   Permission is granted to copy, distribute and/or modify this document
   under the terms of the GNU Free Documentation License, Version 1.2
   or any later version published by the Free Software Foundation;
   with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
   A copy of the license is included in the section entitled “GNU
   Free Documentation License”.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

   with the Invariant Sections being LIST THEIR TITLES, with the
   Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

openSUSE Security Guide 12.2