pam_apparmor
Introduces basic concepts of system security, covering both local and network security aspects. Shows how to use the product inherent security software like AppArmor, SELinux, or the auditing system that reliably collects information about any security-relevant events. Supports the administrator with security-related choices and decisions in installing and setting up a secure SUSE Linux Enterprise Server and additional processes to further secure and harden that installation.
root
Loginspam_apparmor
auditctl
ausearch
autrace
cryptctl
(Model Without Connection to KMIP Server)aa-notify Message in GNOME
/etc/pam.d/sshd
)auth
Section (common-auth
)account
Section (common-account
)password
Section (common-password
)session
Section (common-session
).dsrc
File for Remote Administration.dsrc
File for Local Administration/etc/krb5.conf
nfs
Kernel Module in /etc/modprobe.d/60-nfs.conf
firewalld
RPC Service for NFSaa-unconfined
ls -Z
ps Zaux
/etc/audit/audit.log
auditctl
-s
auditctl
-l
Copyright © 2006– 2020 SUSE LLC 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 SUSE trademarks, see https://www.suse.com/company/legal/. All other third-party trademarks are the property of their respective owners. Trademark symbols (®, ™ etc.) denote trademarks of SUSE and its affiliates. Asterisks (*) denote third-party trademarks.
All information found in this book has been compiled with utmost attention to detail. However, this does not guarantee complete accuracy. Neither SUSE LLC, its affiliates, the authors nor the translators shall be held liable for possible errors or the consequences thereof.
This manual introduces the basic concepts of system security on openSUSE Leap. It covers extensive documentation about the authentication mechanisms available on Linux, such as NIS or LDAP. It deals with aspects of local security like access control lists, encryption and intrusion detection. In the network security part you learn how to secure computers with firewalls and masquerading, and how to set up virtual private networks (VPN). This manual shows how to use security software like AppArmor (which lets you specify per program which files the program may read, write, and execute) or the auditing system that collects information about security-relevant events.
Documentation for our products is available at http://doc.opensuse.org/, where you can also find the latest updates, and browse or download the documentation in various formats. The latest documentation updates are usually available in the English version of the documentation.
The following documentation is available for this product:
This manual will see you through your initial contact with openSUSE® Leap. Check out the various parts of this manual to learn how to install, use and enjoy your system.
Covers system administration tasks like maintaining, monitoring and customizing an initially installed system.
Describes virtualization technology in general, and introduces libvirt—the unified interface to virtualization—and detailed information on specific hypervisors.
AutoYaST is a system for unattended mass deployment of openSUSE Leap systems using an AutoYaST profile containing installation and configuration data. The manual guides you through the basic steps of auto-installation: preparation, installation, and configuration.
Introduces basic concepts of system security, covering both local and network security aspects. Shows how to use the product inherent security software like AppArmor, SELinux, or the auditing system that reliably collects information about any security-relevant events. Supports the administrator with security-related choices and decisions in installing and setting up a secure SUSE Linux Enterprise Server and additional processes to further secure and harden that installation.
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.
Introduces the GNOME desktop of openSUSE Leap. It guides you through using and configuring the desktop and helps you perform key tasks. It is intended mainly for end users who want to make efficient use of GNOME as their default desktop.
The release notes for this product are available at https://www.suse.com/releasenotes/.
Your feedback and contributions to this documentation are welcome! Several channels are available:
Report issues with the documentation at https://bugzilla.opensuse.org/. To simplify this process, you can use the links next to headlines in the HTML version of this document. These preselect the right product and category in Bugzilla and add a link to the current section. You can start typing your bug report right away. A Bugzilla account is required.
To contribute to this documentation, use the
links next to headlines in the HTML version of this document. They take you to the source code on GitHub, where you can open a pull request. A GitHub account is required.For more information about the documentation environment used for this documentation, see the repository's README.
Alternatively, you can report errors and send feedback concerning the documentation to <doc-team@suse.com>. Make sure to include the document title, the product version and the publication date of the documentation. Refer to the relevant section number and title (or include the URL) and provide a concise description of the problem.
If you need further help on openSUSE Leap, see https://en.opensuse.org/Portal:Support.
The following notices and typographical conventions are used in this documentation:
/etc/passwd
: directory names and file names
PLACEHOLDER: replace PLACEHOLDER with the actual value
PATH
: the environment variable PATH
ls
, --help
: commands, options, and
parameters
user
: users or groups
package name : name of a package
Alt, Alt–F1: a key to press or a key combination; keys are shown in uppercase as on a keyboard
, › : menu items, buttons
Dancing Penguins (Chapter Penguins, ↑Another Manual): This is a reference to a chapter in another manual.
Commands that must be run with root
privileges. Often you can also
prefix these commands with the sudo
command to run them
as non-privileged user.
root #
command
tux >
sudo
command
Commands that can be run by non-privileged users.
tux >
command
Notices
Vital information you must be aware of before proceeding. Warns you about security issues, potential loss of data, damage to hardware, or physical hazards.
Important information you should be aware of before proceeding.
Additional information, for example about differences in software versions.
Helpful information, like a guideline or a piece of practical advice.
This chapter introduces basic concepts of computer security. Threats and basic mitigation techniques are described. The chapter also provides references to other chapters, guides and Web sites with further information.
One main characteristic of Linux is its ability to handle multiple users at the same time (multiuser) and to allow these users to simultaneously perform tasks (multitasking) on the same computer. To users, there is no difference between working with data stored locally and data stored in the network.
Because of the multiuser capability, data from different users has to be stored separately to guarantee security and privacy. Also important is the ability to keep data available in spite of a lost or damaged data medium, for example a hard disk.
This chapter is primarily focused on confidentiality and privacy. But a comprehensive security concept includes a regularly updated, workable, and tested backup. Without a backup, restoring data after it has been tampered with or after a hardware failure is very hard.
Use a defense-in-depth approach to security: Assume that no single threat mitigation can fully protect your systems and data, but multiple layers of defense will make an attack much harder. Components of a defense-in-depth strategy can be the following:
Hashing passwords (for example with PBKDF2, bcrypt, or scrypt) and salting them
Encrypting data (for example with AES)
Logging, monitoring, and intrusion detection
Firewall
Antivirus scanner
Defined and documented emergency procedures
Backups
Physical security
Audits, security scans, and intrusion tests
openSUSE Leap includes software that addresses the requirements of the list above. The following sections provide starting points for securing your system.
On a Linux system, only hashes of passwords are stored. Hashes are one-way algorithms that make it easy to encrypt data. At the same time, hash algorithms make it very hard to compute the original secret from the hash.
The hashes are stored in the file /etc/shadow
,
which cannot be read by normal users. Because restoring passwords is
possible with powerful computers, hashed passwords should not be
visible to regular users.
The National Institute of Standards and Technology (NIST) publishes a guideline for passwords, which is available at https://pages.nist.gov/800-63-3/sp800-63b.html#sec5
For details about how to set a password policy, see Section 18.3, “. For general information about authentication on Linux, see ”Part I, “Authentication”.
If it is possible to physically access a computer, the firmware and boot process can be manipulated to gain access when an authorized person boots the machine. While not all computers can be locked into inaccessible rooms, your first step should be physically locking the server room.
Consider taking the following additional measures:
Configure your system so it cannot be booted from a removable device, either by removing the drives entirely or by setting a UEFI password and configuring the UEFI to allow booting from a hard disk only.
To make the boot procedure more tamper-resistant, enable the UEFI secure boot feature. For more information about Secure Boot, see Book “Reference”, Chapter 14 “UEFI (Unified Extensible Firmware Interface)”.
Linux systems are started by a boot loader that usually allows
passing additional options to the booted kernel. You can prevent others
from using such parameters during boot by setting an additional
password for the boot loader. This is crucial to system
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.
For more information about setting a password in the boot loader, see Book “Reference”, Chapter 12 “The Boot Loader GRUB 2”, Section 12.2.6 “Setting a Boot Password”.
Enable hard disk encryption. For more information, see Chapter 14, Encrypting Partitions and Files.
Use cryptctl
to encrypt hosted storage. For more
information, see Chapter 15, Storage Encryption for Hosted Applications with cryptctl.
Use AIDE to detect any changes in your system configuration. For more information, see Chapter 22, Intrusion Detection with AIDE.
Because of the everything is a file approach in
Linux, file permissions are important for controlling access to most
resources. This means that by using file permissions, you can define
access to regular files, directories, and hardware devices.
By default, most hardware devices are only accessible for
root
. However, some devices, for example serial ports, can be
accessible for normal users.
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 at the time of the
attack. By following the above rule, minimize the possible damage.
For details, see Section 20.1, “Traditional File Permissions” and Section 20.2, “Advantages of ACLs”.
AppArmor and SELinux allow you to set constraints for applications and users. For details, see Part IV, “Confining Privileges with AppArmor” and Part V, “SELinux”.
If there is a chance that hard disks could be accessed outside of the installed operating system, for example by booting a live system or removing the hardware, encrypt the data. openSUSE Leap allows you to encrypt partitions containing data and the operating system. For details, see Chapter 14, Encrypting Partitions and Files.
Securing network services is a crucial task. Aim to secure as many layers of the OSI model as possible.
All communication should be authenticated and encrypted with up-to-date cryptographic algorithms on the transport or application layer. Use a Virtual Private Network (VPN) as an additional secure layer on physical networks.
openSUSE Leap provides many options for securing your network:
Use openssl
to create X509 certificates. These certificates can be
used for encryption and authentication of many services.
You can set up your own certificate authority
(CA) and use it as a source of trust in your
network. For details, see man openssl
.
Usually, at least parts of networks are exposed to the public Internet. Reduce attack surfaces by closing ports with firewall rules and by uninstalling or at least disabling unrequired services. For details, see Chapter 25, Masquerading and Firewalls.
Use OpenVPN to secure communication channels over insecure physical networks. For details, see Chapter 26, Configuring a VPN Server.
Use strong authentication for network services. For details, see Part I, “Authentication”.
Software vulnerabilities are issues in software that can be exploited to obtain unauthorized access or misuse systems. Vulnerabilities are especially critical if they affect remote services, such as HTTP servers. Computer systems are very complex, therefore they always include certain vulnerabilities.
When such issues become known, they must usually be fixed in the software by software developers. The resulting update must then be installed by system administrators in a timely and safe manner on affected systems.
Vulnerabilities are usually announced on centralized databases, for example the National Vulnerability Database, which is maintained by the US government. You can subscribe to feeds to stay informed about newly discovered vulnerabilities. In some cases the problems induced by the bugs can be mitigated until a software update is provided. Vulnerabilities are assigned a Common Vulnerabilities and Exposures (CVE) number and a Common Vulnerability Scoring System (CVSS) score. The score helps identify the severity of vulnerabilities.
SUSE provides a feed of security advisories. It is available at https://www.suse.com/en-us/support/update/. There is also a list of security updates by CVE number available at https://www.suse.com/en-us/security/cve/.
In general, administrators should be prepared for severe vulnerabilities in their systems. This includes hardening all computers as far as possible. Also, we recommend to have predefined procedures in place for quickly installing updates for severe vulnerabilities.
To reduce the damage of possible attacks, use restrictive file permissions. See Section 20.1, “Traditional File Permissions”.
Other useful links:
http://lists.opensuse.org/opensuse-security-announce/, mailing list with openSUSE security announcements
https://nvd.nist.gov/home, the National Vulnerability Database
https://cve.mitre.org/, MITRE's CVE database
https://www.bsi.bund.de/DE/Service/Aktuell/Cert_Bund_Meldungen/cert_bund_meldungen_node.html, German Federal Office for Information Security vulnerability feed
https://www.first.org/cvss/, information about the Common Vulnerability Scoring System
Malware is software that is intended to interrupt the normal functioning of a computer or steal data. This includes viruses, worms, ransomware, or rootkits. Sometimes malware uses software vulnerabilities to attack a computer. However, often it is accidentally executed by a user, especially when installing third-party software from unknown sources. openSUSE Leap provides an extensive list of programs (packages) in its download repositories. This reduces the need to download third-party software. All packages provided by SUSE are signed. The package manager of openSUSE Leap checks the signatures of packages after the download to verify their integrity.
The command rpm
--checksig
RPM_FILE
shows whether the
checksum and the signature of a package are correct.
You can find the signing key on the first DVD of openSUSE Leap and
on most key servers worldwide.
You can use the ClamAV antivirus software to detect malware on your system. ClamAV can be integrated into several services, for example mail servers and HTTP proxies. This can be used to filter malware before it reaches the user.
Restrictive user privileges can reduce the risk of accidental code execution.
The following tips are a quick summary of the sections above:
Stay informed about the latest security issues. Get and install the updated packages recommended by security announcements as quickly as possible.
Avoid using root
privileges whenever possible. Set
restrictive file permissions.
Only use encrypted protocols for network communication.
Disable any network services you do not absolutely require.
Conduct regular security audits. For example, scan your network for open ports.
Monitor the integrity of files on your systems with
AIDE
(Advanced Intrusion Detection
Environment).
Take proper care when installing any third-party software.
Check all your backups regularly.
Check your log files, for example with logwatch.
Configure the firewall to block all ports that are not explicitly whitelisted.
Design your security measures to be redundant.
Use encryption where possible, for example for hard disks of mobile computers.
If you discover a security-related problem, first check the available update packages. If no update is available, write an e-mail to <security@suse.de>. Include a detailed description of the problem and the version number of the package concerned. We encourage you to encrypt e-mails with GPG.
You can find a current version of the SUSE GPG key at https://www.suse.com/support/security/contact/.
Common Criteria is the best known and most widely used methodology to evaluate and measure the security value of an IT product. The methodology aims to be independent, as an independent laboratory conducts the evaluation, which a certification body will certify afterward. Security Functional Requirements (SFR) are summarized in so-called Protection Profiles (PP). If the definition of a Security Target (ST) and the Evaluation Assurance Levels (EAL) are comparable, this allows the comparison of security functions of different products. (The definition of a Security Target typically references the PP—if one exists that fits the purpose of the product.)
A clear definition of security in IT products is challenging. Security should be considered a process that never ends, not a static condition that can be met or not. A Common Criteria certificate (below EAL7) does not make a clear statement about the error-proneness of the system, but it adds an important value to the product that cannot be described with the presence of technology alone: That someone has independently inspected the design of the system in such way that it corresponds to the claims that are made, and that explicit care has been taken in producing and maintaining the product.
The certificate states a degree of maturity of both the product with its security functions and the processes of the company that has designed, built and engineered the product, and that will maintain the product across its lifecycle. As such, Common Criteria aims to be fairly holistic with its approach to take everything into account that is relevant for the security of an IT product.
The Evaluation Assurance Level denotes the degree of confidence that the product fulfills the described claims. The levels are from 1 through 7:
EAL1: Functionally tested
EAL2: Structurally tested
EAL3: Methodically tested and checked
EAL4: Methodically designed, tested and reviewed
EAL5: Semi-formally designed and tested
EAL6: Semi-formally verified design and tested
EAL7: Formally verified design and tested
While EAL1 only provides basic assurance for products to meet security requirements, EAL2 to 4 are medium assurance levels. EAL5-EAL7 describe medium-to-high and high assurance. EAL4 is expected to be the highest level of assurance that a product can have if it has not been designed from the start to achieve a higher level of assurance.
Much of the advice in this guide is based on the following guidelines. Consider them when defining your own security processes or deciding about configurations that are not explicitly covered here.
Refer to the , the limitations of cryptography are briefly outlined.
Be aware that cryptography is certainly useful, but only for the specific purposes that it is good for. Using cryptography is not a generic recipe for better security in a system, its use may even impose additional risk on the system. Make informed decisions about the use of cryptography, and feel obliged to have a reason for your decisions. A false sense of security can be more harmful than the weakness itself.
openSUSE Leap supports encryption for:
Network connections (the
openssl
command,
stunnel
), for remote login
(openssh
, man ssh(1)
)
Files (gpg
)
Entire file systems at block layer
(dm-crypt
, cryptsetup
)
VPN (ipsec
, openvpn
)
It is useful to restrict the installed packages in your system to a minimum. Binaries not installed cannot be executed.
During installation of the system, you can limit the set of packages that
is installed. For example, you can deselect all packages and select only
those that you want to use. For example, the selection of the
apache2-mod_perl
package in YaST would automatically cause all packages to be selected
for installation that are needed for the Apache package to operate.
Dependencies have often been artificially cut down to handle the system's
dependency tree more flexibly. You can chose the minimal system, and
build the dependency tree from there with your (leaf) package selection.
Whenever possible, a server should be dedicated to serving exactly one service or application. This limits the number of other services that could be compromised if an attacker can successfully exploit a software flaw in one service (assuming that flaw allows access to others).
The use of AppArmor for services that are provided on a system is an
effective means of containment. For more information, see
Part IV, “Confining Privileges with AppArmor” and the man page of
apparmor
.
The use of virtualization technology is supported with openSUSE Leap. While virtualization is generally designed for server consolidation purposes, it is also usefulness for service isolation. However, virtualization technology cannot match or substitute the separation strength that is given by running services on different physical machines! Be aware that the capability of the hypervisor to separate virtual machines is not higher or stronger than the Linux kernel's capability to separate processes and their address spaces.
Doing regular backups and having a fingerprint of your system is vital, especially in the case of a successful attack against your system. Make it an integral part of your security routine to verify that your backups work.
A fast and directly accessible backup adds confidence about
the integrity of your system. However, it is important that the backup
mechanism/solution has adequate versioning support so that you can
trace changes in the system. As an example: The installation times of
packages (rpm
-q
--queryformat='%{INSTALLTIME} %{NAME}\n'
PACKAGE NAME) must correspond to the changed
files in the backup log files.
Several tools exist on openSUSE Leap 15.2 which can be used for the detection of unknown, yet successful attacks. It does not take much effort to configure them.
In particular, we recommend using the file and directory integrity checker
AIDE
(Advanced Intrusion Detection Environment).
When run for initialization, it creates a hash database of all files in the system that
are listed in its configuration file. This allows verifying the integrity
of all cataloged files at a later time.
If you use AIDE, copy the hash database to a place that is inaccessible for potential attackers. Otherwise, the attacker may modify the integrity database after planting a backdoor, thereby defeating the purpose of the integrity measurement.
An attacker may also have planted a backdoor in the kernel. Apart from being very hard to detect, the kernel-based backdoor can effectively remove all traces of the system compromise, so system alterations become almost invisible. Consequently, an integrity check needs to be done from a rescue system (or any other independent system with the target system's file systems mounted manually).
Be aware that the application of security updates invalidates the integrity
database. rpm
-qlv packagename
lists the files that are contained in a package. The RPM subsystem is
very powerful with the data that it maintains. It is accessible with the
--queryformat
command line option. A differential
update of integrity database with the changed files becomes more
manageable with some fine-grained usage of RPM.
The Common Criteria evaluations inspect a specific configuration of the product in an evaluated setup. How to install and configure the reference system that was used as baseline in the Common Criteria evaluation is documented in the “Administrator's Guide” part of the Common Criteria evaluation documentation.
However, it would be incorrect to understand the evaluated configuration as a hardened configuration. The removal of setuid bits and the prescription of administrative procedures after installation help to reach a specific configuration that is sane. But this is not sufficient for a hardening claim.
For more information about openSUSE Leap security certifications and features, see https://www.suse.com/support/security/certifications/.
Find a list of SUSE security resources at https://www.suse.com/support/security/.
Apart from the documentation that comes with the Common Criteria effort, see also the following manual pages:
pam(8), pam(5) |
apparmor(7) and referred man pages |
rsyslogd(8), syslog(8), syslogd(8) |
fstab(5), mount(8), losetup(8), cryptsetup(8) |
haveged(8), random(4) |
ssh(1), sshd(8), ssh_config(5), sshd_config(5), ssh-agent(1), ssh-add(1), ssh-keygen(1) |
cron(1), crontab(5), at(1), atd(8) |
systemctl(1), daemon(7), systemd.unit(5), systemd.special(5), kernel-command-line(7), bootup(7), systemd.directives |
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 system-wide basis, so they can be requested by any application. This chapter describes how the modular authentication mechanism works and how it is configured.
When 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 Book “Reference”, Chapter 22 “Sharing File Systems with NFS”.
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.”
Whereas Kerberos is used for authentication, LDAP is used for authorization and identification. Both can work together. For more information about LDAP, see Chapter 6, LDAP—A Directory Service, and about Kerberos, see Chapter 7, Network Authentication with Kerberos.
The Lightweight Directory Access Protocol (LDAP) is a protocol 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 LDAP works.
Kerberos is a network authentication protocol which also provides encryption. This chapter describes how to set up Kerberos and integrate services like LDAP and NFS.
Active Directory* (AD) is a directory-service based on LDAP, Kerberos, and other services. It is used by Microsoft* Windows* to manage resources, services, and people. In a Microsoft Windows network, Active Directory provides information about these objects, restricts access to them, and enforces po…
The RADIUS (Remote Authentication Dial-In User Service) protocol has long been a standard service for manage network access. It performs authentication, authorization, and accounting (AAA) protocol for very large businesses such as Internet service providers and cellular network providers, and is al…
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 system-wide basis, so they can be requested by any application. This chapter describes how the modular authentication mechanism works and how it is configured.
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. However, this process is 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.
PAM can be configured in two ways:
/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 Leap.
/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:
TYPE CONTROL MODULE_PATH MODULE_ARGS
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:
auth
Check the user's authenticity, traditionally by querying a password. However, this can also be achieved with a chip card or through biometrics (for example, fingerprints or iris scan).
account
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 user name of an expired account.
password
The purpose of this type of module is to enable the change of an authentication token. Usually this is a password.
session
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:
required
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.
requisite
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, 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.
sufficient
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.
optional
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.
include
If this flag is given, the file specified as argument is inserted at this place.
Contains a full file name 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® Leap, 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 simplify 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.
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 also install the 32-bit version of the PAM modules.
Consider the PAM configuration of sshd as an example:
/etc/pam.d/sshd
) ##%PAM-1.0 1 auth requisite pam_nologin.so 2 auth include common-auth 3 account requisite pam_nologin.so 2 account include common-account 3 password include common-password 3 session required pam_loginuid.so 4 session include common-session 3 session optional pam_lastlog.so silent noupdate showfailed 5
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 | |
Refers to the configuration files of four module types:
| |
Sets the login UID process attribute for the process that was authenticated. | |
Displays information about the last login of a user. |
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 needed 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:
pam_env.so
,
pam_gnome_keyring.so
and
pam_unix.so
. See
Example 3.2, “Default Configuration for the auth
Section (common-auth
)”.
auth
Section (common-auth
) #auth required pam_env.so 1 auth optional pam_gnome_keyring.so 2 auth required pam_unix.so try_first_pass 3
| |
| |
|
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.
When all modules of the auth
type have been
successfully processed, another include statement is processed, in this
case, that in Example 3.3, “Default Configuration for the account
Section (common-account
)”.
common-account
contains only one module,
pam_unix
. If pam_unix
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 3.4, “Default Configuration for the password
Section (common-password
)”.
account
Section (common-account
) #account required pam_unix.so try_first_pass
password
Section (common-password
) #password requisite pam_cracklib.so password optional pam_gnome_keyring.so use_authtok password required pam_unix.so use_authtok nullok shadow try_first_pass
Again, the PAM configuration of
sshd
involves only 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_cracklib
module. The pam_unix
module used afterward carries
over any old and new passwords from pam_cracklib
, 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_cracklib
. Whenever the
account
or the auth
type are
configured to complain about expired passwords, the
password
modules should also be used.
session
Section (common-session
) #session required pam_limits.so session required pam_unix.so try_first_pass session optional pam_umask.so session optional pam_systemd.so session optional pam_gnome_keyring.so auto_start only_if=gdm,gdm-password,lxdm,lightdm session optional pam_env.so
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_unix
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.
Some 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
.
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:
VARIABLE [DEFAULT=VALUE] [OVERRIDE=VALUE]
Name of the environment variable to set.
[DEFAULT=<value>]
Default VALUE the administrator wants to set.
[OVERRIDE=<value>]
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 3.6, “pam_env.conf”.
REMOTEHOST DEFAULT=localhost OVERRIDE=@{PAM_RHOST} DISPLAY DEFAULT=${REMOTEHOST}:0.0 OVERRIDE=${DISPLAY}
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
.
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 for
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 Book “Reference”, Chapter 21 “Samba”, Section 21.5 “Configuring Clients”.
› › to add the file server; see
LUKS2 support was added to cryptsetup
2.0, and openSUSE Leap has included support for LUKS2 in
pam_mount
since openSUSE Leap
42.3.
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, 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
.
The pam-config
tool helps you configure the global PAM
configuration files (/etc/pam.d/common-*
) and
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:
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
.
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.
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 the systemd
journal (see
Book “Reference”, Chapter 11 “journalctl
: Query the systemd
Journal”).
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 command lists both the type and
the options for the queried PAM module.
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)
.
If you prefer to manually create or maintain your PAM configuration
files, 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.
pam_systemd.so
in Configuration
If you are creating your own PAM configuration, make sure to include
pam_systemd.so
configured as session
optional
. Not including the pam_systemd.so
can
cause problems with systemd
task limits. For details, refer to the man
page of pam_systemd.so
.
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.
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.
This document summarizes the topic from the developer's point of view, with information about how to write standard-compliant PAM modules.
This document comprises everything needed by an application developer who wants to use the PAM libraries.
PAM in general and the individual modules come with manual pages that provide a good overview of the functionality of all the components.
When 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 Book “Reference”, Chapter 22 “Sharing File Systems with NFS”.
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.”
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.
To configure just one NIS server for your network, proceed with Section 4.1.1, “Configuring a NIS Master Server”.
If your NIS master server needs to export its data to slave servers, set up the master server as described in Section 4.1.1, “Configuring a NIS Master Server” and set up slave servers in the subnets as described in Section 4.1.2, “Configuring a NIS Slave Server”.
To manage the NIS Server functionality with YaST, install the yast2-nis-server
package by running the zypper in yast2-nis-server
command as root. To configure a NIS master server for your network, proceed as follows:
Start
› › .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
. 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
.Determine basic NIS setup options:
Enter the NIS domain name.
Define whether the host should also be a NIS client (enabling users to log in and access data from the NIS server) by selecting
.If your NIS server needs to act as a master server to NIS slave servers in other subnets, select
.The option
is only useful with . It speeds up the transfer of maps to the slaves.
Select yppasswd
). This makes the options
and 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
.
Select
to have YaST adapt the firewall settings for the NIS server.Leave this dialog with
or click to make additional settings.
/etc
by default).
In addition, passwords can be merged here. The setting should be
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
to confirm your settings and return to the
previous screen.
If you previously enabled
, enter the host names used as slaves and click . If no slave servers exist, this configuration step is skipped.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 .
Check which maps should be available and click
to continue.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:
255.0.0.0 127.0.0.0 0.0.0.0 0.0.0.0
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.
Click
to save your changes and exit the setup.To configure additional NIS slave servers in your network, proceed as follows:
Start
› › .Select
and click .If NIS server software is already installed on your machine, initiate the creation of a NIS slave server by clicking
.Complete the basic setup of your NIS slave server:
Enter the NIS domain.
Enter host name or IP address of the master server.
Set
if you want to enable user logins on this server.Adapt the firewall settings with
.Click
.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:
255.0.0.0 127.0.0.0 0.0.0.0 0.0.0.0
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.
Click
to save changes and exit the setup.To use NIS on a workstation, do the following:
Start
› › .Activate the
button.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 Book “Reference”, Chapter 20 “DHCP”.
Enter your NIS servers and separate their addresses by spaces. If you do not know your NIS server, click
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. asks for a NIS server in the local network after the specified servers fail to respond.Depending on your local installation, you may also want to activate the automounter. This option also installs additional software if required.
If you do not want other hosts to be able to query which server your
client is using, go to the man
ypbind
.
Click
to save them and return to the YaST control center. Your client is now configured with NIS.Whereas Kerberos is used for authentication, LDAP is used for authorization and identification. Both can work together. For more information about LDAP, see Chapter 6, LDAP—A Directory Service, and about Kerberos, see Chapter 7, Network Authentication with Kerberos.
YaST allows setting up authentication to clients using different modules:
Use both an identity service (usually LDAP) and a user authentication service (usually Kerberos). This option is based on SSSD and in the majority of cases is best suited for joining Active Directory domains. .
This module is described in Section 8.3.2, “Joining Active Directory Using . ”
Join an Active Directory (which entails use of Kerberos and LDAP). This option is
based on . winbind
and is best suited for joining an
Active Directory domain if support for NTLM or cross-forest trusts is necessary.
This module is described in Section 8.3.3, “Joining Active Directory Using . ”
Allows setting up LDAP identities and Kerberos authentication independently from each other and provides fewer options. While this module also uses SSSD, it is not as well suited for connecting to Active Directory as the previous two options. .
This module is described in:
Two of the YaST modules are based on SSSD:
and .SSSD stands for System Security Services Daemon. SSSD talks to remote directory services that provide user data and provides various authentication methods, such as LDAP, Kerberos, or Active Directory (AD). It also provides an NSS (Name Service Switch) and PAM (Pluggable Authentication Module) interface.
SSSD can locally cache user data and then allow users to use the data, even if the real directory service is (temporarily) unreachable.
After running one of the YaST authentication modules, you can check whether SSSD is running with:
root #
systemctl status sssd
sssd.service - System Security Services Daemon Loaded: loaded (/usr/lib/systemd/system/sssd.service; enabled) Active: active (running) since Thu 2015-10-23 11:03:43 CEST; 5s ago [...]
To allow logging in when the authentication back-end is unavailable, SSSD will use its cache even if it was invalidated. This happens until the back-end is available again.
To invalidate the cache, run sss_cache -E
(the
command sss_cache
is part of the package
sssd-tools).
To completely remove the SSSD cache, run:
tux >
sudo
systemctl stop sssd
tux >
sudo
rm -f /var/lib/sss/db/*
tux >
sudo
systemctl start sssd
The Lightweight Directory Access Protocol (LDAP) is a protocol 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 LDAP works.
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 such as LDAP ensures that as many 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. The type of data stored in a directory tends to be long lived and changes infrequently. This allows the LDAP service to be optimized for high performance concurrent reads, whereas conventional databases are optimized for accepting many writes to data in a short time.
This section introduces the layout of an LDAP directory tree, and provides the basic terminology used with regard to LDAP. If you are familiar with LDAP, read on at Section 6.3, “Manually Configuring a 389 Directory 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. An object in the tree is identified by its relative distinguished name (RDN). The distinguished name is built from the RDNs of all entries on the path to the entry.
The relations within an LDAP directory tree become more evident in the following example, shown in Figure 6.1, “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 may be assigned. The Schema contains all object classes and attributes which can be used by the LDAP server. Attributes are a structured data type. Their syntax, ordering and other behavior is defined by the Schema. LDAP servers supply a core set of Schemas which can work in a broad variety of environments. If a custom Schema is required, you can upload it to an LDAP server.
Table 6.1, “Commonly Used Object Classes and Attributes” offers a small overview of the object
classes from 00core.ldif
and
06inetorgperson.ldif
used in the example, including
required attributes (Req. Attr.) and valid attribute values. After installing
389-ds
, these can be found in
usr/share/dirsrv/schema
.
Object Class |
Meaning |
Example Entry |
Req. Attr. |
---|---|---|---|
|
name components of the domain |
example |
displayName |
|
organizational unit |
doc |
ou |
|
person-related data for the intranet or Internet |
Geeko Linux |
cn |
Example 6.1, “Excerpt from CN=schema” shows an excerpt from a Schema directive with explanations.
attributetype (1.2.840.113556.1.2.102 NAME 'memberOf' 1 DESC 'Group that the entry belongs to' 2 SYNTAX 1.3.6.1.4.1.1466.115.121.1.12 3 X-ORIGIN 'Netscape Delegated Administrator') 4 objectclass (2.16.840.1.113730.3.2.333 NAME 'nsPerson' 5 DESC 'A representation of a person in a directory server' 6 SUP top STRUCTURAL 7 MUST ( displayName $ cn ) 8 MAY ( userPassword $ seeAlso $ description $ legalName $ mail \ $ preferredLanguage ) 9 X-ORIGIN '389 Directory Server Project’ ...
The name of the attribute, its unique object identifier (OID, numerical), and the abbreviation of the attribute. | |
A brief description of the attribute with | |
The type of data that can be held in the attribute. In this case, it is a case-insensitive directory string. | |
The source of the schema element (for example, the name of the project). | |
The definition of the object class | |
A brief description of the object class. | |
The | |
With | |
With |
The 389-ds
package contains the 389 Directory Server and the
administration tools. If the package is not installed yet, install it with
the following command:
tux >
sudo
zypper install 389-ds
After installation, you can set up the server either manually (as described in Section 6.3) or create a very basic setup with YaST (as described in Section 6.4).
Setting up the 389 Directory Server takes the following basic steps:
The 389 Directory Server is controlled by three primary commands:
dsctl
Manages a local instance and requires root
permissions. Requires you
to be connected to a terminal which is running the directory server instance.
Used for starting, stopping, backing up the database, and more.
dsconf
The primary tool used for administration and configuration of the server. Manages an instance's configuration via its external interfaces. This allows you to make configuration changes remotely on the instance.
dsidm
Used for identity management (managing users, groups, passwords etc.). The permissions are granted by access controls, so, for example, users can reset their own password or change details of their own account.
You create the instance with the dscreate
command.
It can take a configuration file (*.inf
) which
defines the instance configuration settings. Alternatively, the command can
be run interactively.
If not specified otherwise, the default instance name is
localhost
. The instance name cannot
be changed after the instance has been created.
To check the name of an instance you have not created yourself, use
the dsctl -l
command.
Example 6.2 shows an
example configuration file that you can use as a starting point. Alternatively,
use dscreate create-template
to create a template
*.inf
file. The template is commented and pre-filled,
so you can adjust its variables to your needs. For more details, see the
man page of dscreate
.
If you want to set up a trial instance, start an editor and save the
following as /tmp/instance.inf
:
# /tmp/instance.inf
[general]
config_version = 2
[slapd]
root_password = YOUR_PASSWORD_FOR_CN=DIRECTORY_MANAGER1
[backend-userroot]
sample_entries = yes
suffix = dc=example,dc=com
Set the |
To create the 389 Directory Server instance from Example 6.2, run:
tux >
sudo
dscreate from-file /tmp/instance.inf
This creates a working LDAP server.
If dscreate
should fail, the messages will tell you why.
For more details, repeat the command with the -v
option:
tux >
sudo
dscreate -v from-file /tmp/instance.inf
Check the status of the server with:
tux >
sudo
dsctl localhost status instance 'Localhost' is running
In case you want to delete the instance later on:
tux >
sudo
dsctl localhost remove --do-it
With this command, you can also remove partially installed or corrupted instances.
You can manage the CA certificates for 389 Directory Server with the following command
line tools: certutil
, openssl
, and
pk12util
.
For testing purposes, you can create a self-signed certificate with
dscreate
. Find the certificate at
/etc/dirsrv/slapd-localhost/ca.crt
. For remote administration,
copy the certificate to a readable location. For production environments,
contact a CA authority of your organization's choice and request a server
certificate, a client certificate, and a root certificate.
Make sure to meet the following requirements before executing the procedure below:
You have a server certificate and a private key to use for the TSL connection.
You have set up an NSS (Network Security Services) database (for example,
with the certutil
command).
Before you can import an existing private key and certificate into the NSS
(Network Security Services) database, you need to create a bundle of the
private key and the server certificate. This results in a *.p12
file.
*.p12
File and Friendly Name
When creating the PKCS12 bundle, you must encode a friendly name
in the *.p12
file.
Make sure to use Server-Cert
as the friendly name. Otherwise
the TLS connection will fail, because the 389 Directory Server searches for this exact string.
Keep in mind that the friendly name cannot be changed after you
import the *.p12
file into the NSS
database.
Use the following command to create the PKCS12 bundle with the required friendly name:
root #
openssl pkcs12 -export -in SERVER.crt \
-inkey SERVER.key -out SERVER.p12 \
-name Server-Cert
Replace SERVER.crt with the server certificate
and SERVER.key with the private key to be bundled.
With -out
, specify the name of the *.p12
file. Use -name
to set the friendly name to use:
Server-Cert
.
Before you can import the file into the NSS database, you need to obtain its password. To do this, use the following command:
pk12util -i PATH_TO_SERVER.p12 -d sql:PATH_TO_NSS_DB -n Server-cert -W SERVER.p12_PASSWORD
You can then find the password in the
pwdfile.txt
file in the
PATH_TO_NSS_DB directory.
Now import the SERVER.p12 file into your NSS database:
pk12util -i SERVER.p12 -d PATH_TO_NSS_DB
For remote or local administration of the 389 Directory Server, you can create a
.dsrc
configuration file in your home directory. This
saves you typing your user name and connection details with every command.
Example 6.3 shows an example configuration file for remote
administration, whereas Example 6.4 shows one for local administration.
.dsrc
File for Remote Administration ## cat ~/.dsrc [localhost] uri = ldaps://REMOTE_URI 1 basedn = dc=example,dc=com binddn = cn=Directory Manager tls_cacertdir = PATH_TO_CERTDIR 2
If you want to administer the instance on the same host where the 389 Directory Server runs, use the configuration file in Example 6.4.
.dsrc
File for Local Administration ## cat ~/.dsrc
[localhost]
# Note that '/' is replaced with '%%2f'.
uri = ldapi://%%2fvar%%2frun%%2fslapd-localhost.socket 1
basedn = dc=example,dc=com
binddn = cn=Directory Manager
When using |
Users and groups can be created and managed with the dsidm
command. It either runs interactively or you can use it with arguments from
the command line.
In the following example, we add two users, wilber
and geeko
,
by specifying their data via command-line arguments.
Create the user wilber
:
tux >
sudo
dsidm
localhost user create --uidwilber
\ --cnwilber
--displayName 'Wilber Fox' --uidNumber 1000 --gidNumber 1000 \ --homeDirectory /home/wilber
To look up a user's distinguished name
(fully qualified
name to the directory object, which is guaranteed unique):
tux >
sudo
dsidm localhost user getwilber
dn: uid=wilber
,ou=people,dc=example,dc=com [...]
The system prompts you for the directory server root
user password
(unless you configured remote or local access as described in Section 6.3.3, “Configuring Admin Credentials for Remote/Local Access”).
You need the distinguished name for actions such as changing the password for a user.
To set or change the password for wilber
:
tux >
sudo
dsidm localhost account reset_password \ uid=wilber
,ou=people,dc=example,dc=com
The system prompts you for the directory server root
user password
(unless you configured remote or local access as described in Section 6.3.3, “Configuring Admin Credentials for Remote/Local Access”).
Enter the new password for wilber
twice.
If the action was successful, you get the following message:
reset password for uid=wilber
,ou=people,dc=example,dc=com
Create the user geeko
:
tux >
sudo
dsidm
localhost user create --uid \ --cngeeko
--displayName 'Suzanne Geeko' \ --uidNumber 1001 --gidNumber 1001 --homeDirectory /home/geeko
In the following, we create a group, server_admins
, and assign the user
wilber
to this group.
Create the group:
tux >
sudo
dsidm localhost group create
You will be prompted for a group name:
Enter value for cn :
Enter the name for the group, for example: server_admins
.
Add the user wilber
to the group:
tux >
sudo
dsidm localhost group add_member server_admins uid=wilber
,ou=people,dc=example,dc=com added member: uid=wilber
,ou=people,dc=example,dc=com
Verify if authentication works:
tux >
sudo
ldapwhoami -H ldaps://localhost -D \ uid=wilber
,ou=people,dc=example,dc=com -W -x
If you are prompted for the LDAP password of wilber
, authentication works.
If the command fails with the following error, you are probably using a self-signed certificate:
ldap_sasl_bind(SIMPLE): Can't contact LDAP server (-1)
In that case, edit /etc/openldap/ldap.conf
and add the
path to the certificate. For example:
TLS_CACERT /etc/dirsrv/slapd-localhost/ca.crt
Alternatively, include the path to the certificate in the
whoami
command:
tux >
sudo
LDAPTLS_CACERT=/etc/dirsrv/slapd-localhost/ca.crt \ ldapwhoami -H ldaps://localhost -D \ uid=wilber
,ou=people,dc=example,dc=com -W -x
SSSD (System Security Services Daemon) is a daemon that communicates with
remote identity providers and allows pam
and
nsswitch
to consume that data. SSSD can have multiple
back-ends, cache users and groups and provides features like SSH key
distributions.
On a separate server, install the sssd
package:
tux >
sudo
zypper in sssd
Disable and stop the nscd
daemon
because it conflicts with sssd
:
tux >
sudo
systemctl disable nscd && systemctl stop nscd
Create the SSSD configuration and restrict the login to the members of the group server_admins
that we created in Procedure 6.2:
The memberOf
plugin needs to be enabled, so that
clients can log in and authorise.
tux >
sudo
dsidm localhost client_config sssd.conf server_admins
Review the output and paste (or redirect) it to /etc/sssd/sssd.conf
.
If required, edit the configuration file according to your needs.
To configure the certificates on your client, copy ca.crt
from the LDAP server to your client:
tux >
sudo
mkdir -p /etc/openldap/certs cp [...]>/ca.crt /etc/openldap/certs/ /usr/bin/c_rehash /etc/openldap/certs
Enable and start SSSD:
tux >
sudo
systemctl enable sssd systemctl start sssd
To make sure SSSD is part of PAM and NSS, follow the instructions in sections Configure PAM (SUSE) and Configure NSS (SUSE) at http://www.port389.org/docs/389ds/howto/howto-sssd.html.
Verify if the client can provide the details for user wilber
:
tux >
sudo
idwilber
uid=1000(wilber
) gid=100(users) groups=100(users)
If everything is set up correctly, wilber
can access the 389 Directory Server
instance via SSH to the machine where you have installed and configured
SSSD. However, geeko
will fail to do so, because geeko
does not belong to the group server_admins
that we have
configured in Procedure 6.2.
You can use YaST to quickly create a very basic setup of the 389 Directory Server.
In YaST, click yast2 ldap-server
.
In the window that opens, you need to fill in all mandatory text fields.
Enter the
of the 389 Directory Server. It must be resolvable from the host.In
, enter a local name for the LDAP server instance.
The instance name cannot be changed after the instance
has been created. If you plan for only one LDAP server, use the default
instance name localhost
. However, if you plan to host
multiple LDAP servers, use meaningful names for the individual instances.
In example.com
becomes dc=example,dc=com
.
In the mandatory security options, enter the password for the directory manager (LDAP's root/admin account) and repeat the password in the next step. The password must be at least 8 characters long.
To run 389 Directory Server with a CA certificate, specify both of the following options:
Enter the path to the
, with which the server certificates have been signed.
Enter the path to the *.p12
file contains the server's private key and
certificate. These must have been signed by the CA in PEM format that you
have specified above. The must be
Server-Cert
, see Section 6.3.2, “Using CA Certificates for TSL”
for details.
If you do not specify a CA certificate here, a self-signed certificate
will be created automatically. After the instance has been created,
find the related files in
/etc/dirsrv/slapd-INSTANCENAME
.
If you are ready to create the instance, click
.YaST displays a message stating whether the creation was successful and where to find the log files.
The setup with YaST provides only a very basic configuration of the 389 Directory Server. To fine-tune more settings, see Section 6.3, “Manually Configuring a 389 Directory Server” or the documentation mentioned in Section 6.6, “For More Information”.
YaST includes the module
that helps define authentication scenarios involving either LDAP or Kerberos.It can also be used to join Kerberos and LDAP separately. However, in many such cases, using this module may not be the first choice, such as for joining Active Directory (which uses a combination of LDAP and Kerberos). For more information, see Section 5.1, “Configuring an Authentication Client with YaST”.
Start the module by selecting
› .To configure an LDAP client, follow the procedure below:
In the window
, click .Make sure that the tab
is chosen.
Specify one or more LDAP server URLs or host names under
LDAPS://
URLs only. When specifying
multiple addresses, separate them with spaces.
Specify the appropriate LDAP distinguished name (DN) under
dc=example,dc=com
.
If your LDAP server supports TLS encryption, choose the appropriate security option under
.To first ask the server whether it supports TLS encryption and be able to downgrade to an unencrypted connection if it does not, use
.Activate other options as necessary:
You can
and on the local computer for them.If you want to cache LDAP entries locally, use
.Using the cache incurs security risks, depending on the mechanism used.
If you define an authorization rule (for example, members of group
admin
can log in), and you remove a user from that
group, the client cache will not see that change until the cache expires
or refreshes. So a user whose account has been revoked can still log in
later.
This caching mechanism constantly checks if group memberships are still
valid. Thus the cache risk only exists if the sssd
daemon is disconnected from the LDAP server for any reason.
Specify the types of data that should be used from the LDAP source, such as
and , , and (network-shared drives that can be automatically mounted on request).Specify the distinguished name (DN) and password of the user under whose name you want to bind to the LDAP directory in
and .Otherwise, if the server supports it, you can also leave both text boxes empty to bind anonymously to the server.
When using authentication without enabling transport encryption using TLS or StartTLS, the password will be transmitted in the clear.
Under
, you can additionally configure timeouts for BIND operations.To check whether the LDAP connection works, click
.To leave the dialog, click
. Then wait for the setup to complete.Finally, click
.
The command line tools provided by the openldap2-client
package (like ldapsearch
or ldapmodify
)
can be used for administration of data in the LDAP directory. However, they
are low-level tools and hard to use. For details about their use, refer to
the respective man pages and documentation.
For more information about 389 Directory Server, see the upstream documentation, available at http://www.port389.org/docs/389ds/documentation.html.
Kerberos is a network authentication protocol which also provides encryption. This chapter describes how to set up Kerberos and integrate services like LDAP and NFS.
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.
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:
USER/INSTANCE@REALM
primary: The first part of the principal. In the case of users, this is usually the same as the user name.
instance (optional):
Additional information characterizing the
primary. This string is separated from the
primary by a /
.
tux@example.org
and
tux/admin@example.org
can both exist on the same
Kerberos system and are treated as different principals.
realm: Specifies the Kerberos realm. Normally, your realm is your domain name in uppercase letters.
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 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.
Service is used to refer to a specific action to perform. The process behind this action is called a server.
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
.
Your first contact with Kerberos is quite similar to any login procedure at a normal networking system. Enter your user name. 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.
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.
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.
Tickets are designed to be used for one server at a time. Therefore, you need 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.
Ideally, a user 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 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 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 need 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 and GDM offer Kerberos
support, too. Read more about the Kerberos network applications in
Kerberos V5 UNIX User's Guide at
http://web.mit.edu/kerberos.
A Kerberos environment consists of several 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:
Check your network setup and make sure it meets the minimum requirements outlined in Section 7.5.1, “Kerberos Network Topology”. Choose an appropriate realm for your Kerberos setup, see Section 7.5.2, “Choosing the Kerberos Realms”. Carefully set up the machine that is to serve as the KDC and apply tight security, see Section 7.5.3, “Setting Up the KDC Hardware”. Set up a reliable time source in your network to make sure all tickets contain valid time stamps, see Section 7.5.4, “Configuring Time Synchronization”.
Configure the KDC and the clients, see Section 7.5.5, “Configuring the KDC” and Section 7.5.6, “Configuring Kerberos Clients”. Enable remote administration for your Kerberos service, so you do not need physical access to your KDC machine, see Section 7.5.7, “Configuring Remote Kerberos Administration”. Create service principals for every service in your realm, see Section 7.5.8, “Creating Kerberos Service Principals”.
Various services in your network can use Kerberos. To add Kerberos password-checking to applications using PAM, proceed as outlined in Section 7.5.9, “Enabling PAM Support for Kerberos”. To configure SSH or LDAP with Kerberos authentication, proceed as outlined in Section 7.5.10, “Configuring SSH for Kerberos Authentication” and Section 7.5.11, “Using LDAP and Kerberos”.
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 Book “Reference”, Chapter 19 “The Domain Name System” 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 Book “Reference”, Chapter 18 “Time Synchronization with NTP” 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 only the minimum components needed to build a Kerberos infrastructure. Depending on the size and topology of your deployment, your setup may vary.
For a setup similar to the one in Figure 7.1, “Kerberos Network Topology”, configure routing between the two subnets (192.168.1.0/24 and 192.168.2.0/24). Refer to Book “Reference”, Chapter 13 “Basic Networking”, Section 13.4.1.5 “Configuring Routing” for more information on configuring routing with YaST.
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
example.com
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. So if you have a
realm named EXAMPLE.COM
with two
“subrealms” named DEVELOPMENT
and
ACCOUNTING
, these subordinate realms do not inherit
principals from EXAMPLE.COM
. Instead, you would have
three separate realms, and you would need to configure cross-realm
authentication for each realm, so that users from one realm can interact
with servers or other users from another realm.
For the sake of simplicity, let us assume you are setting up only one realm
for your entire organization. For the remainder of this section, the realm
name EXAMPLE.COM
is used in all examples.
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:
Put the server machine into a physically secured location, such as a locked server room to which only a very few people have access.
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.
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, plus anything
X-based. Even installing an SSH server should be considered a potential
security risk.
No graphical login is provided on this machine as an X server is a potential security risk. Kerberos provides its own administration interface.
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).
Disable all user accounts except root
's account by editing
/etc/shadow
and replacing the hashed passwords with
*
or !
characters.
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 during 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
by running an NTP daemon chronyd
as a client on all these machines. 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 chronyd -q
via a cron job. To configure your machine as an NTP client, proceed as
outlined in Book “Reference”, Chapter 18 “Time Synchronization with NTP”, Section 18.1 “Configuring an NTP Client with YaST”.
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 and 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 7.5.6.3, “Adjusting the Clock Skew”.
This section covers the initial configuration and installation of the KDC, including the creation of an administrative principal. This procedure consists of several steps:
Install the RPMs.
On a machine designated as the KDC, install the following software
packages: krb5
,
krb5-server
and
krb5-client
packages.
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. See
Section 7.5.5.1, “Configuring the Server”.
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 7.5.5.2, “Setting Up the Database” for details.
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 them to manage the database. The Kerberos
ACL file is located under
/var/lib/kerberos/krb5kdc/kadm5.acl
. Refer to
Section 7.5.7, “Configuring Remote Kerberos Administration” for details.
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 7.5.5.3, “Creating a Principal” for details.
Start the Kerberos Daemon. After the KDC software is installed and properly configured, start the Kerberos daemon to provide Kerberos service for your realm. Refer to Section 7.5.5.4, “Starting the KDC” for details.
Create a Principal for Yourself. You need a principal for yourself. Refer to Section 7.5.5.3, “Creating a Principal” for details.
Configuring a Kerberos server is highly variable, dependent on your network architecture, DNS and DHCP configuration, realms, and other considerations. You must have a default realm, and domain- to-realm mappings. The following example demonstrates a minimal configuration. This is not a copy-and-paste example; see https://web.mit.edu/kerberos/krb5-latest/doc/admin/conf_files/index.html for detailed information on Kerberos configuration.
/etc/krb5.conf
#[libdefaults] dns_canonicalize_hostname = false rdns = false default_realm = example.com ticket_lifetime = 24h renew_lifetime = 7d [realms] example.com = { kdc = kdc.example.com.:88 admin_server = kdc.example.com default_domain = example.com } [logging] kdc = FILE:/var/log/krb5kdc.log admin_server = FILE:/var/log/kadmind.log default = SYSLOG:NOTICE:DAEMON [domain_realm] .example.com = example.com example.com = example.com
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:
tux >
sudo
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
To verify, use the list command:
tux >
kadmin.localkadmin>
listprincs
You will see several principals in the database, which are for internal use by Kerberos:
K/M@EXAMPLE.COM kadmin/admin@EXAMPLE.COM kadmin/changepw@EXAMPLE.COM krbtgt/EXAMPLE.COM@EXAMPLE.COM
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:
tux >
kadmin.localkadmin>
ank geeko
You will see the following output:
geeko@EXAMPLE.COM's Password: 1 Verifying password: 2
Next, create another principal named
geeko/admin
by typing
ank
geeko/admin
at
the kadmin
prompt. The admin
suffixed to your user name is a role. Later, use this
role when administering the Kerberos database. A user can have several roles
for different purposes. Roles act like completely different accounts that
have similar names.
Start the KDC daemon and the kadmin daemon. To start the daemons manually, enter:
tux >
sudo
systemctl start krb5kdc sudo systemctl start kadmind
Also make sure that the services KDC (krb5kdc
)
and kadmind (kadmind
) are started by default when
the server machine is rebooted. Enable them by entering:
tux >
sudo
systemctl enable krb5kdc kadmind
or by using the YaST
.When the supporting infrastructure is in place (DNS, NTP) and the KDC has been properly configured and started, configure the client machines. To configure a Kerberos client, use one of the two manual approaches described below.
When configuring Kerberos, there are 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 host names 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 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 host names.
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
kdc.example.com
is the host
name of the KDC):
[libdefaults] default_realm = EXAMPLE.COM [realms] EXAMPLE.COM = { kdc = kdc.example.com admin_server = kdc.example.com }
The default_realm
line sets the default realm for Kerberos
applications. If you have several realms, add additional statements to the
[realms]
section.
Also add a statement to this file that tells applications how to map host
names 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 www.example.org = EXAMPLE.COM
This tells the library that all hosts in the
example.com
DNS domains are in the
EXAMPLE.COM
Kerberos realm. In addition, one external
host named www.example.org
should also be considered
a member of the EXAMPLE.COM
realm.
DNS-based Kerberos configuration makes heavy use of SRV records. See (RFC2052) A DNS RR for specifying the location of services at http://www.ietf.org.
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 host name. 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 kdc.example.com. _kerberos._tcp.EXAMPLE.COM. IN SRV 0 0 88 kdc.example.com.
This describes the location of the remote administration service. Typical records look like this:
_kerberos-adm._tcp.EXAMPLE.COM. IN SRV 0 0 749 kdc.example.com.
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 example.com
DNS
domain. This can be done by attaching a TXT record to
_kerberos.host_name
, as shown here:
_kerberos.www.example.org. IN TXT "EXAMPLE.COM"
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:
[libdefaults] clockskew = 60
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, grant yourself the privilege to administer the database by putting the following line into the file:
geeko/admin *
Replace the user name 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:
tux >
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
kadmin:
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
:
tux >
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
Attributes:
Policy: [none]
kadmin: modify_principal -maxlife "8 hours" geeko
Principal "geeko@EXAMPLE.COM" modified.
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 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
Attributes:
Policy: [none]
kadmin:
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 or refer to
the man
8 kadmin
manual page.
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 in the
Kerberos database for each service offered in the realm. For example, if
ldap.example.com offers an LDAP service, you need a service principal,
ldap/ldap.example.com@EXAMPLE.COM
, 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
host name.
Valid service descriptors are:
Service Descriptor |
Service |
---|---|
|
Telnet, RSH, SSH |
|
NFSv4 (with Kerberos support) |
|
HTTP (with Kerberos authentication) |
|
IMAP |
|
POP3 |
|
LDAP |
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, they needs to type their password, so Kerberos can decrypt the ticket. It would be inconvenient for system administrators 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 jupiter.example.com
enter
the following commands during your kadmin session:
tux >
kadmin -p geeko/admin
Authenticating as principal geeko/admin@EXAMPLE.COM with password.
Password for geeko/admin@EXAMPLE.COM:
kadmin: addprinc -randkey host/jupiter.example.com
WARNING: no policy specified for host/jupiter.example.com@EXAMPLE.COM;
defaulting to no policy
Principal "host/jupiter.example.com@EXAMPLE.COM" 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/jupiter.example.com Entry for principal host/jupiter.example.com with kvno 3, encryption type Triple DES cbc mode with HMAC/sha1 added to keytab WRFILE:/etc/krb5.keytab. Entry for principal host/jupiter.example.com with kvno 3, encryption type DES cbc mode with CRC-32 added to keytab WRFILE:/etc/krb5.keytab. kadmin:
When completed, make sure that you destroy the admin ticket obtained with
kinit above with kdestroy
.
An incomplete Kerberos configuration may completely lock you out of your
system, including the root user. To prevent this, add the
ignore_unknown_principals
directive to the
pam_krb5
module after you have added
the pam_krb5
module to the existing PAM configuration
files as described below.
tux >
sudo
pam-config --add --krb5-ignore_unknown_principals
This will direct the pam_krb5
module to ignore some
errors that would otherwise cause the account phase to fail.
openSUSE® Leap 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 GDM. That is, it can be used in all cases where the
user enters a password and expects the authenticating application to obtain
an initial Kerberos ticket on their behalf. To configure PAM support for
Kerberos, use the following command:
tux >
sudo
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.
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 sudo systemctl restart
sshd
.
To use Kerberos authentication with protocol version 2, enable it on the
client side as well. Do this either in the system-wide 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.
The file
/usr/share/doc/packages/openssh/README.kerberos
discusses the interaction of OpenSSH and Kerberos in more detail.
The GSSAPIKeyExchange
mechanism (RFC 4462) is
supported. This directive specifies how host keys are exchanged. For more
information, see the sshd_config manual page (man
sshd_config
).
While Kerberos provides authentication, LDAP is used for authorization and identification. Both services can work together.
For secure connections,389 Directory Server supports different ways of encrypting data:
SSL/TLS connections, Start TLS connections, and SASL authentication. Simple
Authentication and Security Layer (SASL) is a network protocol designed for
authentication. The SASL implementation used on openSUSE Leap is
cyrus-sasl
. Kerberos authentication is performed through
GSS-API (General Security Services API), provided by the
cyrus-sasl-gssapi package. Using GSS-API, 389 Directory Server uses
Kerberos tickets to authenticate sessions and encrypt data.
With the SASL framework you can use different mechanisms to authenticate a user to the server. In Kerberos, authentication is always mutual. This means that not only have you authenticated yourself to the 389 Directory Server, but also the 389 Directory Server has authenticated itself to you. In particular, this means communication is with the desired server, rather than with a random service set up by an attacker.
To enable Kerberos to bind to the 389 Directory Server, create a principal
ldap/ldap.example.com
and add that to the keytab. The
credentials used by the 389 Directory Server to authenticate are given to other servers
by the keytab. 389 Directory Server assigns a keytab through the
KRB5_KTNAME
environment variable.
To set the variable, proceed as follows:
tux >
sudo
systemctl edit dirsrv@INSTANCE
If you used the default name for the 389 Directory Server instance, replace
INSTANCE with localhost
.
Add the following:
[Service] Environment=KRB5_KTNAME=/etc/dirsrv/slapd-INSTANCE/krb5.keytab
The keytab file needs to be readable by the account under which the
389 Directory Server runs (for example, dirserv
):
tux >
sudo
chown dirsrv:dirsrv /etc/dirsrv/slapd-INSTANCE/krb5.keytabtux >
sudo
chmod 600 /etc/dirsrv/slapd-INSTANCE/krb5.keytab
To obtain and cache an initial ticket-granting ticket, use the principal that has been created in Section 7.5.5.3, “Creating a Principal”:
tux >
kinitgeeko
@EXAMPLE.COM
To check if GSSAPI authentication works, run:
tux >
ldapwhoami -Y GSSAPI -H ldap://ldapkdc.example.com
dn: uid=testuser,ou=People,dc=example,dc=com
GSSAPI uses the ccache
to authenticate the user to the
389 Directory Server without the user's password.
When processing a SASL bind request, the 389 Directory Server maps the SASL
authentication ID (used to authenticate to the Directory Server) with an
LDAP entry stored within the server. When using Kerberos, the SASL user ID
usually has the following format:
userid@REALM
,
such as tux
@example.com. This ID must be converted into the
DN of the user's Directory Server entry, such as
uid=tux,ou=people,dc=example,dc=com
.
The 389 Directory Server comes with some default maps for most common configurations.
However, you can create customized maps.
Procedure 7.1, “Managing Maps” shows how to list and display a
map, how to delete a map and how to create a custom map.
To list the existing SASL maps:
tux >
dsconf INSTANCE sasl list
Kerberos uid mapping
rfc 2829 dn syntax
rfc 2829u syntax
uid mapping
To display a map:
tux >
sudo
dsconf INSTANCE sasl get "Kerberos uid mapping" dn: cn=Kerberos uid mapping,cn=mapping,cn=sasl,cn=config cn: Kerberos uid mapping nsSaslMapBaseDNTemplate: dc=\2,dc=\3 nsSaslMapFilterTemplate: (uid=\1) nsSaslMapRegexString: \(.*\)@\(.*\)\.\(.*\) objectClass: top objectClass: nsSaslMapping
The default map only works if your dc has two components. To delete the map (if it does not work for you):
tux >
sudo
dsconf INSTANCE sasl delete "Kerberos uid mapping" Deleting SaslMapping cn=Kerberos uid mapping,cn=mapping,cn=sasl,cn=config : Successfully deleted cn=Kerberos uid mapping,cn=mapping,cn=sasl,cn=config
To create a new map:
tux >
sudo
dsconf localhost sasl create --cn=bhgssapi --nsSaslMapRegexString "\ (.*\)@EXAMPLE.NET.DE" --nsSaslMapBaseDNTemplate="dc=example,dc=net,dc=de" --nsSaslMapFilterTemplate="(uid=\1)"tux >
sudo
Enter value for nsSaslMapPriority : Successfully created bhgssapi
Display the newly created map with:
tux >
sudo
dsconf localhost sasl get "bhgssapi" dn: cn=bhgssapi,cn=mapping,cn=sasl,cn=config cn: bhgssapi nsSaslMapBaseDNTemplate: dc=example,dc=net,dc=de nsSaslMapFilterTemplate: (uid=\1) nsSaslMapPriority: 100 nsSaslMapRegexString: \(.*\)@EXAMPLE.NET.DE objectClass: top objectClass: nsSaslMapping
With this, you can check only the users of a specific realm and remap
them to a different dc base. As you can see, the new map has 3
dc
components, so the default maps would not have
worked for this realm (EXAMPLE.NET.DE
), only for a
realm like EXAMPLE.NET
.
YaST includes the module
that helps define authentication scenarios involving either LDAP or Kerberos.It can also be used to join Kerberos and LDAP separately. However, in many such cases, using this module may not be the first choice, such as for joining Active Directory (which uses a combination of LDAP and Kerberos). For more information, see Section 5.1, “Configuring an Authentication Client with YaST”.
Start the module by selecting
› .To configure a Kerberos client, follow the procedure below:
In the window
, click .Choose the tab
.Click
.In the appearing dialog, specify the correct
. Usually, the realm name is an uppercase version of the domain name. Additionally, you can specify the following:To apply mappings from the realm name to the domain name, activate
and/or .You can specify the
, the and additional .
All of these items are optional if they can be automatically discovered
via the SRV
and TXT
records in
DNS.
To manually map Principals to local user names, use
.
You can also use auth_to_local
rules to supply such
mappings using . For more information about using such rules, see the
official documentation at
https://web.mit.edu/kerberos/krb5-current/doc/admin/conf_files/krb5_conf.html#realms
and an article at
https://community.hortonworks.com/articles/14463/auth-to-local-rules-syntax.html.
Continue with
.To add more realms, repeat from Step 2.
Enable Kerberos users logging in and creation of home directories by activating
and .If you left empty the optional text boxes in Step 3, make sure to enable automatic discovery of realms and key distribution centers by activating and .
You can additionally activate the following:
http://web.mit.edu/kerberos/krb5-current/doc/admin/conf_files/kdc_conf.html#encryption-types.
allows the encryption types listed as weak atallows forwarding of tickets.
allows the use of proxies between the computer of the user and the key distribution center.
allows granting tickets to users behind networks using network address translation.
To set up allowed encryption types and define the name of the keytab file which lists the names of principals and their encrypted keys, use the
.Finish with
and .YaST may now install extra packages.
To check if the setup of the Kerberos back-end inside of LDAP was successful, proceed as follows:
Directly access the KDC database on the host of the 389 Directory Server:
tux >
sudo
kadmin.local
List the principals:
kadmin.local > listprincs
Create a principal:
kadmin.local > ank admin@EXAMPLE.COM
It is written to the 389 Directory Server database.
tux >
sudo
ldapsearch -D 'cn=Directory Manager' -w password -b 'cn=EXAMPLE.COM,cn=kdc,dc=example,dc=com' -H ldaps://localhost
Check if the principal data from Kerberos is stored in LDAP. If yes, you get an output similar to the following:
tux >
sudo
admin@EXAMPLE.COM, EXAMPLE.COM, kdc, example.com dn: krbprincipalname=admin@EXAMPLE.COM,cn=EXAMPLE.COM,cn=kdc,dc=example,dc=com krbLoginFailedCount: 0 krbPrincipalName: admin@EXAMPLE.COM krbPrincipalKey:: MIG2oAMCAQGhAwIBAaIDAgEBowMCAQGkgZ8wgZwwVKAHMAWgAwIBAKFJMEeg AwIBEqFABD4gAKXAsMf7oV5vITzV5OpclhdomR+SdIRCkouS2GeNF9lVgxjT29RpnipNlCjgGOkpr 93d0nh82WhrrAF6bzBEoAcwBaADAgEAoTkwN6ADAgERoTAELhAAFiGRiI0yUjBteGHhTB6ESJYsYJ WxFa4UslUNZD1GEQGlZ/0nltLsyD2ytGc= krbLastPwdChange: 20190702032802Z krbExtraData:: AAJCzxpdcm9vdC9hZG1pbkBFWEFNUExFLkNPTQA= krbExtraData:: AAgBAA== objectClass: krbprincipal objectClass: krbprincipalaux objectClass: krbTicketPolicyAux objectClass: top
Obtain and cache an initial ticket-granting ticket:
tux >
sudo
kinit admin@EXAMPLE.COM
Display a list of currently cached Kerberos tickets:
tux >
sudo
klist Ticket cache: DIR::/run/user/0/krb5cc/tkt Default principal: admin@EXAMPLE.COM Valid starting Expires Service principal 07/02/19 13:29:04 07/03/19 13:29:04 krbtgt/EXAMPLE.COM@EXAMPLE.COM
Most NFS servers can export file systems using any combination of the
default “trust the network” form of security, known as
sec=sys
, and three different levels of Kerberos-based
security, sec=krb5
, sec=krb5i
, and
sec=krb5p
. The sec
option is set as a
mount option on the client. It is often the case that the NFS service will
first be configured and used with sec=sys
, and then Kerberos
can be imposed afterwards. In this case it is likely that the server will be
configured to support both sec=sys
and one of the Kerberos
levels, and then after all clients have transitioned, the
sec=sys
support will be removed, thus achieving true
security. The transition to Kerberos should be fairly transparent if done in an
orderly manner. However there is one subtle detail of NFS behavior that
works differently when Kerberos is used, and the implications of this need to
be understood and possibly addressed. See
Section 7.7.1, “Group Membership”.
The three Kerberos levels indicate different levels of security. With more security comes a need for more processor power to encrypt and decrypt messages. Choosing the right balance is an important consideration when planning a roll-out of Kerberos for NFS.
krb5
provides only authentication. The server can know
who sent a request, and the client can know that the server sent a reply. No
security is provided for the content of the request or reply, so an attacker
with physical network access could transform the request or reply, or both,
in various ways to deceive either server or client. They cannot directly
read or change any file that the authenticated user could not read or
change, but almost anything is theoretically possible.
krb5i
adds integrity checks to all messages. With
krb5i
, an attacker cannot modify any request or reply,
but they can view all the data exchanged, and so could discover the content
of any file that is read.
krb5p
adds privacy to the protocol. As well as reliable
authentication and integrity checking, messages are fully encrypted so an
attacker can only know that messages were exchanged between client and
server, and cannot extract other information directly from the message.
Whether information can be extracted from message timing is a separate
question that Kerberos does not address.
The one behavioral difference between sec=sys
and the
various Kerberos security levels that might be visible is related to group
membership. In Unix and Linux, each file system access comes from a process
that is owned by a particular user and has a particular group owner and several
supplemental groups. Access rights to files can vary based on the
owner and the various groups.
With sec=sys
, the user-id, group-id, and a list of up to
16 supplementary groups are sent to the server in each request.
If a user is a member of more than 16 supplementary groups, the extra groups are lost and some files may not be accessible over NFS that the user would normally expect to have access to. For this reason, most sites that use NFS find a way to limit all users to at most 16 supplementary groups.
If the user runs the newgrp
command or runs a
set-group-id program, either of which can change the list of groups they
are a member of, these changes take effect immediately and provide
different accesses over NFS.
With Kerberos, group information is not sent in requests. Only the user is identified (using a Kerberos “principal”), and the server performs a lookup to determine the user ID and group list for that principal. This means that if the user is a member of more than 16 groups, all of these group memberships will be used in determining file access permissions. However it also means that if the user changes a group-id on the client in some way, the server will not notice the change and will not take it into account in determining access rights.
Usually the improvement of having access to more groups brings a real benefit, and the loss of not being able to change groups is not noticed as it is not widely used. A site administrator considering the use of Kerberos should be aware of the difference though and ensure that it will not actually cause problems.
Using Kerberos for security requires extra CPU power for encrypting and
decrypting messages. How much extra CPU power is required and whether the
difference is noticeable will vary with different hardware and different
applications. If the server or client are already saturating the available
CPU power, it is likely that a performance drop will be measurable when
switching from sec=sys
to Kerberos. If there is spare CPU
capacity available, it is quite possible that the transition will not
result in any throughput change. The only way to be sure how much impact
the use of Kerberos will have is to test your load on your hardware.
The only configuration options that might reduce the load will also reduce
the quality of the protection offered. sec=krb5
should
produce noticeably less load than sec=krb5p
but, as
discussed above, it does not produce very strong security. Similarly it is
possible to adjust the list of ciphers that Kerberos can choose from, and this
might change the CPU requirement. However the defaults are carefully chosen
and should not be changed without similar careful consideration.
The other possible performance issue when configuring NFS to use Kerberos involves availability of the Kerberos authentication servers, known as the KDC or Key Distribution Center.
The use of NFS adds load to such servers to the same degree that adding the
use of Kerberos for any other services adds some load. Every time a given user
(Kerberos principal) establishes a session with a service, for example by
accessing files exported by a particular NFS server, the client needs to
negotiate with the KDC. Once a session key has been negotiated, the client
server can communicate without further help for many hours, depending on
details of the Kerberos configuration, particularly the
ticket_lifetime
setting.
The concerns most likely to affect the provisioning of Kerberos KDC servers are availability and peak usage.
As with other core services such as DNS, LDAP or similar name-lookup
services, having two servers that are reasonably "close" to every client
provides good availability for modest resources. Kerberos allows for multiple
KDC servers with flexible models for database propagation, so distributing
servers as needed around campuses, buildings, and even cabinets is fairly
straightforward. The best mechanism to ensure each client finds a nearby
Kerberos server is to use split-horizon DNS with each building (or similar)
getting different details from the DNS server. If this is not possible,
then managing the /etc/krb5.conf
file to be different
at different locations is a suitable alternative.
As access to the Kerberos KDC is infrequent, load is only likely to be a problem at peak times. If thousands of people all log in between 9:00 and 9:05, then the servers will receive many more requests-per-minute than they might in the middle of the night. The load on the Kerberos server is likely to be more than that on an LDAP server, but not orders of magnitude more. A sensible guideline is to provision Kerberos replicas in the same manner that you provision LDAP replicas, and then monitor performance to determine if demand ever exceeds capacity.
One service of the Kerberos KDC that is not easily distributed is the handling of updates, such as password changes and new user creation. These must happen at a single master KDC.
These updates are not likely to happen with such frequency that any significant load will be generated, but availability could be an issue. It can be annoying to create a new user or change a password, and the master KDC on the other side of the world is temporarily unavailable.
When an organization is geographically distributed and has a policy of handling administration tasks locally at each site, it can be beneficial to create multiple Kerberos domains, one for each administrative center. Each domain would then have its own master KDC which would be geographically local. Users in one domain can still get access to resources in another domain by setting up trust relationships between domains.
The easiest arrangement for multiple domains is to have a global domain (for example, EXAMPLE.COM) and various local domains (for example, ASIA.EXAMPLE.COM, EUROPE.EXAMPLE.COM). If the global domain is configured to trust each local domain, and each local domain is configured to trust the global domain, then fully transitive trust is available between any pair of domains, and any principal can establish a secure connection with any service. Ensuring appropriate access rights to resources, for example files provided by that service, will be dependent on the user name lookup service used, and the functionality of the NFS file server, and is beyond the scope of this document.
The official site of MIT Kerberos is http://web.mit.edu/kerberos. There, find links to any other relevant resource concerning Kerberos, including Kerberos installation, user, and administration guides.
The book Kerberos—A Network Authentication System by Brian Tung (ISBN 0-201-37924-4) offers extensive information.
Active Directory* (AD) is a directory-service based on LDAP, Kerberos, and other services. It is used by Microsoft* Windows* to manage resources, services, and people. In a Microsoft Windows network, Active Directory provides information about these objects, restricts access to them, and enforces policies. openSUSE® Leap lets you join existing Active Directory domains and integrate your Linux machine into a Windows environment.
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 Leap Linux client:
GNOME Files (previously called Nautilus) supports browsing shared resources through SMB.
GNOME Files supports sharing directories and files as in Windows.
Through GNOME Files, users can access their Windows user data and can edit, create, and delete files and directories on the Windows server. Users can access their data without having to enter their password multiple times.
Users can log in and access their local data on the Linux machine even if they are offline or the Active Directory server is unavailable for other reasons.
This port of Active Directory 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.
Many desktop applications 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.
In Windows Server 2016 and later, Microsoft has removed the role IDMU/NIS Server and along with it the Unix Attributes plug-in for the Active Directory Users and Computers MMC snap-in.
However, Unix attributes can still be managed manually when Active Directory Users and Computers MMC snap-in. For more information, see Clarification regarding the status of Identity Management for Unix (IDMU) & NIS Server Role in Windows Server 2016 Technical Preview and beyond.
are enabled in theAlternatively, use the method described in Procedure 8.1, “Joining an Active Directory Domain Using to complete attributes on the client side (in particular, see ”Step 6.c).
The following section contains technical background for most of the previously named features. For more information about file and printer sharing using Active Directory, see Book “GNOME User Guide”.
Many system components need to interact flawlessly to integrate a Linux client into an existing Windows Active Directory domain. The following sections focus on the underlying processes of the key events in Active Directory server and client interaction.
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 Active Directory can use the LDAP protocol to exchange directory information with the clients. To learn more about LDAP in general, refer to Chapter 6, LDAP—A Directory Service.
Kerberos is a third-party trusted authentication service. All its clients trust Kerberos 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. To learn more about Kerberos in Linux, refer to Chapter 7, Network Authentication with Kerberos.
Depending on which YaST module you use to set up Kerberos authentication, different client components process account and authentication data:
The sssd
daemon is the
central part of this solution. It handles all communication with the
Active Directory server.
To gather name service information,
sssd_nss
is used.
To authenticate users, the
pam_sss
module for PAM
is used. The creation of user homes for the Active Directory users on the Linux
client is handled by pam_mkhomedir
.
For more information about PAM, see Chapter 3, Authentication with PAM.
The winbindd
daemon is the
central part of this solution. It handles all communication with the
Active Directory server.
To gather name service information,
nss_winbind
is used.
To authenticate users, the
pam_winbind
module for PAM
is used. The creation of user homes for the Active Directory users on the Linux
client is handled by pam_mkhomedir
.
For more information about PAM, see Chapter 3, Authentication with PAM.
Figure 8.1, “Schema of Winbind-based Active Directory Authentication” highlights the most prominent components of Winbind-based Active Directory authentication.
Applications that are PAM-aware, like the login routines and the GNOME display manager, 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.
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 Windows 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:
The Windows domain controller providing both LDAP and KDC (Key Distribution Center) services is located.
A machine account for the joining client is created in the directory service.
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.
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.
The login manager of GNOME (GDM) has been extended to allow the handling of Active Directory domain login. Users can choose to log in to 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 several PAM modules as described in Section 8.2, “Background Information for Linux Active Directory Support”. If there are errors, the error codes are translated into user-readable error messages that PAM gives at login through any of the supported methods (GDM, 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 Leap 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, the client 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 Leap supports local home directories for Active Directory users. If configured through YaST as described in Section 8.3, “Configuring a Linux Client for Active Directory”, user home directories are created when a Windows/Active Directory user first logs in to the Linux client. These home directories look and feel identical to standard Linux user home directories and work independently of the Active Directory Domain Controller.
Using a local user home, it is possible to access a user's data on this machine (even when the Active Directory server is disconnected) as long as the Linux client has been configured to perform offline authentication.
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 Active Directory 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 Active Directory 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 Leap acquires a new Kerberos ticket when the user has locked and unlocked the desktop (for example, using a desktop screen saver).
Before your client can join an Active Directory 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 Active Directory DNS server. Alternatively, configure your machine to use the Active Directory 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 8.2, “Joining an Active Directory Domain Using . ”
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
. Select your network interface from the
list of interfaces and click . Select
and apply your settings with
. Leave the firewall settings with › . To
disable the firewall, check the option, and leave the firewall module with
› .
You cannot log in to an Active Directory domain unless the Active Directory administrator has provided you with a valid user account for that domain. Use the Active Directory user name and password to log in to the Active Directory domain from your Linux client.
YaST contains multiple modules that allow connecting to an Active Directory:
Use both an identity service (usually LDAP) and a user authentication service (usually Kerberos). This option is based on SSSD and in the majority of cases is best suited for joining Active Directory domains. .
This module is described in Section 8.3.2, “Joining Active Directory Using . ”
Join an Active Directory (which entails use of Kerberos and LDAP). This option is
based on . winbind
and is best suited for joining an
Active Directory domain if support for NTLM or cross-forest trusts is necessary.
This module is described in Section 8.3.3, “Joining Active Directory Using . ”
Allows setting up LDAP identities and Kerberos authentication independently from each other and provides fewer options. While this module also uses SSSD, it is not as well suited for connecting to Active Directory as the previous two options. .
This module is described in:
The YaST module
supports authentication at an Active Directory. Additionally, it also supports the following related authentication and identification providers:Support for legacy NSS providers via a proxy. .
FreeIPA and Red Hat Enterprise Identity Management provider. .
An LDAP provider. For more information about configuring LDAP, see
. man 5 sssd-ldap
.
An SSSD-internal provider for local users. .
Relay authentication to another PAM target via a proxy. .
FreeIPA and Red Hat Enterprise Identity Management provider. .
An LDAP provider. .
Kerberos authentication. .
An SSSD-internal provider for local users. .
Disables authentication explicitly. .
To join an Active Directory domain using SSSD and the
module of YaST, proceed as follows:Open YaST.
To be able to use DNS auto-discovery later, set up the Active Directory Domain Controller (the Active Directory server) as the name server for your client.
In YaST, click
.Select
, then enter the IP address of the Active Directory Domain Controller into the text box .Save the setting with
.From the YaST main window, start the module
.The module opens with an overview showing different network properties of your computer and the authentication method currently in use.
To start editing, click
.Now join the domain.
Click
.In the appearing dialog, specify the correct
. Then specify the services to use for identity data and authentication: Select for both.Ensure that
is activated.Click
.(Optional) Usually, you can keep the default settings in the following dialog. However, there are reasons to make changes:
If the Local Host Name Does Not Match the Host Name Set on the
Domain Controller.
Find out if the host name of your computer matches what the name
your computer is known as to the Active Directory Domain Controller. In a
terminal, run the command hostname
, then compare
its output to the configuration of the Active Directory Domain Controller.
If the values differ, specify the host name from the Active Directory configuration under
. Otherwise, leave the appropriate text box empty.If You Do Not Want to Use DNS Auto-Discovery. Specify the that you want to use. If there are multiple Domain Controllers, separate their host names with commas.
To continue, click
.If not all software is installed already, the computer will now install missing software. It will then check whether the configured Active Directory Domain Controller is available.
If everything is correct, the following dialog should now show that it has discovered an
but that you are .
In the dialog, specify the Administrator
).
To make sure that the current domain is enabled for Samba, activate
.To enroll, click
.You should now see a message confirming that you have enrolled successfully. Finish with
.After enrolling, configure the client using the window
.To allow logging in to the computer using login data provided by Active Directory, activate
.(Optional)
Optionally, under ,
activate additional data sources such as information on which users are
allowed to use sudo
or which network drives are
available.
To allow Active Directory users to have home directories, activate
. The path for home directories can be set in multiple ways—on the client, on the server, or both ways:
To configure the home directory paths on the Domain Controller, set
an appropriate value for the attribute
UnixHomeDirectory
for each user. Additionally,
make sure that this attribute replicated to the global catalog. For
information on achieving that under Windows, see
https://support.microsoft.com/en-us/kb/248717.
To configure home directory paths on the client in such a way that
precedence will be given to the path set on the domain controller,
use the option fallback_homedir
.
To configure home directory paths on the client in such a way that
the client setting will override the server setting, use
override_homedir
.
As settings on the Domain Controller are outside of the scope of this documentation, only the configuration of the client-side options will be described in the following.
From the side bar, select
fallback_homedir
or
override_homedir
, then click .
Specify a value. To have home directories follow the format
/home/USER_NAME
, use
/home/%u
.
For more information about possible variables, see the man page
sssd.conf
(man 5 sssd.conf
),
section
override_homedir.
Click
.Save the changes by clicking
. Then make sure that the values displayed now are correct. To leave the dialog, click .
To join an Active Directory domain using winbind
and the
module of YaST, proceed as
follows:
Log in as root
and start YaST.
Start
› .
Enter the domain to join at Figure 8.5, “Determining Windows Domain Membership”). If the DNS settings on your host
are properly integrated with the Windows DNS server, enter the Active Directory
domain name in its DNS format
(mydomain.mycompany.com
). 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.
To use the SMB source for Linux authentication, activate
.To automatically create a local home directory for Active Directory users on the Linux machine, activate
.Check
to allow your domain users to log in even if the Active Directory server is temporarily unavailable, or if you do not have a network connection.To change the UID and GID ranges for the Samba users and groups, select
. Let DHCP retrieve the WINS server only if you need it. This is the case when some machines are resolved only by the WINS system.Configure NTP time synchronization for your Active Directory environment by selecting
and entering an appropriate server name or IP address. This step is obsolete if you have already entered the appropriate settings in the stand-alone YaST NTP configuration module.Click
and confirm the domain join when prompted for it.Provide the password for the Windows administrator on the Active Directory server and click Figure 8.6, “Providing Administrator Credentials”).
(seeAfter you have joined the Active Directory domain, you can log in to it from your workstation using the display manager of your desktop or the console.
Joining a domain may not succeed if the domain name ends with
.local
. Names ending in .local
cause conflicts with Multicast DNS (MDNS) where
.local
is reserved for link-local host names.
Only a domain administrator account, such as
Administrator
, can join openSUSE Leap into Active
Directory.
To check whether you are successfully enrolled in an Active Directory domain, use the following commands:
klist
shows whether the current user has a valid
Kerberos ticket.
getent passwd
shows published LDAP data for all
users.
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 Active Directory credentials. Login is supported for GNOME, the console, SSH, and any other PAM-aware application.
openSUSE Leap supports offline authentication, allowing you to log in to your client machine even when it is offline. See Section 8.2.3, “Offline Service and Policy Support” for details.
To authenticate a GNOME client machine against an Active Directory server, proceed as follows:
Click
.
In the text box DOMAIN_NAME\USER_NAME
.
Enter your Windows password.
If configured to do so, openSUSE Leap creates a user home directory on the local machine on the first login of each user authenticated via Active Directory. This allows you to benefit from the Active Directory support of openSUSE Leap while still having a fully functional Linux machine at your disposal.
Besides logging in to the Active Directory client machine using a graphical front-end, you can log in using the text-based console or even remotely using SSH.
To log in to your Active Directory client from a console, enter
DOMAIN_NAME\USER_NAME
at the login:
prompt and provide the password.
To remotely log in to your Active Directory client machine using SSH, proceed as follows:
At the login prompt, enter:
tux >
ssh DOMAIN_NAME\\USER_NAME@HOST_NAME
The \
domain and login delimiter is escaped with
another \
sign.
Provide the user's password.
openSUSE Leap helps the 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 Leap 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 provides feedback about password expiration and the prompt for new passwords in an interactive mode. To change passwords in the display managers, 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:
Log in at the console.
Enter passwd
.
Enter your current password when prompted.
Enter the new password.
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:
Click the
icon on the left edge of the panel.Select
.From the
section, select › .Enter your old password.
Enter and confirm the new password.
Leave the dialog with
to apply your settings.The RADIUS (Remote Authentication Dial-In User Service) protocol has long been a standard service for manage network access. It performs authentication, authorization, and accounting (AAA) protocol for very large businesses such as Internet service providers and cellular network providers, and is also popular for small networks. It authenticates users and devices, authorizes those users and devices for certain network services, and tracks use of services for billing and auditing. You don't have to use all three of the AAA protocols, but only the ones you need. For example, you may not need accounting but only client authentication, or perhaps all you want is accounting, and client authorization is managed by something else.
It is extremely efficient and manages thousands of requests on modest hardware. Of course it works for all network protocols and not just dialup, but the name remains the same.
RADIUS operates in a distributed architecture, sitting separately from the Network Access Server (NAS). User access data is stored on a central RADIUS server that is available to multiple NAS. The NAS provide the physical access to the network, such as a managed Ethernet switch, or wireless access point.
FreeRADIUS is the open source RADIUS implementation, and is the most widely-used RADIUS server. In this chapter you will learn how to install and test a FreeRADIUS server. Because of the numerous possible use cases, after your initial setup is working correctly your next stop is the official documentation, which is detailed and thorough (see https://freeradius.org/documentation/).
The following steps set up a simple test system. When you have verified that the server is operating correctly and you are ready to create a production configuration, you will have several undo steps to perform before starting your production configuration.
First install the freeradius-server
and
freeradius-server-utils
packages.
Then enter /etc/raddb/certs
and run the bootstrap
script to create a set of test certificates:
root #
zypper in freeradius-serverroot #
cd /etc/raddb/certsroot #
./bootstrap
The README in the certs
directory contains a great deal of useful
information. When the bootstrap
script has completed, start the server
in debugging mode:
root #
radiusd -X
[...]
Listening on auth address * port 1812 bound to server default
Listening on acct address * port 1813 bound to server default
Listening on auth address :: port 1812 bound to server default
Listening on acct address :: port 1813 bound to server default
Listening on auth address 127.0.0.1 port 18120 bound to server inner-tunnel
Listening on proxy address * port 54435
Listening on proxy address :: port 58415
Ready to process requests
When you see the "Listening" and "Ready to process requests" lines, your server has
started correctly. If it does not start, read the output carefully because it tells you
what went wrong. You may direct the output to a text file with
tee
:
tux >
radiusd -X | tee radiusd.text
The next step is to test authentication with a test client and user.
The client is a client of the RADIUS server, such as a wireless access point
or switch. Clients are configured in
/etc/raddb/client.conf
. Human users are configured
in /etc/raddb/mods-config/files/authorize
.
Open
/etc/raddb/mods-config/files/authorize
and uncomment the
following lines:
bob Cleartext-Password := "hello" Reply-Message := "Hello, %{User-Name}"
A test client, client localhost
, is provided in
/etc/raddb/client.conf
, with a secret of
testing123
. Open a second terminal, and as an
unprivileged user use the radtest
command to log in
as bob:
tux >
radtest bob hello 127.0.0.1 0 testing123
Sent Access-Request Id 241 from 0.0.0.0:35234 to 127.0.0.1:1812 length 73
User-Name = "bob"
User-Password = "hello"
NAS-IP-Address = 127.0.0.1
NAS-Port = 0
Message-Authenticator = 0x00
Cleartext-Password = "hello"
Received Access-Accept Id 241 from 127.0.0.1:1812 to 0.0.0.0:0 length 20
In your radius -X
terminal, a successful login looks
like this:
(3) pap: Login attempt with password (3) pap: Comparing with "known good" Cleartext-Password (3) pap: User authenticated successfully (3) [pap] = ok [...] (3) Sent Access-Accept Id 241 from 127.0.0.1:1812 to 127.0.0.1:35234 length 0 (3) Finished request Waking up in 4.9 seconds. (3) Cleaning up request packet ID 241 with timestamp +889
Now run one more login test from a different computer on your network.
Create a client configuration on your server by uncommenting and
modifying the following entry in clients.conf
:
client private-network-1 } ipaddr = 192.0.2.0/24 secret = testing123-1 {
Enter the IP address of your test client machine. On the client machine,
install freeradius-server-utils
, which
provides a number of useful test commands. Try logging in from the client
as bob, using the radtest
command. It is better to use the
IP address of the RADIUS server rather than the hostname because it is faster:
tux >
radtest bob hello 192.168.2.100 0 testing123-1
If your test logins fail, review all the output to learn what went wrong.
There are several test users and test clients provided. The configuration
files are full of useful information, and we recommend studying them. When
you are satisfied with your testing and ready to create a production
configuration, remove all the test certificates in
/etc/raddb/certs
and replace them with your own
certificates, comment out all the test users and clients, and stop
radiusd
by pressing
Ctrl–c. Manage the
radiusd.service
with systemctl
, just
like any other service.
To learn how to fit a FreeRADIUS server in your network, see https://freeradius.org/documentation/ and https://networkradius.com/freeradius-documentation/ for in-depth references and howtos.
Physical security should be one of the utmost concerns. Linux production servers should be in locked data centers where only people have access that have passed security checks. Depending on the environment and circumstances, you can also consider boot loader passwords.
The seccheck
SUSE Security Checker is a set of
shell scripts designed to automatically check the local security of a system
on a regular schedule, and emails reports to the root user, or any user
as configured by the administrator.
A very important step in securing a Linux system is to determine the primary function(s) or role(s) of the Linux server. Otherwise, it can be difficult to understand what needs to be secured and securing these Linux systems can prove ineffective. Therefore, it is critical to look at the default list…
Servers should have separate file systems for at least /, /boot, /usr, /var, /tmp, and /home. This prevents, for example, that logging space and temporary space under /var and /tmp fill up the root partition. Third-party applications should be on separate file systems as well, for example under /opt…
Encrypting files, partitions, and entire disks prevents unauthorized access to your data and protects your confidential files and documents.
Databases and similar applications are often hosted on external servers that are serviced by third-party staff. Certain data center maintenance tasks require third-party staff to directly access affected systems. In such cases, privacy requirements necessitate disk encryption.
It is important that all system and vendor accounts that are not used for logins are locked. To get a list of unlocked accounts on your system, you can check for accounts that do not have an encrypted password string starting with ! or * in the /etc/shadow file. If you lock an account using passwd -…
spectre-meltdown-checker
is a shell script to test if
your system is vulnerable to the several speculative execution
vulnerabilities that are in nearly all CPUs manufactured in the
past 20 years. This is a hardware flaw that potentially allows an attacker
to read all data on the system. On cloud computing services, where multiple
virtual machines are on a single physical host, an attacker can gain access
to all virtual machines. Fixing these vulnerabilities requires redesigning
and replacing CPUs. Until this happens, there are several software patches
that mitigate these vulnerabilities. If you have kept your SUSE systems
updated, all of these patches should already be installed.
spectre-meltdown-checker
generates a detailed report. It
is impossible to guarantee that your system is secure, but it shows you
which mitigations are in place, and potential vulnerabilities.
The YaST module openSUSE Leap. 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 › . The dialog always starts with the , and other configuration dialogs are available from the right pane.
offers a central clearinghouse to configure security-related settings for
PolKit (formerly known as PolicyKit) is an application framework that
acts as a negotiator between the unprivileged user session and the
privileged system context. Whenever a process from the user session
tries to carry out an action in the system context, PolKit is queried.
Based on its configuration—specified in a so-called
“policy”—the answer could be “yes”,
“no”, or “needs authentication”. Unlike
classical privilege authorization programs such as sudo, PolKit does
not grant root
permissions to an entire session, but only to
the action in question.
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.
Certificates play an important role in the authentication of companies and individuals. Usually certificates are administered by the application itself. In some cases, it makes sense to share certificates between applications. The certificate store is a common ground for Firefox, Evolution, and NetworkManager. This chapter explains some details.
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.
Physical security should be one of the utmost concerns. Linux production servers should be in locked data centers where only people have access that have passed security checks. Depending on the environment and circumstances, you can also consider boot loader passwords.
Additionally, consider questions like:
Who has direct physical access to the host?
Of those that do, should they?
Can the host be protected from tampering and should it be?
The amount of physical security needed on a particular system depends on the situation, and can also vary widely by available funds.
Most server racks in data centers include a locking feature. Usually this will be a hasp/cylinder lock on the front of the rack that allows you to turn an included key to a locked or unlocked position – granting or denying entry. Cage locks can help prevent someone from tampering or stealing devices/media from the servers, or opening the cases and directly manipulating/sabotaging the hardware. Preventing system reboots or the booting from alternate devices is also important (for example CD/DVDs/USB drives/etc.).
Some servers also have case locks. These locks can do different things according to the designs of the system vendor and construction. Many systems are designed to self-disable if attempts are made to open the system without unlocking. Others have device covers that will not let you plug in or unplug keyboards or mice. While locks are sometimes a useful feature, they are usually lower quality and easily defeated by attackers with ill intent.
This section describes only basic methods to secure the boot process. To find out about more advanced boot protection using UEFI and the secure boot feature, see Book “Reference”, Chapter 14 “UEFI (Unified Extensible Firmware Interface)”, Section 14.1 “Secure Boot”.
The BIOS (Basic Input/Output System) or its successor UEFI (Unified Extensible Firmware Interface) is the lowest level of software/firmware on PC class systems. Other hardware types (POWER, IBM Z) that run Linux also have low-level firmware that performs similar functions as the PC BIOS. When this document references the BIOS, it usually means BIOS and/or UEFI. The BIOS dictates system configuration, puts the system into a well defined state and provides routines for accessing low-level hardware. The BIOS executes the configured Linux boot loader (like GRUB 2) to boot the host.
Most BIOS implementations can be configured to prevent unauthorized users from manipulating system and boot settings. This is typically done by setting a BIOS admin or boot password. The admin password only needs to be entered for changing the system configuration but the boot password will be required during every normal boot. For most use cases it is enough to set an admin password and restrict booting to the built-in hard disk. This way an attacker will not be able to simply boot a Linux live CD or USB thumb drive, for example. Although this does not provide a high level of security (a BIOS can be reset, removed or modified – assuming case access), it can be another deterrent.
Many BIOS firmwares have various other security related settings. Check with the system vendor, the system documentation or examine the BIOS during a system boot to find out more.
If a system has been set up with a boot password, the host will not boot up unattended (for example in case of a system reboot or power failure). This is a trade-off.
Once a system is set up for the first time, the BIOS admin password will not be required often. Don't forget the password or you will need to clear the BIOS memory via hardware manipulation to get access again.
The Linux boot loader GRUB 2, which is used by default in openSUSE Leap, can have a boot passwords set. It also provides a password feature, so that only administrators can start the interactive operations (for example editing menu entries and entering the command line interface). If a password is specified, GRUB 2 will disallow any interactive control until you press the key C and E and enter a correct password.
You can refer to the GRUB 2 man page for examples.
It is very important to keep in mind that when setting these passwords they will need to be remembered! Also, enabling these passwords might merely slow an intrusion, not necessarily prevent it. Again, someone could boot from a removable device, and mount your root partition. If you are using BIOS-level security and a boot loader, it is a good practice to disable the ability to boot from removable devices in your computer's BIOS, and then also password-protecting the BIOS itself.
Also keep in mind that the boot loader configuration files will need to be
protected by changing their mode to 600
(read/write for
root
only), or others will be able to read your passwords or hashes!
Security policies usually contain some procedures for the treatment of
storage media that is going to be retired or disposed of. Disk and media
wipe procedures are frequently prescribed as is complete destruction of
the media. You can find several free tools on the Internet. A search of
“dod disk wipe utility” will yield several variants. To
retire servers with sensitive data, it is important to ensure that data
cannot be recovered from the hard disks. To ensure that all traces of data
are removed, a wipe utility—such as
scrub
—can be used. Many wipe utilities overwrite
the data several times. This assures that even sophisticated methods are
not able to retrieve any parts of the wiped data. Some tools can even be
operated from a bootable removable device and remove data according to the
U.S. Department of Defense (DoD) standards. Note that many government
agencies specify their own standards for data security. Some standards are
stronger than others, yet may require more time to implement.
Some devices, like SSDs, use wear leveling and do not necessarily write new data in the same physical locations. Such devices usually provide their own erasing functionality.
scrub
overwrites hard disks, files, and other devices
with repeating patterns intended to make recovering data from these
devices more difficult. It operates in three basic modes: on a character
or block device, on a file, or on a directory specified. For more
information, set the manual page man 1 scrub
.
4-pass NNSA Policy Letter NAP-14.1-C (XVI-8) for sanitizing removable and non-removable hard disks, which requires overwriting all locations with a pseudo random pattern twice and then with a known pattern: ran- dom(x2), 0x00, verify.
4-pass DoD 5220.22-M section 8-306 procedure (d) for sanitizing removable and non-removable rigid disks which requires overwriting all addressable locations with a character, its complement, a random character, then verify. Note: scrub performs the random pass first to make verification easier: random, 0x00, 0xff, verify.
9-pass method recommended by the German Center of Security in Information Technologies (http://www.bsi.bund.de): 0xff, 0xfe, 0xfd, 0xfb, 0xf7, 0xef, 0xdf, 0xbf, 0x7f.
The canonical 35-pass sequence described in Gutmann's paper cited below.
7-pass method described by Bruce Schneier in "Applied Cryptography" (1996): 0x00, 0xff, random(x5)
Roy Pfitzner's 7-random-pass method: random(x7).
Roy Pfitzner's 33-random-pass method: random(x33).
US Army AR380-19 method: 0x00, 0xff, random. (Note: identical to DoD 522.22-M section 8-306 procedure (e) for sanitizing magnetic core memory).
1-pass pattern: 0x00.
1-pass pattern: 0xff.
1-pass pattern: random(x1).
2-pass pattern: random(x2).
6-pass pre-version 1.7 scrub method: 0x00, 0xff, 0xaa, 0x00, 0x55, verify.
5-pass pattern: 0x00, 0xff, 0xaa, 0x55, verify.
1-pass custom pattern. String may contain C-style numerical escapes: \nnn (octal) or \xnn (hex).
In some environments it is required to restrict access to removable
media such as USB storage or optical devices. The tools coming with the
udisks2
package help with such a
configuration.
Create a user group whose users will be allowed to mount and eject removable devices, for example mmedia_all:
tux >
sudo
groupadd mmedia_all
Add a specific user tux
to the new group:
tux >
sudo
usermod -a -G mmedia_alltux
Create the /etc/polkit-1/rules.d/10-mount.rules
file with the following content:
tux >
cat /etc/polkit-1/rules.d/10-mount.rules
polkit.addRule(function(action, subject) {
if (action.id =="org.freedesktop.udisks2.eject-media"
&& subject.isInGroup("mmedia_all")) {
return polkit.Result.YES;
}
});
polkit.addRule(function(action, subject) {
if (action.id =="org.freedesktop.udisks2.filesystem-mount"
&& subject.isInGroup("mmedia_all")) {
return polkit.Result.YES;
}
});
The name of a rules file must start with a digit, otherwise it will be ignored.
Rules files are processed in alphabetical order. Functions are called
in the order they were added until one of the functions returns a
value. Therefore, to add an authorization rule that is processed before
other rules, put it in a file in /etc/polkit-1/rules.d with a name
that sorts before other rules files, for example
/etc/polkit-1/rules.d/10-mount.rules
. Each function
should return a value from polkit.Result
.
Restart udisks2
:
root #
systemctl restart udisks2
Restart polkit
root #
systemctl restart polkit
The seccheck
SUSE Security Checker is a set of
shell scripts designed to automatically check the local security of a system
on a regular schedule, and emails reports to the root user, or any user
as configured by the administrator.
If seccheck is not installed on your system, install it with
sudo zypper in seccheck
. These scripts are controlled by systemd
timers, which are not enabled by default, but must be enabled by the administrator.
There are four seccheck
timers:
/usr/lib/systemd/system/seccheck-daily.timer
/usr/lib/systemd/system/seccheck-monthly.timer
/usr/lib/systemd/system/seccheck-weekly.timer
/usr/lib/systemd/system/seccheck-autologout.timer
seccheck-daily.timer
,
seccheck-monthly.timer
, and
seccheck-weekly.timer
run multiple checks as
described in Section 11.3, “Daily, Weekly, and Monthly Checks”.
seccheck-autologout.timer
logs out inactive users, see
Section 11.4, “Automatic Logout”.
You can change the recipient of the seccheck mails from root to
any user in
/etc/sysconfig/seccheck
.
The following example changes
it to an admin user named firewall
:
SECCHK_USER="firewall"
Manage your timers with systemctl
, just like any other
systemd timer. The following example enables and starts
seccheck-daily.timer
:
tux >
sudo
systemctl enable --now seccheck-daily.timer
List all active timers:
tux >
sudo
systemctl list-timers
List all enabled timers, active and inactive:
tux >
sudo
systemctl list-timers --all
seccheck
performs the following daily checks:
|
length/number/contents of fields, accounts with same UID accounts with UID/GID of 0 or 1 beside root and bin |
|
length/number/contents of fields, accounts with no password |
|
length/number/contents of fields |
user root checks |
secure umask and |
|
checks if important system users are put there |
|
checks for mail aliases which execute programs |
|
checks if users' |
home directory |
checks if home directories are writable or owned by someone else |
dot-files check |
checks many dot-files in the home directories if they are writable or owned by someone else |
mailbox check |
checks if user mailboxes are owned by user and are readable |
NFS export check |
exports should not be exported globally |
NFS import check |
NFS mounts should have the |
promisc check |
checks if network cards are in promiscuous mode |
list modules |
lists loaded modules |
list sockets |
lists open ports |
The following table lists the weekly checks:
password check |
runs |
RPM md5 check |
checks for changed files via RPM's MD5 checksum feature |
suid/sgid check |
lists all suid and sgid files |
exec group write |
lists all executables which are group/world-writable |
writable check |
lists all files which are world-writable (including executables) |
device check |
lists all devices |
john
To enable password auditing, it is necessary to first install the package john, the John the Ripper fast password cracker. The package is available on the openSUSE Build Service at https://build.opensuse.org/package/show/security/john.
The monthly check prints a complete report, and the daily and weekly checks print diffs.
The seccheck-autologout.timer
timer runs every 10 minutes,
checks both remote and local terminal sessions for inactivity, and terminates them if
an idle time is exceeded.
Configure your desired timeouts in
/etc/security/autologout.conf
file. Parameters
include default idle and logout delay times, and the configuration for
limiting maximum idle times specific to users, groups, TTY devices and
SSH sessions. /etc/security/autologout.conf
includes
several configuration examples.
A very important step in securing a Linux system is to determine the primary function(s) or role(s) of the Linux server. Otherwise, it can be difficult to understand what needs to be secured and securing these Linux systems can prove ineffective. Therefore, it is critical to look at the default list of software packages and remove any unnecessary packages or packages that do not comply with your defined security policies.
Generally, an RPM software package consists of the following:
The package's meta data that is written to the RPM database upon installation.
The package's files and directories.
Scripts that are being executed before and after installation and removal.
Packages generally do not impose any security risk to the system unless they contain:
setuid or setgid bits on any of the installed files
group- or world-writable files or directories
a service that is activated upon installation/activated by default.
Assuming that neither of the three conditions above apply, a package is merely a collection of files. Neither installation nor deinstallation of such packages has influence on the security value of the system.
Nevertheless, it is useful to restrict the installed packages in your system to a minimum. Doing this will result in fewer packages that require updates and will simplify maintenance efforts when security alerts and patches are released. It is a best practice not to install, among others, development packages or desktop software packages (for example, an X Server) on production servers. If you do not need them, you should also not install, for example, the Apache Web server or Samba file sharing server.
Many third-party vendors like Oracle and IBM require a desktop environment and development libraries to run installers. To avoid this from having an impact on the security of their production servers, many organizations work around this by creating a silent installation (response file) in a development lab.
Also, other packages like FTP and Telnet daemons should not be installed
as well unless there is a justified business reason for it.
ssh
, scp
or sftp
should be used as replacements, see
.
One of the first action items should be to create a Linux image that only contains RPMs needed by the system and applications, and those needed for maintenance and troubleshooting purposes. A good approach is to start with a minimum list of RPMs and then add packages as needed. This process is time-consuming but usually worth the effort.
The SUSE Appliance Program includes a component called JeOS (Just Enough Operating System). JeOS has a very small footprint and can be customized to fit the specific needs of a system developer. Main uses of JeOS are for hardware/software appliance or virtual machine development. Key benefits of JeOS are efficiency, higher performance, increased security and simplified management.
If JeOS is not an option for you, a good choice is the minimal installation pattern.
To generate a list of all installed packages, use the following command:
root #
zypper packages -i
To retrieve details about a particular package, run:
root #
zypper info PACKAGE_NAME
To check for and report potential conflicts and dependencies when deleting a package, run:
root #
zypper rm -D PACKAGE_NAME
This can be very useful, as running the removal command without a test can often yield a lot of complaints and require manual recursive dependency hunting.
When removing packages, be careful not to remove any essential system packages. This could put your system into a broken state in which it can no longer be booted or repaired. If you are uncertain about this, then it is best to do a complete backup of your system before you start to remove any packages.
For the final removal of one or more packages use the following
zypper
command with the added “-u”
switch, which causes any dependencies that are becoming unused by
removing the named packages, to be removed as well:
root #
zypper rm -u PACKAGE_NAME
Building an infrastructure for patch management is another very important part of a proactive and secure production Linux environment.
It is recommended to have a written security policy and procedure to handle Linux security updates and issues. For example, a security policy should detail the time frame for assessment, testing, and roll out of patches. Network related security vulnerabilities should get the highest priority and should be addressed immediately within a short time frame. The assessment phase should occur within a testing lab, and initial roll out should occur on development systems first
A separate security log file should contain details on which Linux security announcements have been received, which patches have been researched and assessed, when patches have been applied, etc.
SUSE releases their patches in three categories, security, recommended and optional. There are a few options that can be used to keep systems patched, up to date and secure. Each system can register and then retrieve updates via the SUSE Update Web site using the included YaST tool—YaST Online Update. SUSE has also created the Repository Mirroring Tool (RMT), an efficient way to maintain a local repository of available/released patches/updates/fixes that systems can then pull from (reducing Internet traffic). SUSE also offers SUSE Manager for the maintenance, patching, reporting and centralized management of Linux systems, not only SUSE, but other distributions as well.
On a per-server basis, installation of important updates and improvements is possible using the YaST Online Update tool. Current updates for the SUSE Linux Enterprise family are available from the product specific update catalogs containing patches. Installation of updates and improvements is accomplished using YaST and selecting
in the Group. All new patches (except the optional ones) that are currently available for your system will already be marked for installation. Clicking will then automatically install these patches.YaST also offers the possibility to set up an automatic update. Select
› . Configure a Daily or a Weekly update. Some patches, such as kernel updates, require user interaction, which would cause the automatic update procedure to stop. Check for the update procedure to proceed automatically.In this case, run a manual Online Update from time to install patches that require interaction.
When rpm
or
zypper
.
The Repository Mirroring Tool for SUSE Linux Enterprise goes one step further than the Online Update process by establishing a proxy system with repository and registration targets. This helps customers centrally manage software updates within the firewall on a per-system basis, while maintaining their corporate security policies and regulatory compliance.
The downloadable RMT (http://download.suse.com/) is integrated with SUSE Customer Center (https://scc.suse.com/) and provides a repository and registration target that is synchronized with it. This can be very helpful in tracking entitlements in large deployments. The RMT maintains all the capabilities of SUSE Customer Center, while allowing a more secure centralized deployment. It is included with every SUSE Linux Enterprise subscription and is therefore fully supported.
The RMT provides an alternative to the default configuration, which requires opening the firewall to outbound connections for each device to receive updates. That requirement often violates corporate security policies and can be seen as a threat to regulatory compliance by some organizations. Through its integration with SUSE Customer Center, the RMT ensures that each device can receive its appropriate updates without the need to open the firewall, and without any redundant bandwidth requirements.
The RMT also enables customers to locally track their SUSE Linux Enterprise devices (that is servers, desktops, or Point of Service terminals) throughout their enterprise. Now they can easily determine how many entitlements are in need of renewal at the end of a billing cycle without having to physically walk through the data center to manually update spreadsheets.
The RMT informs the SUSE Linux Enterprise devices of any available software updates. Each device then obtains the required software updates from the RMT. The introduction of the RMT improves the interaction among SUSE Linux Enterprise devices within the network and simplifies how they receive their system updates. The RMT enables an infrastructure for several hundred SUSE Linux Enterprise devices per instance of each installation (depending on the specific usage profile). This offers more accurate and efficient server tracking.
In a nutshell, the Repository Mirroring Tool for SUSE Linux Enterprise provides customers with:
Assurance of firewall and regulatory compliance
Reduced bandwidth usage during software updates
Full support under active subscription from SUSE
Maintenance of existing customer interface with SUSE Customer Center
Accurate server entitlement tracking and effective measurement of subscription usage
Automated process to easily tally entitlement totals (no more spreadsheets!)
Simple installation process that automatically synchronizes server entitlement with SUSE Customer Center
SUSE Manager automates Linux server management, allowing you to provision and maintain your servers faster and more accurately. It monitors the health of each Linux server from a single console so you can identify server performance issues before they impact your business. And it lets you comprehensively manage your Linux servers across physical, virtual and cloud environments while improving data center efficiency. SUSE Manager delivers complete lifecycle management for Linux:
Asset management
Provisioning
Package management
Patch management
Configuration management
Redeployment
For more information on SUSE Manager refer to https://www.suse.com/products/suse-manager/.
Servers should have separate file systems for at least
/
, /boot
,
/usr
, /var
,
/tmp
, and /home
. This prevents,
for example, that logging space and temporary space under
/var
and /tmp
fill up the root
partition. Third-party applications should be on separate file systems as
well, for example under /opt
.
Another advantage of separate file systems is the possibility to choose special mount options that are only suitable for certain regions in the file system hierarchy. A number of interesting mount options are:
noexec
: prevents execution of files.
nodev
: prevents character or block special devices
from being usable.
nosuid
: prevents the set-user-ID
or set-group-ID
bits from being effective.
ro
: mounts the file system
read-only
.
Each of these options needs to be carefully considered before applying
it to a partition mount. Applications may stop working, or the support
status may be violated. When applied correctly, mount options can help
against some types of security attacks or misconfigurations. For
example, there should be no need for set-user-ID
binaries to be placed in /tmp
.
You are advised to review Chapter 2, Common Criteria. It is
important to understand the need to separate the partitions that could
impact a running system (for example, log files filling up
/var/log
are a good reason to separate
/var
from the /
partition).
Another thing to keep in mind is that you will likely need to leverage LVM
or another volume manager or at the very least the extended partition type
to work around the limit of four primary partitions on PC class systems.
Another capability in openSUSE Leap is encrypting a partition or even a single directory or file as a container. Refer to Chapter 14, Encrypting Partitions and Files for details.
The following sections deal with some ways the default permissions and
file settings can be modified to enhance the security of a host. It is
important to note that the use of the default openSUSE Leap utilities like
seccheck
- can be run to lock down and improve the
general file security and user environment. However, it is beneficial to
understand how to modify these things.
openSUSE Leap hosts include three default settings for file permissions:
permissions.easy
,
permissions.secure
, and
permissions.paranoid
, all located in the
/etc
directory. 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, usually root
).
Administrators can use the file
/etc/permissions.local
to add their own settings. The
easiest way to implement one of the default permission rule-sets above is
to use the module in YaST.
Each of the following topics will be modified by a selected rule-set, but the information is important to understand on its own.
The umask
(user file-creation mode mask) command is a
shell built-in command which determines the default file permissions for
newly created files and directories. This can be overwritten by system calls but many
programs and utilities use umask
. By default,
umask
is set to 022
. You can modify
this globally by changing the value in /etc/profile
or for each user in the startup files of the shell.
To determine the active umask, use the umask
command:
tux >
umask
022
The umask is subtracted from the access mode 777
if at
least one bit is set.
With the default umask you see the behavior most users expect to see on a Linux system.
tux >
touch atux >
mkdir btux >
ls -on total 16 -rw-r--r--. 1 17086 0 Nov 29 15:05 a drwxr-xr-x. 2 17086 4096 Nov 29 15:05 b
You can specify arbitrary umask values, depending on your needs.
tux >
umask 111tux >
touch ctux >
mkdir dtux >
ls -on total 16 -rw-rw-rw-. 1 17086 0 Nov 29 15:05 c drw-rw-rw-. 2 17086 4096 Nov 29 15:05 d
Based on your thread model you can use a stricter umask like 037
to prevent accidental data leakage.
tux >
umask 037tux >
touch etux >
mkdir ftux >
ls -on total 16 -rw-r-----. 1 17086 0 Nov 29 15:06 e drwxr-----. 2 17086 4096 Nov 29 15:06 f
When the SUID (set user ID) or SGID (set group ID) bits are set on an
executable, it executes with the UID or GID of the owner of the executable
rather than that of the person executing it. This means that, for example,
all executables that have the SUID bit set and are owned by root
are
executed with the UID of root
. A good example is the passwd
command that
allows ordinary users to update the password field in the /etc/shadow
file
which is owned by root
.
But SUID/SGID bits can be misused when the executable has a security hole. Therefore, you should search the entire system for SUID/SGID executables and document it. To search the entire system for SUID or SGID files, you can run the following command:
root #
find /bin /boot /etc /home /lib /lib64 /opt /root /sbin /srv /tmp /usr /var -type f -perm '/6000' -ls
You might need to extend the list of directories that are searched if you have a different file system structure.
SUSE only sets the SUID/SGID bit on binary if it is really necessary. Ensure that code developers do not set SUID/SGID bits on their programs if it is not an absolute requirement. Very often you can use workarounds like removing the executable bit for world/others. However, a better approach is to change the design of the software or use capabilities.
openSUSE Leap supports file capabilities to allow more fine grained
privileges to be given to programs rather than the full power of root
:
root #
getcap -v /usr/bin/ping
/usr/bin/ping = cap_new_raw+eip
The previous command only grants the CAP_NET_RAW capability to whoever
executes ping
. In case of vulnerabilities inside
ping
, an attacker can gain at most this capability in
contrast with full root
. Whenever possible, file capabilities should
be chosen in favor of the SUID bit. But this only applies when the binary
is suid to root
, not to other users such as news
,
lp
and similar.
World-writable files are a security risk since they can be modified by any user on the system. Additionally, world-writable directories allow anyone to add or delete files. To locate world-writable files and directories, you can use the following command:
root #
find /bin /boot /etc /home /lib /lib64 /opt /root /sbin /srv /tmp /usr /var -type f -perm -2 ! -type l -ls
You might need to extend the list of directories that are searched if you have a different file system structure.
The ! -type l
parameter skips all symbolic links since
symbolic links are always world-writable. However, this is not a problem
as long as the target of the link is not world-writable, which is checked
by the above find command.
World-writable directories with the sticky bit such as the /tmp
directory do not allow anyone except the owner of a file to delete or rename it in
this directory.
The sticky bit makes files stick to the user who created it and it prevents other users from
deleting and renaming the files. Therefore, depending on the purpose of
the directory, world-writable directories with sticky are usually not an
issue. An example is the /tmp
directory:
tux >
ls -ld /tmp
drwxrwxrwt 18 root root 16384 Dec 23 22:20 /tmp
The t
mode bit in the output denotes the sticky bit.
Files not owned by any user or group might not necessarily be a security problem in itself. However, unowned files could pose a security problem in the future. For example, if a new user is created and the new users happens to get the same UID as the unowned files have, then this new user will automatically become the owner of these files.
To locate files not owned by any user or group, use the following command:
root #
find /bin /boot /etc /home /lib /lib64 /opt /root /sbin /srv /tmp /usr /var -nouser -o -nogroup
You might need to extend the list of directories that are searched if you have a different file system structure.
A different problem is files that were not installed via the packaging system and therefore don't receive updates. You can check for such files with the following command:
tux >
find /bin /lib /lib64 /usr -path /usr/local -prune -o -type f -a -exec /bin/sh -c "rpm -qf {} &> /dev/null || echo {}" \;
Run this command as an untrusted user (for example nobody) since crafted
file names might lead to command
execution. This shouldn't be a problem since these directories should only be writeable by root
, but
it's still a good security precaution.
This will show you all files under /bin
,
/lib
, /lib64
and
/usr
(with the
exception of files in /usr/local
) that are not tracked
by the package manager. These files might not represent a security issue, but
you should be aware of what is not tracked and take the necessary precautions to
keep these files up to date.
Encrypting files, partitions, and entire disks prevents unauthorized access to your data and protects your confidential files and documents.
You can choose between the following encryption options:
It is possible to create an encrypted partition with YaST during installation or in an already installed system. For further info, see Section 14.1.1, “Creating an Encrypted Partition during Installation” and Section 14.1.2, “Creating an Encrypted Partition on a Running System”. This option can also be used for removable media, such as external hard disks, as described in Section 14.1.3, “Encrypting the Content of Removable Media”.
To quickly encrypt one or more files, you can use the GPG tool. See Section 14.2, “Encrypting Files with GPG” for more information.
Encryption methods described in this chapter cannot protect your running system from being compromised. After the encrypted volume is successfully mounted, everybody with appropriate permissions can access 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.
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 need 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 an encrypted partition manually in the partitioning dialog.
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:
Run the YaST Expert Partitioner with
› .Select a hard disk, click
, and select a primary or an extended partition.Select the partition size or the region to use on the disk.
Select the file system, and mount point of this partition.
Activate the
check box.After checking
, a pop-up window asking for installing additional software may appear. Confirm to install all the required packages to ensure that the encrypted partition works well.If the encrypted file system needs to be mounted only when necessary, enable
in the . otherwise enable and enter the mount point.Click
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.Complete the process by clicking
. 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 when it has been mounted.
To skip mounting the encrypted partition during start-up, press 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.
To mount an encrypted partition which is not mounted during the boot process, open a file manager and click the partition entry in the pane listing common places on your file system. 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 14.1.2, “Creating an Encrypted Partition on a Running System” and be aware that this action destroys all data on the existing partition.
It is also possible to create encrypted partitions on a running system. However, encrypting an existing partition destroys all data on it, and requires re-sizing and restructuring of existing partitions.
On a running system, select Section 14.1.1, “Creating an Encrypted Partition during Installation”.
› in the YaST control center. Click to proceed. In the , select the partition to encrypt and click . The rest of the procedure is the same as described inYaST treats removable media (like external hard disks or flash disks) the same as any other storage device. Virtual disks or partitions on external media can be encrypted as described above. However, you should disable mounting at boot time, because removable media is usually connected only when the system is up and running.
If you encrypted your removable device with YaST, the GNOME desktop
automatically recognizes the encrypted partition and prompts for the
password when the device is detected. If you plug in a FAT-formatted
removable device when running GNOME, the desktop user entering the
password automatically becomes the owner of the device.
For devices with a file system other than FAT, change the
ownership explicitly for users other than root
to give them
read-write access to the device.
The GPG encryption software can be used to encrypt individual files and documents.
To encrypt a file with GPG, you need to generate a key pair first. To do
this, run the gpg --gen-key
and follow the on-screen
instructions. When generating the key pair, GPG creates a user ID (UID) to
identify the key based on your real name, comments, and email address. You
need this UID (or just a part of it like your first name or email address)
to specify the key you want to use to encrypt a file. To find the UID of an
existing key, use the gpg --list-keys
command. To encrypt
a file use the following command:
tux >
gpg -e -r UID
FILE
Replace UID with part of the UID (for example, your first name) and FILE with the file you want to encrypt. For example:
tux >
gpg -e -r Tux secret.txt
This command creates an encrypted version of the specified file
recognizable by the .gpg
file extension (in
this example, it is secret.txt.gpg
).
To decrypt an encrypted file, use the following command:
tux >
gpg -d -o DECRYPTED_FILE
ENCRYPTED_FILE
Replace DECRYPTED_FILE with the desired name for the decrypted file and ENCRYPTED_FILE with the encrypted file you want to decrypt.
Keep in mind that the encrypted file can be only decrypted using the same key that was used for encryption. If you want to share an encrypted file with another person, you have to use that person's public key to encrypt the file.
Databases and similar applications are often hosted on external servers that are serviced by third-party staff. Certain data center maintenance tasks require third-party staff to directly access affected systems. In such cases, privacy requirements necessitate disk encryption.
cryptctl
allows encrypting sensitive directories using
LUKS and offers the following additional features:
Encryption keys are located on a central server, which can be located on customer premises.
Encrypted partitions are automatically remounted after an unplanned reboot.
cryptctl
consists of two components:
A client is a machine that has one or more encrypted partitions but does not permanently store the necessary key to decrypt those partitions. For example, clients can be cloud or otherwise hosted machines.
The server holds encryption keys that can be requested by clients to unlock encrypted partitions.
You can also set up the cryptctl
server to store
encryption keys on a KMIP 1.3-compatible (Key Management
Interoperability Protocol) server. In that case, the
cryptctl
server will not store the encryption keys of
clients and is dependent upon the KMIP-compatible server to provide
these.
cryptctl
Server Maintenance
Since the cryptctl
server manages timeouts for the
encrypted disks and, depending on the configuration, can also hold
encryption keys, it
should be under your direct control and managed only by trusted
personnel.
Additionally, it should be backed up regularly. Losing the server's data means losing access to encrypted partitions on the clients.
To handle encryption, cryptctl
uses LUKS with
aes-xts-256 encryption and 512-bit keys. Encryption keys are transferred
using TLS with certificate verification.
cryptctl
(Model Without Connection to KMIP Server) #cryptctl
Before continuing, make sure the package cryptctl is installed on all machines you intend to set up as servers or clients.
cryptctl
Server #Edit source
Before you can define a machine as a cryptctl
client,
you need to set up a machine as a cryptctl
server.
Before beginning, choose whether to use a self-signed certificate to secure communication between the server and clients. If not, generate a TLS certificate for the server and have it signed by a certificate authority.
Additionally, you can have clients authenticate to the server using certificates signed by a certificate authority. To use this extra security measure, make sure to have a CA certificate at hand before starting this procedure.
As root
, run:
root #
cryptctl init-server
Answer each of the following prompts and press Enter after every answer. If there is a default answer, it is shown in square brackets at the end of the prompt.
Choose a password with at least 10 characters and confirm it. This password assumes the role of a master password, able to unlock all partitions that are registered on the server.
Specify the path to a PEM-encoded TLS certificate or certificate chain file or leave the field empty to create a self-signed certificate. If you specify a path, use an absolute path.
If you want the server to be identified by a host name other than the
default shown, specify a host name. cryptctl
will
then generate certificates which include the host name.
Specify the IP address that belongs to the network interface that you want to listen on for decryption requests from the clients, then set a port number (the default is port 3737).
The default IP address setting,
0.0.0.0
means that
cryptctl
will listen on
all network interfaces for client requests using IPv4.
Specify a directory on the server that will hold the decryption keys for clients.
Specify whether clients need to authenticate to the server using a TLS certificate. If you choose
, this means that clients authenticate using disk UUIDs only. (However, communication will be encrypted using the server certificate in any case.)If you choose
, pick a PEM-encoded certificate authority to use for signing client certificates.Specify whether to use a KMIP 1.3-compatible server (or multiple such servers) to store encryption keys of clients. If you choose this option, provide the host names and ports for one or multiple KMIP-compatible servers.
Additionally, provide a user name, password, a CA certificate for the
KMIP server, and a client identity certificate for the
cryptctl
server.
The setting to use a KMIP server cannot easily be changed later. To
change this setting, both the cryptctl
server and
its clients need to be configured afresh.
Finally, configure an SMTP server for e-mail notifications for encryption and decryption requests or leave the prompt empty to skip setting up e-mail notifications.
cryptctl
currently cannot send e-mail using
authentication-protected SMTP servers. If that is necessary, set up
a local SMTP proxy.
When asked whether to start the cryptctl
server,
enter y
.
To check the status of the service
cryptctl-server
, use:
root #
systemctl status cryptctl-server
To reconfigure the server later, do either of the following:
Run the command cryptctl init-server
again.
cryptctl
will then propose the existing settings as
the defaults, so that you only need to specify the values that you want
to change.
Make changes directly in the configuration file
/etc/sysconfig/cryptctl-server
.
However, to avoid issues, do not change the settings
AUTH_PASSWORD_HASH
and
AUTH_PASSWORD_SALT
manually. The values of these
options need to be calculated correctly.
cryptctl
Client #Edit source
The following interactive setup of cryptctl
is
currently the only setup method.
Make sure the following preconditions are fulfilled:
A cryptctl
server is available over the network.
There is a directory to encrypt.
The client machine has an empty partition available that is large enough to fit the directory to encrypt.
When using a self-signed certificate, the certificate
(*.crt
file) generated on the server is
available locally on the client. Otherwise, the certificate authority
of the server certificate must be trusted by the client.
If you set up the server to require clients to authenticate using a client certificate, prepare a TLS certificate for the client which is signed by the CA certificate you chose for the server.
As root
, run:
root #
cryptctl encrypt
Answer each of the following prompts and press Enter after every answer. If there is a default answer, it is shown in square brackets at the end of the prompt.
Specify the host name and port to connect to on the
cryptctl
server.
If you configured the server to have clients authenticate to it using a TLS certificate, specify a certificate and a key file for the client. The client certificate must be signed by the certificate authority chosen when setting up the server.
Specify the absolute path to the server certificate (the
*.crt
file).
Enter the encryption password that you specified when setting up the server.
Specify the path to the directory to encrypt. Specify the path to the empty partition that will contain the encrypted content of the directory.
Specify the number of machines that are allowed to decrypt the partition simultaneously.
Then specify the timeout in seconds before additional machines are allowed to decrypt the partition after the last vital sign was received from the client or clients.
When a machine unexpectedly stops working and then reboots, it needs to be able to unlock its partitions again. That means this timeout should be set to a time slightly shorter than the reboot time of the client.
If the time is set too long, the machine cannot decrypt
encrypted partitions on the first try. cryptctl
will
then continue to periodically check whether the encryption key has
become available. However, this will introduce a delay.
If the timeout is set too short, machines with a copy of the encrypted partition have an increased chance of unlocking the partition first.
To start encryption, enter yes
.
cryptctl
will now encrypt the specified directory to
the previously empty partition and then mount the newly encrypted
partition. The file system type will be of the same type as the
original unencrypted file system.
Before creating the encrypted partition,
cryptctl
moves the unencrypted content of the
original directory to a location prefixed with
cryptctl-moved-
.
To check that the directory is indeed mounted correctly, use:
tux >
lsblk -o NAME,MOUNTPOINT,UUID
NAME MOUNTPOINT UUID [...] sdc └─sdc1 PARTITION_UUID └─cryptctl-unlocked-sdc1 /secret-partition UNLOCKED_UUID
cryptctl
identifies the encrypted partition by its
UUID. For the previous example, that is the UUID displayed next to
sdc1
.
On the server, you can check whether the directory was decrypted using
cryptctl
.
root #
cryptctl list-keys
For a successfully decrypted partition, you will see output like:
2019/06/06 15:50:00 ReloadDB: successfully loaded database of 1 records Total: 1 records (date and time are in zone EDT) Used By When UUID Max.Users Num.Users Mount Point IP_ADDRESS 2019-06-06 15:00:50 UUID 1 1 /secret-partition
For a partition not decrypted successfully, you will see output like:
2019/06/06 15:50:00 ReloadDB: successfully loaded database of 1 records Total: 1 records (date and time are in zone EDT) Used By When UUID Max.Users Num.Users Mount Point 2019-06-06 15:00:50 UUID 1 1 /secret-partition
See the difference in the empty Used by
column.
Verify that the UUID shown is that of the previously encrypted partition.
After verifying that the encrypted partition works, delete the
unencrypted content from the client. For example, use rm
.
For more
safety, overwrite the content of the files before deleting them, for
example, using shred -u
.
shred
Does Not Guarantee That Data Is Completely Erased
Depending on the type of storage media, using
shred
is not a guarantee that all data is
completely removed. In particular, SSDs usually employ wear leveling
strategies that render shred
ineffective.
The configuration for the connection from client to server is stored in
/etc/sysconfig/cryptctl-client
and can be edited
manually.
The server stores an encryption key for the client partition in
/var/lib/cryptctl/keydb/PARTITION_UUID
.
When a cryptctl
client is active, it will send a
“heartbeat” to the cryptctl
server every
10 seconds. If the server does not receive a heartbeat from the client
for the length of the timeout configured during the client setup, the
server will assume that the client is offline. It will then allow another
client to connect (or allow the same client to reconnect after a reboot).
To see the usage status of all keys, use:
root #
cryptctl list-keys
The information under Num. Users
shows whether the key
is currently in use. To see more detail on a single key, use:
root #
cryptctl show-key UUID
This command will show information about mount point, mount options, usage options, the last retrieval of the key, and the last three heartbeats from clients.
Additionally, you can use journalctl
to find logs of
when keys were retrieved.
There are two ways of unlocking a partition manually, both of which are run on a client:
Online Unlocking. Online unlocking allows circumventing timeout or user limitations. This method can be used when there is a network connection between client and server but the client could not (yet) unlock the partition automatically. This method will unlock all encrypted partitions on a machine.
To use it, run cryptctl online-unlock
. Be prepared to
enter the password specified when setting up the server.
Offline Unlocking. This method can be used when a client cannot or must not be brought online to communicate with its server. The encryption key from the server must still be available. This method is meant as a last resort only and can only unlock a single partition at a time.
To use it, run cryptctl offline-unlock
. The
server's key file for the requisite partition
(/var/lib/cryptctl/keydb/PARTITION_UUID
)
needs to be available on the client.
To ensure that partitions cannot be decrypted during a maintenance
downtime, turn off the client and disable the
cryptctl
server. You can do so by either:
Stopping the service
cryptctl-server
:
root #
systemctl stop cryptctl-server
Unplugging the cryptctl
server from the network.
For more information, also see the project home page https://github.com/HouzuoGuo/cryptctl/.
root
Logins
It is important that all system and vendor accounts that are not used for
logins are locked. To get a list of unlocked accounts on your system, you
can check for accounts that do not have an encrypted
password string starting with !
or
*
in the /etc/shadow
file. If you
lock an account using passwd
-l
, it
will put a !!
in front of the encrypted password,
effectively disabling the password. If you lock an account using
usermod
-L
, it will put a
!
in front of the encrypted password. Many system and
shared accounts are usually locked by default by having a
*
or !!
in the password field which
renders the encrypted password into an invalid string. Hence, to get a
list of all unlocked (encryptable) accounts, run
(egrep
is used to allow use of regular-expressions):
root #
egrep -v ':\*|:\!' /etc/shadow | awk -F: '{print $1}'
Also make sure all accounts have a x
in the password
field in /etc/passwd
. The following command lists
all accounts that do not have a x
in the password
field:
root #
grep -v ':x:' /etc/passwd
An x
in the password fields means that the password
has been shadowed, for example the encrypted password needs to be looked
up in the /etc/shadow
file. If the password field in
/etc/passwd
is empty, then the system will not look
up the shadow file and it will not prompt the user for a password at the
login prompt.
All system or vendor accounts that are not being used by users, applications, by the system or by daemons should be removed from the system. You can use the following command to find out if there are any files owned by a specific account:
root #
find / -path /proc -prune -o -user ACCOUNT -ls
The -prune
option in this example is used to skip the
/proc file system. If you are sure that an account can be deleted, you
can remove the account using the following command:
root #
userdel -r ACCOUNT
Without the -r
option userdel
will
not delete the user's home directory and mail spool
(/var/spool/mail/USER
).
Note that many system accounts have no home directory.
Password expiration is a general best practice—but might need to be excluded for some system and shared accounts (for example Oracle, etc.). Expiring password on those accounts could lead to system outages if the application account expires.
Typically a corporate policy should be developed that dictates rules/procedures regarding password changes for system and shared accounts. However, normal user account passwords should expire automatically. The following example shows how password expiration can be set up for individual user accounts.
The following files and parameters in the table can be used when a new
account is created with the useradd
command. Settings
such as these are stored for each user account in the
/etc/shadow
file. If using the YaST tool
( ) to add users, the settings
are available on a per-user basis. Here are the various
settings—some of which can also be system-wide (for example
modification of /etc/login.defs
and
/etc/default/useradd
):
|
|
Maximum number of days a password is valid. |
|
|
Minimum number of days before a user can change the password since the last change. |
|
|
Number of days when the password change reminder starts. |
|
|
Number of days after password expiration that account is disabled. |
|
|
Account expiration date in the format YYYY-MM-DD. |
Users created prior to these modifications will not be affected.
Ensure that the above parameters are changed in the
/etc/login.defs
and
/etc/default/useradd
files. Review of the
/etc/shadow
file will show how these settings get
stored after adding a user.
To create a new user account, execute the following command:
root #
useradd -c "TEST_USER" -g USERS TEST
The -g
option specifies the primary group for this
account:
root #
id TEST
uid=509(test) gid=100(users) groups=100(users)
The settings in /etc/login.defs
and
/etc/default/useradd
are recorded for the test user
in the /etc/shadow
file as follows:
root #
grep TEST /etc/shadow
test:!!:12742:7:60:7:14::
Password aging can be modified at any time by use of the
chage
command. To disable password aging for system and
shared accounts, you can run the following chage
command:
root #
chage -M -1 SYSTEM_ACCOUNT_NAME
To get password expiration information:
root #
chage -l SYSTEM_ACCOUNT_NAME
For example:
root #
chage -l TEST
Minimum: 7
Maximum: 60
Warning: 7
Inactive: 14
Last Change: Jan 11, 2015
Password Expires: Mar 12, 2015
Password Inactive: Mar 26, 2015
Account Expires: Never
On an audited system it is important to restrict people from using simple passwords that can be cracked too easily. Writing down complex passwords is all right as long as they are stored securely. Some will argue that strong passwords protect you against dictionary attacks and those type of attacks can be defeated by locking accounts after a few failed attempts. However, this is not always an option. If set up like this, locking system accounts could bring down your applications and systems which would be nothing short of a denial of service attack – another issue.
At any rate, it is important to practice effective password management safety. Most companies require that passwords have at the very least a number, one lowercase letter, and one uppercase letter. Policies vary, but maintaining a balance between password strength/complexity and management is sometimes difficult.
Linux-PAM (Pluggable Authentication Modules for Linux) is a suite of shared libraries that enable the local system administrator to choose how applications authenticate users.
It is strongly recommended to familiarize oneself with the capabilities of PAM and how this architecture can be leveraged to provide the best authentication setup for an environment. This configuration can be done once – and implemented across all systems (a standard) or can be enhanced for individual hosts (enhanced security – by host / service / application). The key is to realize how flexible the architecture is.
To learn more about the PAM architecture, find PAM documentation in
the /usr/share/doc/packages/pam
directory (in
a variety of formats).
The following discussions are examples of how to modify the default PAM stacks—specifically around password policies—for example password strength, password re-use and account locking. While these are only a few of the possibilities, they serve as a good start and demonstrate PAM's flexibility.
pam-config
Limitations
The pam-config
tool can be used to configure the
common-{account,auth,password,session} PAM configuration files, which
contain global options. These files include the following comment:
# This file is autogenerated by pam-config. All changes # will be overwritten.
Individual service files, such as login, password, sshd
,
and su
must be edited directly. You may elect to edit
all files directly, and not use pam-config
, though
pam-config
includes useful features such as converting
an older configuration, updating your current configuration, and sanity checks.
For more information, see man 8 pam-config
.
openSUSE Leap can leverage the
pam_cracklib
library to test
for weak passwords – and to suggest using a stronger one if it
determines obvious weakness. The following parameters represent an
example that could be part of a corporate password policy or something
required because of audit constraints.
The PAM libraries follow a defined flow. The best way to design the perfect stack usually is to consider all of the requirements and policies and draw out a flow chart.
|
|
Minimum length of password is 8 |
|
|
Minimum number of lowercase letters is 1 |
|
|
Minimum number of uppercase letters is 1 |
|
|
Minimum number of digits is 1 |
|
|
Minimum number of other characters is 1 |
To set up these password restrictions, use the
pam-config
tool to specify the parameters you want
to configure. For example, the minimum length parameter could be
modified like this:
tux >
sudo
pam-config -a --cracklib-minlen=8 --cracklib-retry=3 \ --cracklib-lcredit=-1 --cracklib-ucredit=-1 --cracklib-dcredit=-1 \ --cracklib-ocredit=-1 --cracklib
Now verify that the new password restrictions work for new passwords.
Simply login to a non-root account and change the password using the
passwd
command. Note that the above requirements
are not enforced if you run the passwd
command
under root.
The pam_pwhistory module can be used to configure the number of previous passwords that cannot be reused. The following command implements password restrictions on a system so that a password cannot be reused for at least six months:
tux >
sudo
pam-config -a --pwhistory --pwhistory-remember=26
Recall that in the section
Section 16.2, “Enabling Password Aging” we set
PASS_MIN_DAYS
to 7
, which
specifies the minimum number of days allowed between password changes.
Therefore, if pam_unix
is configured to
remember 26
passwords, then the previously used
passwords cannot be reused for at least six months (26*7 days).
The PAM configuration (/etc/pam.d/common-auth
)
resulting from the pam-config
command looks like the
following:
auth required pam_env.so auth required pam_unix.so try_first_pass account required pam_unix.so try_first_pass password requisit pam_cracklib.so password required pam_pwhistory.so remember=26 password optional pam_gnome_keyring.so use_authtok password required pam_unix.so use_authtok nullok shadow try_first_pass session required pam_limits.so session required pam_unix.so try_first_pass session optional pam_umask.so
Locking accounts after a defined number of failed ssh, login, su, or sudo attempts is a common security practice. However, this could lead to outages if an application, admin, or root user is locked out. In effect this makes it easy to cause denial-of-service attacks by deliberately creating login failures. Fortunately, controlling this with PAM is straightforward.
By default, PAM allows all root logins. Use pam_tally2
to control failed login behavior for all other users, including human and
system users. Add the following line to the top of
/etc/pam.d/login
to lock out all users (except root)
after six failed logins, and to automatically unlock the account after
ten minutes:
auth required pam_tally2.so deny=6 unlock_time=600
This is an example of a complete /etc/pam.d/login
file:
#%PAM-1.0 auth requisite pam_nologin.so auth include common-auth auth required pam_tally2.so deny=6 unlock_time=600 account include common-account account required pam_tally2.so password include common-password session required pam_loginuid.so session include common-session #session optional pam_lastlog.so nowtmp showfailed session optional pam_mail.so standard
You may also lock out root, though obviously you must be very certain you want to do this:
auth required pam_tally2.so deny=6 even_deny_root unlock_time=600
You may define a different lockout time for root:
auth required pam_tally2.so deny=6 root_unlock_time=120 unlock_time=600
If you want to require the administrator to unlock accounts, leave off the
unlock_time
option. The next two example commands
display the number of failed login attempts and how to unlock a user
account:
root #
pam_tally2 -u username
Login Failures Latest failure From username 6 12/17/19 13:49:43 pts/1root #
pam_tally2 -r -u username
The default location for attempted accesses is recorded in
/var/log/tallylog
.
If the user succeeds in logging in after the login timeout expires, or after the administrator resets their account, the counter resets to 0.
Configure other login services to use pam_tally2
in their
individual configuration files in /etc/pam.d/
:
sshd, su, sudo, sudo-i
, and su-l
.
root
Logins #Edit source
By default, the root
user is assigned a password
and can log in using various methods—for example, on a local
terminal, in a graphical session, or remotely via SSH. These methods
should be restricted as far as possible. Shared usage of the root account
should be avoided. Instead, individual administrators should use tools such as
su
or sudo
(for more information,
type man 1 su
or man 8 sudo
) to obtain elevated
privileges. This allows associating root
logins with particular
users. This also adds another layer of security; not only the root
password, but both the root
and the password of
an an administrator's regular account would need to be compromised to gain full root
access. This section explains how to limit direct root logins on the different
levels of the system.
TTY devices provide text-mode system access via the console. For desktop
systems these are accessed via the local keyboard or—in case of server
systems—via input devices connected to a KVM switch or a remote
management card (ILO, DRAC, etc).
By default, Linux offers 6 different consoles, that can be switched to
via the key combinations
Alt–F1 to
Alt–F6, when
running in text mode, or
Ctrl–Alt–F1 to
Ctrl–Alt–F6
when running in a graphical session. The
associated terminal devices are named tty1
.. tty6
accordingly.
The following steps restrict root access to the first TTY. Even this access method is only meant for emergency access to the system and should never be used for everyday system administration tasks.
The steps shown here are tailored towards PC architectures (x86 and
AMD64/Intel 64). On architectures such as POWER, different terminal
device names than tty1
may be used. Be careful not to
lock yourself out completely by specifying wrong terminal device names.
You can determine the device name of the terminal you are currently
logged into by running the tty
command. Be careful
not to do this in a virtual terminal, such as via SSH or in a graphical
session (device names /dev/pts/N
),
but only from an actual login terminal reachable via
Alt–FN.
Ensure that the PAM stack configuration file /etc/pam.d/login
contains the pam_securetty
module in the auth
block:
auth requisite pam_nologin.so auth [user_unknown=ignore success=ok ignore=ignore auth_err=die default=bad] pam_securetty.so noconsole auth include common-auth
This will include the pam_securetty
module during the
authentication process on local consoles, which restricts root
to logging-in
only on TTY devices that are listed in the file /etc/securetty
.
Remove all entries from /etc/securetty
except one.
This limits the access to TTY devices for root.
# # This file contains the device names of tty lines (one per line, # without leading /dev/) on which root is allowed to login. # tty1
Check whether logins to other terminals will be rejected for
root
. A login on tty2
, for example, should be
rejected immediately, without even querying the account password.
Also make sure that you can still successfully login to
tty1
and thus that root
is not locked out of
the system completely.
Do not add the pam_securetty
module to the
/etc/pam.d/common-auth
file. This would
break the su
and sudo
commands,
because these tools would then also reject root
authentications.
These configuration changes will also cause root logins on serial
consoles such as /dev/ttyS0
to be denied. In case you
require such use cases you need to list the respective tty devices
additionally in the /etc/securetty
file.
To improve security on your server, avoid using graphical
environments at all. Graphical programs are often
not designed to be run as root
and are more likely to
contain security issues than console programs. If you require a graphical
login, use a non-root
login. Configure your system to disallow
root
from logging in to graphical sessions.
To prevent root
from logging in to graphical sessions, you
can apply the same basic steps as outlined in
Section 16.5.1, “Restricting Local Text Console Logins”.
Just add the pam_securetty
module to the PAM
stack file belonging to the display manager—for example,
/etc/pam.d/gdm
for GDM. The graphical
session also runs on a TTY device: by default, tty7
.
Therefore, if you restrict root
logins to tty1
,
then root
will be denied login in the graphical session.
By default, the root
user is also allowed to log
into a machine remotely via the SSH network protocol (if the SSH port is
not blocked by the firewall). To restrict this, make the following
change to the OpenSSH configuration:
Edit /etc/ssh/sshd_config
and adjust the following parameter:
PermitRootLogin no
Restart the sshd
service to make the changes effective:
systemctl restart sshd.service
Using the PAM pam_securetty
module is not suitable in
case of OpenSSH, because not all SSH logins go through the PAM
stack during authorization (for example, when using SSH public-key authentication).
In addition, an attacker could differentiate between a wrong password
and a successful login that was only rejected later on by policy.
It can be a good idea to terminate an interactive shell session after a certain period of inactivity. For example, to revent open, unguarded sessions or to avoid waste of system resources.
By default, there is no inactivity timeout for shells. Nothing will happen if a shell stays open and unused for days or even years. It is possible to configure most shells in a way that idle sessions terminate automatically after a certain amount of time. The following example shows how to set an inactivity timeout for a number of common types of shells.
The inactivity timeout can be configured for login shells only or for all interactive shells. In the latter case the inactivity timeout is running individually for each shell instance. This means that timeouts will accumulate. When a sub- or child-shell is started, then a new timeout begins for the sub- or child-shell, and only afterwards will the timeout of the parent continue running.
The following table contains configuration details for a selection of common shells shipped with openSUSE Leap:
package | shell personalities | shell variable | time unit | readonly setting | config path (only login shell) | config path (all shells) |
---|---|---|---|---|---|---|
|
|
| seconds |
|
|
|
|
|
| seconds |
|
|
|
|
|
| minutes |
|
|
|
|
|
| seconds |
|
|
|
Every listed shell supports an internal timeout shell variable that can be set to a specific time value to cause the inactivity timeout. If you want to prevent users from overriding the timeout setting, you can mark the corresponding shell timeout variable as readonly. The corresponding variable declaration syntax is also found in the table above.
This feature is only helpful for avoiding risks if a user is forgetful or follows unsafe practices. It does not protect against hostile users. The timeout only applies to interactive wait states of a shell. A malicious user can always find ways to circumvent the timeout and keep their session open regardless.
To configure the inactivity timeout, you need to add the matching timeout
variable declaration to each shell's startup script. Use either the path for
login shells only, or the one for all shells, as listed in the table. The
following example uses paths and settings that are suitable for
bash
and ksh
to setup a read-only login shell timeout that cannot be overridden by users.
Create the file /etc/profile.d/timeout.sh
with the
following content:
# /etc/profile.d/timeout.sh for SUSE Linux # # Timeout in seconds until the bash/ksh session is terminated # in case of inactivity. # 24h = 86400 sec readonly TMOUT=86400
We recommend using the screen
tool in order to
detach sessions before logging out. screen
sessions
are not terminated and can be re-attached whenever required. An active
session can be locked without logging out (read about Ctrl–a–x / lockscreen
in
man screen
for details).
Linux allows you to set limits on the amount of system resources that users and groups can consume. This is also very handy if bugs in programs cause them to use up too much resources (for example memory leaks), slow down the machine, or even render the system unusable. Incorrect settings can allow programs to use too many resources which may make the server unresponsive to new connections or even local logins (for example if a program uses up all available file handles on the host). This can also be a security concern if someone is allowed to consume all system resources and therefore cause a denial of service attack – either unplanned or worse, planned. Setting resource limits for users and groups may be an effective way to protect systems, depending on the environment.
The following example demonstrates the practical usage of setting or
restricting system resource consumption for an Oracle user account. For a
list of system resource settings, see
/etc/security/limits.conf
or man
limits.conf
.
Most shells like Bash provide control over various resources (for example the maximum allowable number of open file descriptors or the maximum number of processes) that are available on a per/user basis. To examine all current limits in the shell execute:
root #
ulimit -a
For more information on ulimit
for the Bash shell,
examine the Bash man pages.
Setting "hard" and "soft" limits might not behave as expected when using
an SSH session. To see valid behavior it may be necessary to login as
root and then su to the id with limits (for example,
oracle
in these examples). Resource limits should
also work assuming the application has been started automatically during
the boot process. It may be necessary to set
UsePrivilegeSeparation
in
/etc/ssh/sshd_config
to "no" and restart the SSH
daemon (systemctl restart sshd
) if it seems that the
changes to resource limits are not working (via SSH). However this is
not generally recommended – as it weakens a systems security.
ssh
You can add some extra security to your server by disabling password
authentication for SSH. Remember that you need to have SSH keys
configured, otherwise you cannot access the server. To disable password
login, add the following lines to
/etc/ssh/sshd_config
:
UseLogin no UsePAM no PasswordAuthentication no PubkeyAuthentication yes
In this example, a change to the number of file handles or open files
that the user oracle
can use is made by editing
/etc/security/limits.conf
as root
making the
following changes:
oracle soft nofile 4096 oracle hard nofile 63536
The soft limit in the first line defines the limit on the number of file
handles (open files) that the
oracle
user will have after
login. If the user sees error messages about running out of file handles,
then the user can increase the number of file handles like in this
example up to the hard limit (in this example 63536) by executing:
root #
ulimit -n 63536
You can set the soft and hard limits higher if necessary.
It is important to be judicious with the usage of ulimits. Allowing a
"hard" limit for “nofile” for a user that equals the kernel
limit (/proc/sys/fs/file-max
) is very bad! If the
user consumes all the available file handles, the system cannot initiate
new logins as accessing (opening) PAM modules which are required for
performing a login will not be possible.
You also need to ensure that pam_limits
is either
configured globally in /etc/pam.d/common-auth
, or
for individual services like SSH, su, login, and telnet in:
/etc/pam.d/sshd (for SSH) |
/etc/pam.d/su (for su) |
/etc/pam.d/login (local logins and telnet) |
If you do not want to enable it for all logins, there is a specific PAM
module that will read the /etc/security/limits.conf
file. Entries in pam configuration directives will have entries like:
session required /lib/security/pam_limits.so session required /lib/security/pam_unix.so
It is important to note that changes are not immediate and require a new login session:
root #
su – oracletux >
ulimit -n 4096
Note that these examples are specific to the Bash shell -
ulimit
options are different for other shells. The
default limit for the user oracle
is
4096
. To increase the number of file handles the user
oracle
can use to 63536
, do:
root #
su – oracletux >
ulimit -n 4096tux >
ulimit -n 63536tux >
ulimit -n 63536
Making this permanent requires the addition of the setting,
ulimit -n 63536
, (again, for Bash) to the users
profile (~/.bashrc
or
~/.profile
file) which is the user start-up file for
the Bash shell on openSUSE Leap (to verify your shell run: echo
$SHELL
). To do this you could simply type (or copy/paste
– if you are reading this on the system) the following commands for the
Bash shell of the user oracle
:
root #
su - oracletux >
cat >> ~oracle/.bash_profile << EOF ulimit -n 63536 EOF
It is often necessary to place a banner on login screens on all servers for legal/audit policy reasons or to give security instructions to users.
If you want to print a login banner after a user
logs in on a text based terminal for example using SSH or on a local
console, you can leverage the file /etc/motd
(motd =
message of the day). The file exists by default on openSUSE Leap, but
it is empty. Simply add content to the file that is applicable/required
by the organization.
Try to keep the login banner content to a single terminal page (or less), as it will scroll the screen if it does not fit, making it more difficult to read.
You can also have a login banner printed before a
user logs in on a text based terminal. For local console
logins you can edit the /etc/issue
file, which will
cause the banner to be displayed before the login prompt. For logins via
SSH you can edit the “Banner” parameter in the
/etc/ssh/sshd_config
file, which will then
appropriately display the banner text before the SSH login prompt.
For graphical logins via GDM, you can follow
the GNOME admin guide to set up a login banner. Furthermore
you can make the following changes to require a user to acknowledge the
legal banner by selecting or .
Edit the /etc/gdm/Xsession
file and add the
following lines at the beginning of the script:
if ! /usr/bin/gdialog --yesno '\nThis system is classified...\n' 10 10; then /usr/bin/gdialog --infobox 'Aborting login' exit 1; fi
The text This system is classified... needs to be replaced with the desired banner text. It is important to note that this dialog will not prevent a login from progressing. For more information about GDM scripting, refer to the GDM Admin Manual.
Here is a list of commands you can use to get data about user logins:
who
.
Lists currently logged in users.
w
.
Shows who is logged in and what they are doing.
last
.
Shows a list of last logged in users, including login time, logout time,
login IP address, etc.
lastb
.
Same as last
, except that by default it shows
/var/log/btmp
, which contains all the bad login
attempts.
lastlog
.
This command reports data maintained in
/var/log/lastlog
, which is a record of the last
time a user logged in.
ac
.
Available after installing the acct
package.
Prints the connect time in hours on a per-user basis or daily basis,
etc. This command reads /var/log/wtmp
.
dump-utmp
.
Converts raw data from /var/run/utmp or
/var/log/wtmp
into ASCII-parseable format.
Also check the /var/log/messages
file, or the output
of journalctl
if no logging facility is running. See
Book “Reference”, Chapter 11 “journalctl
: Query the systemd
Journal” for more information on the systemd
journal.
spectre-meltdown-checker
is a shell script to test if
your system is vulnerable to the several speculative execution
vulnerabilities that are in nearly all CPUs manufactured in the
past 20 years. This is a hardware flaw that potentially allows an attacker
to read all data on the system. On cloud computing services, where multiple
virtual machines are on a single physical host, an attacker can gain access
to all virtual machines. Fixing these vulnerabilities requires redesigning
and replacing CPUs. Until this happens, there are several software patches
that mitigate these vulnerabilities. If you have kept your SUSE systems
updated, all of these patches should already be installed.
spectre-meltdown-checker
generates a detailed report. It
is impossible to guarantee that your system is secure, but it shows you
which mitigations are in place, and potential vulnerabilities.
spectre-meltdown-checker
#Edit sourceInstall the script, and then run it as root without any options:
root #
zypper in spectre-meltdown-checkerroot #
spectre-meltdown-checker.sh
You will see colorful output like Figure 17.1, “Output from spectre-meltdown-checker”:
spectre-meltdown-checker.sh --help
lists all options. It
is useful to pipe plain text output, with no colors, to a file:
root #
spectre-meltdown-checker.sh --no-color| tee filename.txt
The previous examples are on a running system, which is the default. You may
also run spectre-meltdown-checker
offline by specifying
the paths to the kernel, config, and System.map files:
root #
cd /bootroot #
spectre-meltdown-checker.sh \ --no-color \ --kernel vmlinuz-4.12.14-lp151.28.13-default \ --config config-4.12.14-lp151.28.13-default \ --map System.map-4.12.14-lp151.28.13-default| tee filename.txt
Other useful options are:
Increase verbosity; repeat for more verbosity, for example -v -v
-v
Print human-readable explanations
Format output in various machine-readable formats
spectre-meltdown-checker.sh --disclaimer
provides
important information about what the script does, and does not do.
For more information, see the following references:
SUSE Knowledge Base article #7022937, Security Vulnerability: Spectre Variant 4 (Speculative Store Bypass) aka CVE-2018-3639: https://www.suse.com/support/kb/doc/?id=7022937
speed47/spectre-meltdown-checker source code on GitHub, with detailed references to relevant Common Vulnerabilities and Exposures (CVE): https://github.com/speed47/spectre-meltdown-checker
SUSE Blog article, Meltdown and Spectre Performance: https://www.suse.com/c/meltdown-spectre-performance/
SUSE Knowledge Base article #7022512, providing information on architectures, CVEs, and mitigations: https://www.suse.com/support/kb/doc/?id=7022512
The YaST module openSUSE Leap. 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 › . The dialog always starts with the , and other configuration dialogs are available from the right pane.
offers a central clearinghouse to configure security-related settings forThe
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. Click to open an overview of the setting and information on how to make it secure. To change a setting, click the corresponding link in the Status column. Depending on the setting, the following entries are available:Click this entry to toggle the status of the setting to either enabled or disabled.
Click this entry to 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.
openSUSE Leap comes with three . These configurations affect all the settings available in the module. Each configuration can be modified to your needs using the dialogs available from the right pane changing its state to :
A configuration for a workstation with any kind of network connection (including a connection to the Internet).
This setting is designed for a laptop or tablet that connects to different networks.
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.
A pre-selected
(when opening the dialog) indicates that one of the predefined sets has been modified. Actively choosing this option does not change the current configuration—you will need to change it using the .Passwords that are easy to guess are a major security issue. The
dialog provides the means to ensure that only secure passwords can be used.By activating this option, a warning will be issued if new passwords appear in a dictionary, or if they are proper names (proper nouns).
If the user chooses a password with a length shorter than specified here, a warning will be issued.
When password expiration is activated (via
), this setting stores the given number of a user's previous passwords, preventing their reuse.Choose a password encryption algorithm. Normally there is no need to change the default (Blowfish).
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.
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.
Configure which users can shut down the machine via the graphical login manager in this dialog. You can also specify how Ctrl–Alt–Del will be interpreted and who can hibernate the system.
This dialog lets you configure security-related login settings:
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.
When checked, the graphical login manager (GDM) can be accessed from the network. This is a potential security risk.
Set minimum and maximum values for user and group IDs. These default settings would rarely need to be changed.
Other security settings that do not fit the above-mentioned categories are listed here:
openSUSE Leap 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.
file permissions are suitable for stand-alone machines. These settings
allow regular users to, for example, read most system files. See the
file /etc/permissions.easy
for the complete
configuration. The file permissions are
designed for multiuser machines with network access. A thorough
explanation of these settings can be found in
/etc/permissions.secure
. The
settings are the most restrictive ones and
should be used with care. See
/etc/permissions.paranoid
for more information.
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.
The magic SysRq key is a key combination that enables you to have some control over the system even when it has crashed. The complete documentation can be found at https://www.kernel.org/doc/html/latest/admin-guide/sysrq.html.
PolKit (formerly known as PolicyKit) is an application framework that
acts as a negotiator between the unprivileged user session and the
privileged system context. Whenever a process from the user session
tries to carry out an action in the system context, PolKit is queried.
Based on its configuration—specified in a so-called
“policy”—the answer could be “yes”,
“no”, or “needs authentication”. Unlike
classical privilege authorization programs such as sudo, PolKit does
not grant root
permissions to an entire session, but only to
the action in question.
PolKit works by limiting specific actions by users, by group, or by name. It then defines how those users are allowed to perform this action.
When a user starts a session (using the graphical environment or on the console), each session consists of the authority and an authentication agent. The authority is implemented as a service on the system message bus, whereas the authentication agent is used to authenticate the current user, which started the session. The current user needs to prove their authenticity, for example, using a passphrase.
Each desktop environment has its own authentication agent. Usually it is started automatically, whatever environment you choose.
PolKit's configuration depends on actions and authorization rules:
*.policy
)
Written as XML files and located in
/usr/share/polkit-1/actions
. Each file defines
one or more actions, and each action contains descriptions and
default permissions. Although a system administrator can write their
own rules, PolKit's files must not be edited.
*.rules
)
Written as JavaScript files and located in two places:
/usr/share/polkit-1/rules.d
is used for third
party packages and /etc/polkit-1/rules.d
for
local configurations. Each rule file refers to the action specified
in the action file. A rule determines what restrictions are allowed
to a subset of users. For example, a rule file could overrule a
restrictive permission and allow some users to allow it.
PolKit contains several commands for specific tasks (see also the specific man page for further details):
pkaction
Get details about a defined action. See Section 19.3, “Querying Privileges” for more information.
pkcheck
Checks whether a process is authorized, specified by either
--process
or --system-bus-name
.
pkexec
Allows an authorized user to execute the specific program as another user.
pkttyagent
Starts a textual authentication agent. This agent is used if a desktop environment does not have its own authentication agent.
At the moment, not all applications requiring privileges use PolKit. Find the most important policies available on openSUSE® Leap below, sorted into the categories where they are used.
Set scheduling priorities for the PulseAudio daemon |
Add, remove, edit, enable or disable printers |
Modify schedule |
Modify system and mandatory values with GConf |
Change the system time |
Manage and monitor local virtualized systems |
Apply and modify connections |
Read and change privileges for other users |
Modify defaults |
Update and remove packages |
Change and refresh repositories |
Install local files |
Rollback |
Import repository keys |
Accepting EULAs |
Setting the network proxy |
Wake on LAN |
Mount or unmount fixed, hotpluggable and encrypted devices |
Eject and decrypt removable media |
Enable or disable WLAN |
Enable or disable Bluetooth |
Device access |
Stop, suspend, hibernate and restart the system |
Undock a docking station |
Change power-management settings |
Register product |
Change the system time and language |
Every time a PolKit-enabled process carries out a privileged operation,
PolKit is asked whether this process is entitled to do so. PolKit
answers according to the policy defined for this process. The answers can
be yes
, no
, or
authentication needed
. By default, a policy contains
implicit
privileges, which automatically apply to all
users. It is also possible to specify explicit
privileges which apply to a specific user.
Implicit privileges can be defined for any active and inactive sessions. An active session is the one in which you are currently working. It becomes inactive when you switch to another console for example. When setting implicit privileges to “no”, no user is authorized, whereas “yes” authorizes all users. However, usually it is useful to demand authentication.
A user can either authorize by authenticating as root
or by
authenticating as self. Both authentication methods exist in four
variants:
The user always needs to authenticate.
The authentication is bound to the instance of the program currently running. After the program is restarted, the user is required to authenticate again.
The authentication dialog offers a check button
. If checked, the authentication is valid until the user logs out.The authentication dialog offers a check button
. If checked, the user needs to authenticate only once.Explicit privileges can be granted to specific users. They can either be granted without limitations, or, when using constraints, limited to an active session and/or a local console.
It is not only possible to grant privileges to a user, a user can also be blocked. Blocked users cannot carry out an action requiring authorization, even though the default implicit policy allows authorization by authentication.
Each application supporting PolKit comes with a default set of implicit policies defined by the application's developers. Those policies are the so-called “upstream defaults”. The privileges defined by the upstream defaults are not necessarily the ones that are activated by default on SUSE systems. openSUSE Leap comes with a predefined set of privileges that override the upstream defaults:
/etc/polkit-default-privs.standard
Defines privileges suitable for most desktop systems
/etc/polkit-default-privs.restrictive
Designed for machines administrated centrally
To switch between the two sets of default privileges, adjust the value
of POLKIT_DEFAULT_PRIVS
to either
restrictive
or standard
in
/etc/sysconfig/security
. Then run the command
set_polkit_default_privs
as root
.
Do not modify the two files in the list above. To define your
own custom set of privileges, use
/etc/polkit-default-privs.local
. For details, refer
to
Section 19.4.3, “Modifying Configuration Files for Implicit Privileges”.
To query privileges use the command pkaction
included
in PolKit.
PolKit comes with command line tools for changing privileges and
executing commands as another user (see
Section 19.1.3, “Available Commands” for a short
overview). Each existing policy has a speaking, unique name with which it
can be identified. List all available policies with the command
pkaction
. See man pkaction
for more
information.
If you want to display the needed authorization for a given policy (for
example, org.freedesktop.login1.reboot
) use
pkaction
as follows:
tux >
pkaction -v --action-id=org.freedesktop.login1.reboot
org.freedesktop.login1.reboot:
description: Reboot the system
message: Authentication is required to allow rebooting the system
vendor: The systemd Project
vendor_url: http://www.freedesktop.org/wiki/Software/systemd
icon:
implicit any: auth_admin_keep
implicit inactive: auth_admin_keep
implicit active: yes
The keyword auth_admin_keep
means that users need to
enter a passphrase.
pkaction
on openSUSE Leap
pkaction
always operates on the upstream defaults.
Therefore it cannot be used to list or restore the defaults shipped with
openSUSE Leap. To do so, refer to
Section 19.5, “Restoring the Default Privileges”.
Adjusting privileges by modifying configuration files is useful when you want to deploy the same set of policies to different machines, for example to the computers of a specific team. It is possible to change implicit and explicit privileges by modifying configuration files.
The available actions depend on what additional packages you have
installed on your system. For a quick overview, use
pkaction
to list all defined rules.
To get an idea, the following example describes how the command
gparted
(“GNOME Partition Editor”)
is integrated into PolKit.
The file
/usr/share/polkit-1/actions/org.opensuse.policykit.gparted.policy
contains the following content:
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE policyconfig PUBLIC "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN" "http://www.freedesktop.org/standards/PolicyKit/1.0/policyconfig.dtd"> <policyconfig> 1 <action id="org-opensuse-policykit-gparted"> 2 <message>Authentication is required to run the GParted Partition Editor</message> <icon_name>gparted</icon_name> <defaults> 3 <allow_any>auth_admin</allow_any> <allow_inactive>auth_admin</allow_inactive> < allow_active>auth_admin</allow_active> </defaults> <annotate 4 key="org.freedesktop.policykit.exec.path">/usr/sbin/gparted</annotate> <annotate 4 key="org.freedesktop.policykit.exec.allow_gui">true</annotate> </action> </policyconfig>
Root element of the policy file. | |
Contains one single action. | |
The | |
The |
To add your own policy, create a .policy
file with
the structure above, add the appropriate value into the
id
attribute, and define the default permissions.
Your own authorization rules overrule the default settings. To add your
own settings, store your files under
/etc/polkit-1/rules.d/
.
The files in this directory start with a two-digit number, followed by a
descriptive name, and end with .rules
. Functions
inside these files are executed in the order they have been sorted in.
For example, 00-foo.rules
is sorted (and hence
executed) before 60-bar.rules
or even
90-default-privs.rules
.
Inside the file, the script checks for the specified action ID, which is
defined in the .policy
file. For example, if you
want to allow the command gparted
to be executed by
any member of the admin
group, check for the action ID
org.opensuse.policykit.gparted
:
/* Allow users in admin group to run GParted without authentication */ polkit.addRule(function(action, subject) { if (action.id == "org.opensuse.policykit.gparted" && subject.isInGroup("admin")) { return polkit.Result.YES; } });
Find the description of all classes and methods of the functions in the PolKit API at http://www.freedesktop.org/software/polkit/docs/latest/ref-api.html.
openSUSE Leap ships with two sets of default authorizations, located
in /etc/polkit-default-privs.standard
and
/etc/polkit-default-privs.restrictive
. For more
information, refer to
Section 19.2.3, “Default Privileges”.
Custom privileges are defined in
/etc/polkit-default-privs.local
. Privileges defined
here will always take precedence over the ones defined in the other
configuration files. To define your custom set of privileges,
do the following:
Open /etc/polkit-default-privs.local
. To define a
privilege, add a line for each policy with the following format:
<privilege_identifier> <any session>:<inactive session>:<active session>
For example:
org.freedesktop.policykit.modify-defaults auth_admin_keep_always
The following values are valid for the SESSION placeholders:
yes
grant privilege
no
block
auth_self
user needs to authenticate with own password every time the privilege is requested
auth_self_keep_session
user needs to authenticate with own password once per session, privilege is granted for the whole session
auth_self_keep_always
user needs to authenticate with own password once, privilege is granted for the current and for future sessions
auth_admin
user needs to authenticate with root
password every time
the privilege is requested
auth_admin_keep_session
user needs to authenticate with root
password once per
session, privilege is granted for the whole session
auth_admin_keep_always
user needs to authenticate with root
password once,
privilege is granted for the current and for future sessions
Run as root
for changes to take effect:
# /sbin/set_polkit_default_privs
Optionally check the list of all privilege identifiers with the
command pkaction
.
openSUSE Leap comes with a predefined set of privileges that is activated by default and thus overrides the upstream defaults. For details, refer to Section 19.2.3, “Default Privileges”.
Since the graphical PolKit tools and the command line tools always
operate on the upstream defaults, openSUSE Leap includes an additional
command-line tool, set_polkit_default_privs
. It resets
privileges to the values defined in
/etc/polkit-default-privs.*
. However, the command
set_polkit_default_privs
will only reset policies that
are set to the upstream defaults.
Make sure /etc/polkit-default-privs.local
does not
contain any overrides of the default policies.
Policies defined in
/etc/polkit-default-privs.local
will be applied
on top of the defaults during the next step.
To reset all policies to the upstream defaults first and then apply the openSUSE Leap defaults:
tux >
sudo
rm -f /var/lib/polkit/* && set_polkit_default_privs
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.
The permissions of all files included in openSUSE Leap are
carefully chosen. When installing additional software or files,
take great care when setting the permissions. Always use the
-l
option with the command ls
to detect any incorrect file permissions immediately. An incorrect
file attribute does not only mean that files could be changed or
deleted. Modified files could be executed by root
or
services could be hijacked by modifying configuration files. This
significantly increases the danger of an attack.
A openSUSE® Leap system includes the files
permissions
,
permissions.easy
,
permissions.secure
, 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 launched it, but
with the permissions of the file owner, usually root
. An
administrator can use the file
/etc/permissions.local
to add their own
settings.
To define one of the available profiles, select /etc/permissions
or consult
man chmod
.
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.
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
.
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
can access
this directory even without root privileges.
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
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 needed to use several
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 Ext2, Ext3, Ext4, 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 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.
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 ACLs can only be applied to directories. They determine the permissions a file system object inherits from its parent directory when it is created.
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.
Table 20.1, “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 and 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 20.2, “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.
Type |
Text Form |
---|---|
owner |
|
named user |
|
owning group |
|
named group |
|
mask |
|
other |
|
Entry Type |
Text Form |
Permissions |
---|---|---|
named user |
|
|
mask |
|
|
effective permissions: |
|
Figure 20.1, “Minimum ACL: ACL Entries Compared to Permission Bits” and
Figure 20.2, “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.
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 20.1, “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 20.2, “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.
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 (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 user::rwx group::r-x other::---
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:
root #
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 user::rwx user:geeko:rwx group::r-x group:mascots:rwx mask::rwx other::---
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
. The mask
entry 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 20.2, “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::rwx user:geeko:rwx # effective: r-x group::r-x group:mascots:rwx # effective: r-x mask::r-x other::---
After executing chmod
to remove the write
permission from the group class bits, the output of
ls
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
.
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.
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.
The following three examples show the main operations for directories and default ACLs:
Add a default ACL to the existing directory
mydir
with:
tux >
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:
tux >
getfacl mydir
# file: mydir
# owner: tux
# group: project3
user::rwx
user:geeko:rwx
group::r-x
group:mascots:rwx
mask::rwx
other::---
default:user::rwx
default:group::r-x
default:group:mascots:r-x
default:mask::r-x
default:other::---
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.
In the next example, use mkdir
to create a
subdirectory in mydir
, which inherits the
default ACL.
tux >
mkdir mydir/mysubdir
getfacl mydir/mysubdir
# file: mydir/mysubdir
# owner: tux
# group: project3
user::rwx
group::r-x
group:mascots:r-x
mask::r-x
other::---
default:user::rwx
default:group::r-x
default:group:mascots:r-x
default:mask::r-x
default:other::---
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.
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 user::rw- group::r-x # effective:r-- group:mascots:r-x # effective:r-- mask::r-- other::---
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 20.4.3.1, “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.
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”.
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 Nautilus.
Vi/Vim and emacs both fully support ACLs by preserving the permissions on
writing files including backups. Unfortunately, many editors and file
managers still lack ACL support. 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 file name,
the ACLs may be lost, unless the editor supports ACLs. Except for the
star
archiver, there are currently no backup applications
that preserve ACLs.
For more information about ACLs, see the man pages for
getfacl(1)
, acl(5)
, and
setfacl(1)
.
Certificates play an important role in the authentication of companies and individuals. Usually certificates are administered by the application itself. In some cases, it makes sense to share certificates between applications. The certificate store is a common ground for Firefox, Evolution, and NetworkManager. This chapter explains some details.
The certificate store is a common database for Firefox, Evolution, and NetworkManager at the moment. Other applications that use certificates are not covered but may be in the future. If you have such an application, you can continue to use its private, separate configuration.
The configuration is mostly done in the background. To activate it, proceed as follows:
Decide if you want to activate the certificate store globally (for every user on your system) or specifically to a certain user:
For every user.
Use the file /etc/profile.local
For a specific user.
Use the file ~/.profile
Open the file from the previous step and insert the following line:
export NSS_USE_SHARED_DB=1
Save the file
Log out of and log in to your desktop.
All the certificates are stored under
$HOME/.local/var/pki/nssdb/
.
To import a certificate into the certificate store, do the following:
Start Firefox.
Open the dialog from
› . Change to › and click .Import your certificate depending on your type: use
to import server certificate, to identify other, and to identify yourself.
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.
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 need 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 root-kit 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.
Before you install your system, verify the checksum of your medium (see Book “Start-Up”, Chapter 4 “Troubleshooting”, Section 4.1 “Checking Media”) to make sure you do not use a compromised source. After you have installed the system, initialize the AIDE database. To make 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 Leap. To install it,
either use › , 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
check boxes are used in the files section. Important options include the
following:
Option |
Description |
---|---|
p |
Check for the file permissions of the selected files or directories. |
i |
Check for the inode number. Every file name has a unique inode number that should not change. |
n |
Check for the number of links pointing to the relevant file. |
u |
Check if the owner of the file has changed. |
g |
Check if the group of the file has changed. |
s |
Check if the file size has changed. |
b |
Check if the block count used by the file has changed. |
m |
Check if the modification time of the file has changed. |
c |
Check if the files access time has changed. |
S |
Check for a changed file size. |
I |
Ignore changes of the file name. |
md5 |
Check if the md5 checksum of the file has changed. We recommend to use sha256 or sha512. |
sha1 |
Check if the sha1 (160 Bit) checksum of the file has changed. We recommend to use sha256 or sha512. |
sha256 |
Check if the sha256 checksum of the file has changed. |
sha512 |
Check if the sha512 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 !/sbin/conf.d
To create the AIDE database, proceed as follows:
Open /etc/aide.conf
.
Define which files should be checked with which check boxes. For a
complete list of available check boxes, see
/usr/share/doc/packages/aide/manual.html
. The
definition of the file selection needs some knowledge about regular
expressions. Save your modifications.
To check whether the configuration file is valid, run:
root #
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:
root #
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.
Initialize the AIDE database. Run the command:
root #
aide -i
Copy the generated database to a save location like a CD-R or DVD-R, a remote server or a flash 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.
To perform a file system check, proceed as follows:
Rename the database:
root #
mv /var/lib/aide/aide.db.new /var/lib/aide/aide.db
After any configuration change, you always need to re-initialize 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 22.2, “Setting Up an AIDE Database” for more information.
Perform the check with the following command:
root #
aide --check
If the output is empty, everything is fine. If AIDE found changes, it displays a summary of changes, for example:
root #
aide --check
AIDE found differences between database and filesystem!!
Summary:
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:
root #
aide --check -V
AIDE found differences between database and filesystem!!
Start timestamp: 2009-02-18 15:14:10
Summary:
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.
To avoid risk, 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 its traces.
To accomplish this task, AIDE must be run from a rescue system that is independent of the installed system. With openSUSE Leap 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 http://en.opensuse.org/SDB:Linuxrc. In the following, one possible way to accomplish this task is discussed.
Provide an FTP server as a second machine.
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:
root #
cp DVD1/suse/ARCH/aideVERSION.ARCH.rpm /srv/ftproot #
cp DVD1/suse/ARCH/mhashVERSION.ARCH.rpm /srv/ftp
Create an info file /srv/ftp/info.txt
that
provides the needed boot parameters for the rescue system:
dud:ftp://ftp.example.com/aideVERSION.ARCH.rpm dud:ftp://ftp.example.com/mhashVERSION.ARCH.rpm
Replace your FTP domain name, VERSION and ARCH with the values used on your system.
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:
info=ftp://ftp.example.com/info.txt
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.
Information about AIDE is available at the following places:
The home page of AIDE: http://aide.sourceforge.net
In the documented template configuration
/etc/aide.conf
.
In several files below
/usr/share/doc/packages/aide
after installing the
aide
package.
On the AIDE user mailing list at https://www.ipi.fi/mailman/listinfo/aide.
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 use this feature in an impressive way. With X, it is no problem to log in to a remote host and start a graphical program that is then sen…
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. 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
.
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 differ…
Today, Internet connections are cheap and available almost everywhere. However, not all connections are secure. Using a Virtual Private Network (VPN), you can create a secure network within an insecure network such as the Internet or Wi-Fi. It can be implemented in different ways and serves several purposes. In this chapter, we focus on the OpenVPN implementation to link branch offices via secure wide area networks (WANs).
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 use this feature in an impressive way. With X, it is no problem to log in to 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—like someone spoofing 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, 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 cannot open any new
windows or X clients.
SSH (secure shell) can be used to encrypt a network connection and forward it to an X server transparently. 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 24, SSH: Secure Network Operations.
If you do not consider the computer where you log in to be a secure host, do not use X forwarding. If X forwarding is enabled, an attacker could authenticate via your SSH connection. The attacker could then intrude on your X server and, for example, read your keyboard input.
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. 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 Leap is OpenSSH.
openSUSE Leap installs the OpenSSH package by default providing the
commands ssh
, scp
, and
sftp
. In the default configuration, remote access of a
openSUSE Leap system is only possible with the OpenSSH utilities, and
only if the sshd
is running and
the firewall permits access.
SSH on openSUSE Leap uses cryptographic hardware acceleration if available. As a result, the transfer of large quantities of data through an SSH connection is considerably faster than without cryptographic hardware. As an additional benefit, the CPU will see a significant reduction in load.
ssh
—Secure Shell #Edit source
With ssh
it is possible to log in to remote
systems and to work interactively. To log in to the host
sun
as user tux
enter one of
the following commands:
tux >
ssh tux@suntux >
ssh -l tux sun
If the user name is the same on both machines, you can omit it. Using
ssh sun
is sufficient. 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 run
non-interactive commands on remote systems using ssh
HOST COMMAND.
COMMAND needs to be properly quoted. Multiple
commands can be concatenated as on a local shell.
tux >
ssh root@sun "dmesg -T | tail -n 25"tux >
ssh root@sun "cat /etc/issue && uptime"
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 local machine over the
existing SSH connection. At the same time, X applications started
remotely cannot be intercepted by unauthorized individuals.
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. Refer to
Section 24.5.2, “Copying an SSH Key” for details.
This mechanism is deactivated in the default settings, but can be
permanently activated at any time in the system-wide configuration file
/etc/ssh/sshd_config
by setting
AllowAgentForwarding yes
.
scp
—Secure Copy #Edit source
scp
copies files to or from a remote machine. If
the user name on jupiter is different than the user name on
sun, specify the latter using the
USER_NAME@host
format. If
the file should be copied into a directory other than 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.
tux >
scp ~/MyLetter.tex tux@sun:/tmp 1tux >
scp tux@sun:/tmp/MyLetter.tex ~ 2
-l
Option
With the ssh
command, the option
-l
can be used to specify a remote user (as an
alternative to the
USER_NAME@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
tux >
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.
sftp
—Secure File Transfer #Edit sourcesftp
#Edit source
If you want to copy several files from 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
man page.
tux >
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'
[...]
As with a regular FTP server, a user cannot only download,
but also upload files to a remote machine running an SFTP server
by using the put
command. By default the
files will be uploaded to the remote host with the same
permissions as on the local host. There are two options to
automatically alter these permissions:
A umask works as a filter against the permissions of the original file on the local host. It can only withdraw permissions:
permissions original |
umask |
permissions uploaded |
---|---|---|
0666 |
0002 |
0664 |
0600 |
0002 |
0600 |
0775 |
0025 |
0750 |
To apply a umask on an SFTP server, edit the file
/etc/ssh/sshd_configuration
. Search for the line
beginning with Subsystem sftp
and add the
-u
parameter with the desired setting, for example:
Subsystem sftp /usr/lib/ssh/sftp-server -u 0002
Explicitly setting the permissions sets the same permissions for all
files uploaded via SFTP. Specify a three-digit pattern such as
600
, 644
, or
755
with -u
. When both
-m
and -u
are specified,
-u
is ignored.
To apply explicit permissions for uploaded files on an SFTP server,
edit the file /etc/ssh/sshd_configuration
.
Search for the line beginning with Subsystem sftp
and add the -m
parameter with the desired setting,
for example:
Subsystem sftp /usr/lib/ssh/sftp-server -m 600
sshd
) #Edit source
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 called 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.
To watch the log entries from the sshd
use the following command:
tux >
sudo
journalctl -u sshd
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.
If you install openSUSE Leap 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.
If the public keys of a host have changed (that needs to be verified
before connecting to such a server), the offending keys can be
removed with ssh-keygen -r
HOSTNAME
.
As of version 6.8, OpenSSH comes with a protocol extension that supports host key rotation. It makes sense to replace keys, if you are still using weak keys such as 1024-bit RSA keys. It is strongly recommended to replace such a key and go for 2048-bit DSA keys or something even better. The client will then use the “best” host key.
After installing new host keys on the server, restart sshd.
This protocol extension can
inform a client of all the new host keys on the server, if the user
initiates a connection with ssh
. Then, the
software on the client updates
~/.ssh/known_hosts
, and the user is not
required to accept new keys of previously known and trusted hosts
manually. The local known_hosts
file will
contain all the host keys of the remote hosts, in addition to the
one that authenticated the host during this session.
Once the administrator of the server knows that all the clients have
fetched the new keys, they can remove the old keys. The protocol
extension ensures that the obsolete keys will be removed from the
client's configuration, too. The key removal occurs while initiating
an ssh
session.
For more information, see:
http://blog.djm.net.au/2015/02/key-rotation-in-openssh-68.html
http://heise.de/-2540907 („Endlich neue Schlüssel für SSH-Server,“ German only)
In its 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. What is
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.
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 (id_rsa.pub
or
id_dsa.pub
) and a private key
(id_rsa
or id_dsa
).
To be able to log in without having to specify the remote user's
password, the public key of the “SSH user” must be
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 in to another host on which this feature is enabled using the same user name. openSUSE Leap is set up for using key-based authentication, covering setting up host-based authentication on openSUSE Leap is beyond the scope of this manual.
If the host-based authentication is to be used, the file
/usr/lib/ssh/ssh-keysign
should
have the setuid bit set, which is not the default setting in
openSUSE Leap. 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.
To generate a key with default parameters (RSA, 2048 bits), enter the
command ssh-keygen
.
Accept the default location to store the key
(~/.ssh/id_rsa
) by pressing
Enter (strongly recommended) or enter an
alternative location.
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.
To change the password of an existing key pair, use the command
ssh-keygen -p
.
To copy a public SSH key to ~/.ssh/authorized_keys
of a user on a remote machine, use the command
ssh-copy-id
. To copy your personal key
stored under ~/.ssh/id_rsa.pub
you may use the
short form. To copy DSA keys or keys of other users, you need
to specify the path:
tux >
~/.ssh/id_rsa.pub
ssh-copy-id -i tux@suntux >
~/.ssh/id_dsa.pub
ssh-copy-id -i ~/.ssh/id_dsa.pub tux@suntux >
~notme/.ssh/id_rsa.pub
ssh-copy-id -i ~notme/.ssh/id_rsa.pub tux@sun
To successfully copy the key, you need to enter the remote
user's password. To remove an existing key, manually edit
~/.ssh/authorized_keys
.
ssh-agent
#Edit source
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
the ssh-agent
man page for details.
After the ssh-agent
is started, you need to add your
keys by using ssh-add
. It will prompt for the
passphrase. After the password has been provided once, you can use the
secure shell commands within the running session without having to
authenticate again.
ssh-agent
in an X Session #Edit source
On openSUSE Leap, the ssh-agent
is automatically
started by the GNOME display manager. To also invoke
ssh-add
to add your keys to the agent at the
beginning of an X session, do the following:
Log in as the desired user and check whether the file
~/.xinitrc
exists.
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
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
When starting a new X session, you will be prompted for your SSH passphrase.
ssh-agent
in a Terminal Session #Edit source
In a terminal session you need to manually start the
ssh-agent
and then call ssh-add
afterward. 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.
tux >
ssh-agent -s /bin/bash
eval $(ssh-agent)
After the agent has been started, run ssh-add
to
provide the agent with your keys.
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.
root #
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:
root #
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 in the OpenSSH package
documentation under
/usr/share/doc/packages/openssh
.
In some environments, it is convenient or necessary to log in over SSH. As such, the user needs to provide a public SSH key. To add or remove an SSH key, proceed as follows:
Open YaST.
Under
, open the module.Select the user you want to change and press
.Switch to the
tab.
Add or remove your public key(s). If you add a public SSH key, look
for the file extension .pub
.
Confirm with
.
Your public SSH key is saved in ~/.ssh/authorized_keys
.
The home page of OpenSSH
The OpenSSH Wikibook
man sshd
The man page of the OpenSSH daemon
man ssh_config
The man page 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
Man pages of several binary files to securely copy files
(scp
, sftp
), to log in
(slogin
, ssh
), and to manage
keys.
/usr/share/doc/packages/openssh/README.SUSE
,
/usr/share/doc/packages/openssh/README.FIPS
SUSE package specific documentation; changes in defaults with respect to upstream, notes on FIPS mode etc.
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. Using iptables—a generic table structure for the
definition of rule sets—precisely controls the packets allowed to
pass a network interface. Such a packet filter can be set up using
firewalld
and its graphical interface firewall-config
.
openSUSE Leap 15.0
introduces firewalld
as the new default software firewall, replacing
SuSEfirewall2.
SuSEfirewall2 has not been removed from openSUSE Leap
15.0
and is still part of the main repository, though not installed by default.
This chapter provides guidance for configuring firewalld
, and migrating
from SuSEfirewall2 for users who have upgraded from older openSUSE Leap
releases.
This section discusses the low-level details of packet filtering. The
components netfilter
and
iptables
are responsible for the filtering and
manipulation of network packets and 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 all 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 25.1, “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 case, 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 allow this.
Masquerading is the Linux-specific form of NAT (network address
translation) and can be used to connect a small LAN with the
Internet. LAN hosts use IP
addresses from the private range (see
Book “Reference”, Chapter 13 “Basic Networking”, Section 13.1.2 “Netmasks and Routing”) and on the Internet
official IP addresses are used. To be able to connect
to the Internet, a LAN host's private address is 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.
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, add
the line net.ipv4.ip_forward = 1
in the file
/etc/sysctl.conf
. Alternatively do this via YaST,
for example by calling yast routing ip-forwarding on
.
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 several 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.
Firewall is probably the term most widely used to describe a mechanism that controls the data flow between networks. 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 Leap. For further information about packet filtering and firewalling, read the Firewall HOWTO.
firewalld
#Edit sourcefirewalld
Replaces SuSEfirewall2
openSUSE Leap 15.0
introduces firewalld
as the new default software firewall, replacing
SuSEfirewall2.
SuSEfirewall2 has not been removed from openSUSE Leap
15.0
and is still part of the main repository, though not installed by default.
If you are upgrading from a release older than openSUSE Leap
15.0,
SuSEfirewall2 will be unchanged and you must manually upgrade to
firewalld
(see Section 25.5, “Migrating from SuSEfirewall2”).
firewalld
is a daemon that maintains the system's
iptables
rules and offers a D-Bus interface for
operating on them. It comes with a command line utility
firewall-cmd
and a graphical user interface
firewall-config
for interacting with it. Since
firewalld
is running in the background and provides a well defined
interface it allows other applications to request changes to the iptables
rules, for example to set up virtual machine networking.
firewalld
implements different security zones. Several predefined
zones like internal
and public
exist.
The administrator can define additional custom zones if desired. Each
zone contains its own set of iptables rules. Each network interface is a
member of exactly one zone. Individual connections can also be assigned to
a zone based on the source addresses.
Each zone represents a certain level of trust. For example the
public
zone is not trusted, because other computers in
this network are not under your control (suitable for Internet or wireless
hotspot connections). On the other hand the internal
zone
is used for networks that are under your control, like a
home or company network. By utilizing zones this way, a host can offer
different kinds of services to trusted networks and untrusted networks in a
defined way.
For more information about the predefined zones and their meaning in
firewalld
, refer to its manual at
http://www.firewalld.org/documentation/zone/predefined-zones.html.
The initial state for network interfaces is to be assigned to no zone at
all. In this case the network interface will be implicitly handled in the
default zone, which can be determined by calling firewall-cmd
--get-default-zone
. If not configured otherwise, the default
zone is the public
zone.
The firewalld
packet filtering model allows any outgoing connections to
pass. Outgoing connections are connections that are actively established by
the local host. Incoming connections that are established by remote hosts are
blocked if the respective service is not allowed in the zone in
question. Therefore, each of the interfaces with incoming traffic must be
placed in a suitable zone to allow for the desired services to be
accessible. For each of the zones, define the services or protocols you
need.
An important concept of firewalld
is the distinction between two separate
configurations: the runtime and the
permanent configuration. The runtime configuration
represents the currently active rules, while the permanent configuration
represents the saved rules that will be applied when restarting
firewalld
. This allows to add temporary rules that will be discarded after
restarting firewalld
, or to experiment with new rules while being able to
revert back to the original state. When you are changing the configuration,
you need to be aware of which configuration you are editing. How this is done
is discussed in Section 25.4.1.2, “Runtime Versus Permanent Configuration”.
To perform the firewalld
configuration using the graphical
user interface firewall-config
refer to its documentation.
In the following section we will be looking at how to perform typical
firewalld
configuration tasks using firewall-cmd
on the
command line.
firewalld
will be installed and enabled by default. It is a regular
systemd
service that can be configured via systemctl
or the YaST Services Manager.
After the installation, YaST automatically starts firewalld
and
leaves all interfaces in the default public
zone. If a
server application is configured and activated on the system, YaST can
adjust the firewall rules via the options or in the server configuration modules. Some server module
dialogs include a button for
activating additional services and ports.
By default all firewall-cmd
commands operate on the
runtime configuration. You can apply most operations to the permanent
configuration only by adding the
--permanent
parameter. When doing so the change will
only affect the permanent configuration and will not be effective
immediately in the runtime configuration. There is currently no way to add
a rule to both runtime and permanent configurations in a single invocation.
To achieve this you can apply all necessary changes to the runtime
configuration and when all is working as expected issue the following
command:
root #
firewall-cmd --runtime-to-permanent
This will write all current runtime rules into the permanent configuration.
Any temporary modifications you or other programs may have made to the
firewall in other contexts are made permanent this way. If you are unsure
about this, you can also take the opposite approach to be on the safe side:
Add new rules to the permanent configuration and reload firewalld
to
make them active.
Some configuration items, like the default zone, are shared by both the runtime and permanent configurations. Changing them will reflect in both configurations at once.
To revert the runtime configuration to the permanent configuration and
thereby discard any temporary changes, two possibilities exist, either via
the firewalld
command line interface or via systemd
:
root #
firewall-cmd --reload
root #
systemctl reload firewalld
For brevity the examples in the following sections will always operate on the runtime configuration, if applicable. Adjust them accordingly to make them permanent.
You can list all network interfaces currently assigned to a zone like this:
root #
firewall-cmd --zone=public --list-interfaces
eth0
Similarly you can query which zone a specific interface is assigned to:
root #
firewall-cmd --get-zone-of-interface=eth0
public
The following command lines assign an interface to a zone. The variant
using --add-interface
will only work if
eth0
is not already assigned to another zone. The
variant using --change-interface
will always work,
removing eth0
from its current zone if necessary:
root #
firewall-cmd --zone=internal --add-interface=eth0
root #
firewall-cmd --zone=internal --change-interface=eth0
Any operations without an explicit --zone
argument will
implicitly operate on the default zone. This pair of commands can be used
for getting and setting the default zone assignment:
root #
firewall-cmd --get-default-zone
dmzroot #
firewall-cmd --set-default-zone=public
Any network interfaces not explicitly assigned to a zone will be
automatically part of the default zone. Changing the default zone will
reassign all those network interfaces immediately for the permanent and
runtime configurations. You should never use a trusted zone like
internal
as the default zone, to avoid unexpected
exposure to threats. For example hotplugged network interfaces like USB
Ethernet interfaces would automatically become part of the trusted zone in
such cases.
Also note that interfaces that are not explicitly part of any zone will not appear in the zone interface list. There is currently no command to list unassigned interfaces. Due to this it is best to avoid unassigned network interfaces during regular operation.
firewalld
has a concept of services. A service
consists of definitions of ports and protocols. These definitions
logically belong together in the context of a given network service like
a Web or mail server protocol. The following commands can be used to get
information about predefined services and their details:
root #
firewall-cmd --get-services
[...] dhcp dhcpv6 dhcpv6-client dns docker-registry [...]root #
firewall-cmd --info-service dhcp
dhcp ports: 67/udp protocols: source-ports: modules: destination:
These service definitions can be used for easily making the associated network functionality accessible in a zone. This command line will open the HTTP Web server port in the internal zone, for example:
root #
firewall-cmd --add-service=http --zone=internal
The removal of a service from a zone is performed using the counterpart
command --remove-service
. You can also define custom
services using the --new-service
subcommand. Refer to
http://www.firewalld.org/documentation/howto/add-a-service.html
for more details on how to do this.
If you just want to open a single port by number, you can use the following approach. This will open TCP port 8000 in the internal zone:
root #
firewall-cmd --add-port=8000/tcp --zone=internal
For removal use the counterpart command --remove-port
.
firewalld
supports a --timeout
parameter that allows
to open a service or port for a limited time duration. This can be helpful
for quick testing and makes sure that closing the service or port will not
be forgotten. To allow the imap
service in the
internal
zone for 5 minutes, you would call
root #
firewall-cmd --add-service=imap --zone=internal --timeout=5m
firewalld
offers a lockdown mode that prevents
changes to the firewall rules while it is active. Since applications can
automatically change the firewall rules via the D-Bus interface, and
depending on the PolicyKit rules regular users may be able to do the same,
it can be helpful to prevent changes in some situations. You can find more
information about this at
https://fedoraproject.org/wiki/Features/FirewalldLockdown.
It is important to understand that the lockdown mode feature
provides no real security, but merely protection against accidental or
benign attempts to change the firewall. The way the lockdown mode is
currently implemented in firewalld
provides no security against malicious
intent. as is pointed out at
http://seclists.org/oss-sec/2017/q3/139.
iptables
Rules #Edit source
firewalld
claims exclusive control over the host's
netfilter
rules. You should never modify
firewall rules using other tools like iptables
. Doing
so could confuse firewalld
and break security or functionality.
If you need to add custom firewall rules that aren't covered by
firewalld
features then there are two ways to do so. To directly pass
raw iptables
syntax you can use the
--direct
option. It expects the table, chain, and
priority as initial arguments and the rest of the command line is passed
as is to iptables
. The following example adds a
connection tracking rule for the forwarding filter table:
root #
firewall-cmd --direct --add-rule ipv4 filter FORWARD 0 -i eth0 -o eth1 \ -p tcp --dport 80 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT
Additionally, firewalld
implements so called rich
rules, an extended syntax for specifying
iptables
rules in an easier way. You can find the
syntax specification at
http://www.firewalld.org/documentation/man-pages/firewalld.richlanguage.html.
The following example drops all IPv4 packets originating from a certain
source address:
root #
firewall-cmd --zone=public --add-rich-rule='rule family="ipv4" \ source address="192.168.2.4" drop'
firewalld
is not designed to run as a fully fledged router. The
basic functionality for typical home router setups is available. For a
corporate production router you should not use firewalld
, however, but
use dedicated router and firewall devices instead. The following provides
just a few pointers on what to look for to utilize routing in firewalld
:
First of all IP forwarding needs to be enabled as outlined in Section 25.2, “Masquerading Basics”.
To enable IPv4 masquerading, for example in the
internal
zone, issue the following command.
root #
firewall-cmd --zone=internal --add-masquerade
firewalld
can also enable port forwarding. The following command will
forward local TCP connections on port 80 to another host:
root #
firewall-cmd --zone=public \ --add-forward-port=port=80:proto=tcp:toport=80:toaddr=192.168.1.10
Some network services do not listen on predefined port numbers. Instead
they operate based on the portmapper
or
rpcbind
protocol. We will use the term
rpcbind
from here on. When one of these services
starts, it chooses a random local port and talks to
rpcbind
to make the port number known.
rpcbind
itself is listening on a well known port.
Remote systems can then query rpcbind
about the network
services it knows about and on which ports they are listening. Not many
programs use this approach anymore today. Popular examples are
Network Information Services (NIS; ypserv
and
ypbind
) and the Network File System (NFS) version 3.
The newer NFSv4 only requires the single well known
TCP port 2049. For protocol version 4.0 the kernel parameter
fs.nfs.nfs_callback_tcpport
may need to be set
to a static port (see Example 25.1, “Callback Port Configuration for the nfs
Kernel Module in /etc/modprobe.d/60-nfs.conf
”).
Starting with protocol version 4.1 this setting has also become
unnecessary.
The dynamic nature of the rpcbind
protocol makes
it difficult to make the affected services behind the firewall accessible.
firewalld
does not support these services by itself. For manual configuration,
see Section 25.4.2.1, “Configuring Static Ports”.
Alternatively, openSUSE Leap provides a helper script. For details, see
Section 25.4.2.2, “Using firewall-rpcbind-helper for Configuring Static Ports”.
One possibility is to configure all involved network services to use
fixed port numbers. Once this is done, the fixed ports can be opened in
firewalld
and everything should work. The actual port numbers used are
at your discretion but should not clash with any well known port numbers
assigned to other services. See Table 25.1, “Important Sysconfig Variables for Static Port Configuration”
for a list of the available configuration items for NIS and NFSv3
services. Note that depending on your actual NIS or NFS configuration,
not all of these ports may be required for your setup.
File Path |
Variable Name |
Example Value |
---|---|---|
/etc/sysconfig/nfs
| MOUNTD_PORT | 21001 |
STATD_PORT | 21002 | |
LOCKD_TCPPORT | 21003 | |
LOCKD_UDPPORT | 21003 | |
RQUOTAD_PORT | 21004 | |
/etc/sysconfig/ypbind
| YPBIND_OPTIONS | -p 24500 |
/etc/sysconfig/ypserv
| YPXFRD_ARGS | -p 24501 |
YPSERV_ARGS | -p 24502 | |
YPPASSWDD_ARGS | --port 24503 |
You will need to restart any related services that are affected by these
static port configurations for the changes to take effect. You can see
the currently assigned rpcbind ports by using the command
rpcinfo -p
. On success only the statically configured
ports should show up there.
Apart from the port configuration for network services running in
userspace there are also ports that are used by the Linux kernel directly
when it comes to NFS. One of these ports is
nfs_callback_tcpport
. It is only required for NFS
protocol versions older than 4.1. There is a sysctl named
fs.nfs.nfs_callback_tcpport
to configure this port.
This sysctl node only appears dynamically when NFS mounts are active.
Therefore it is best to configure the port via kernel module
parameters. This can be achieved by creating a file as shown in
Example 25.1, “Callback Port Configuration for the nfs
Kernel Module in /etc/modprobe.d/60-nfs.conf
”.
nfs
Kernel Module in /etc/modprobe.d/60-nfs.conf
#options nfs callback_tcpport=21005
To make this change effective it is easiest to reboot the machine.
Otherwise all NFS services need to be stopped and the
nfs
kernel module needs to be reloaded. To verify the
active NFS callback port, check the output of
cat /sys/module/nfs/parameters/callback_tcpport
.
For easy handling of the now statically configured RPC ports, it is useful
to create a new firewalld
service definition. This service definition
will group all related ports and, for example, makes it easy to make them
accessible in a specific zone. In
Example 25.2, “Commands to Define a new firewalld
RPC Service for NFS” this is done for the
NFS ports as they have been configured in the accompanying examples.
firewalld
RPC Service for NFS #root #
firewall-cmd --permanent --new-service=nfs-rpc
root #
firewall-cmd --permanent --service=nfs-rpc --set-description="NFS related, statically configured RPC ports"
# add UDP and TCP ports for the given sequenceroot #
for port in 21001 21002 21003 21004; do firewall-cmd --permanent --service=nfs-rpc --add-port ${port}/udp --add-port ${port}/tcp done
# the callback port is TCP onlyroot #
firewall-cmd --permanent --service=nfs-rpc --add-port 21005/tcp
# show the complete definition of the new custom serviceroot #
firewall-cmd --info-service=nfs-rpc --permanent -v
nfs-rpc summary: description: NFS and related, statically configured RPC ports ports: 4711/tcp 21001/udp 21001/tcp 21002/udp 21002/tcp 21003/udp 21003/tcp 21004/udp 21004/tcp protocols: source-ports: modules: destination: # reload firewalld to make the new service definition availableroot #
firewall-cmd --reload
# the new service definition can now be used to open the ports for example in the internal zoneroot #
firewall-cmd --add-service=nfs-rpc --zone=internal
The steps to configure static ports as shown in the previous section can
be simplified by using the SUSE helper tool
firewall-rpc-helper.py
. Install it with
zypper in firewalld-rpcbind-helper
.
The tool allows interactive configuration of the service patterns
discussed in the previous section. It can also display current port
assignments and can be used for scripting. For details, see
firewall-rpc-helper.py --help
.
firewalld
Configuration for AutoYaST
See the Firewall Configuration section of the
AutoYaST Guide to learn how to create a firewalld
configuration for AutoYaST.
When upgrading from
any version of openSUSE Leap before 15.0
to openSUSE Leap 15.2, SuSEfirewall2
is not changed and remains active. There is no automatic migration, so you must
migrate to firewalld
manually. firewalld
includes a helper migration
script, susefirewall2-to-firewalld
. Depending on the
complexity of your SuSEfirewall2 configuration, the script may perform a
perfect migration, or it may fail. Most likely it will partially succeed and you
will have to review your new firewalld
configuration and make adjustments.
The resulting configuration will make firewalld
behave somewhat like
SuSEfirewall2. To take full advantage of firewalld
's features you may elect to
create a new configuration, rather than trying to migrate your old configuration.
It is safe to run the susefirewall2-to-firewalld
script
with no options, as it makes no permanent changes to your system. However, if
you are administering the system remotely you could get locked out.
Install and run susefirewall2-to-firewalld
:
root #
zypper in susefirewall2-to-firewalldroot #
susefirewall2-to-firewalld INFO: Reading the /etc/sysconfig/SuSEfirewall2 file INFO: Ensuring all firewall services are in a well-known state. INFO: This will start/stop/restart firewall services and it's likely INFO: to cause network disruption. INFO: If you do not wish for this to happen, please stop the script now! 5...4...3...2...1...Lets do it! INFO: Stopping firewalld INFO: Restarting SuSEfirewall2_init INFO: Restarting SuSEfirewall2 INFO: DIRECT: Adding direct rule="ipv4 -t filter -A INPUT -p udp -m udp --dport 5353 -m pkttype --pkt-type multicast -j ACCEPT" [...] INFO: Enabling direct rule=ipv6 -t filter -A INPUT -p udp -m udp --dport 546 -j ACCEPT INFO: Enabling direct rule=ipv6 -t filter -A INPUT -p udp -m udp --dport 5353 -m pkttype --pkt-type multicast -j ACCEPT INFO: Enable logging for denied packets INFO: ########################################################## INFO: INFO: The dry-run has been completed. Please check the above output to ensure INFO: that everything looks good. INFO: INFO: ########################################################### INFO: Stopping firewalld INFO: Restarting SuSEfirewall2_init INFO: Restarting SuSEfirewall2
This results in a lot of output, which you may wish to direct to a file for easier review:
root #
susefirewall2-to-firewalld | tee newfirewallrules.txt
The script supports these options:
-c
Commit changes. The script will make changes to the system, so make sure you only use
this option if you are really happy with the proposed changes. This will
reset your current firewalld
configuration, so make sure you make backups!
-d
Super noisy. Use it to file bug reports but be careful to mask sensitive information.
-h
This message.
-q
No output. Errors will not be printed either!
-v
Verbose mode. It will print warnings and other informative messages.
The most up-to-date information and other documentation about the firewalld
package is found in /usr/share/doc/packages/firewalld
.
The home page of the netfilter and iptables project, http://www.netfilter.org, provides a large collection of
documents about iptables in general in many languages.
Today, Internet connections are cheap and available almost everywhere. However, not all connections are secure. Using a Virtual Private Network (VPN), you can create a secure network within an insecure network such as the Internet or Wi-Fi. It can be implemented in different ways and serves several purposes. In this chapter, we focus on the OpenVPN implementation to link branch offices via secure wide area networks (WANs).
This section defines some terms regarding VPN and gives a brief overview of some scenarios.
The two “ends” of a tunnel, the source or destination client.
A tap device simulates an Ethernet device (layer 2 packets in the OSI model, such as Ethernet frames). A tap device is used for creating a network bridge. It works with Ethernet frames.
A tun device simulates a point-to-point network (layer 3 packets in the OSI model, such as IP packets). 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.
Whenever you set up a VPN connection, your IP packets are transferred over a secured tunnel. A tunnel can use either a tun or tap device. They are virtual network kernel drivers which implement the transmission of Ethernet frames or IP frames/packets.
Any user space program, such as OpenVPN, can attach itself to a tun or tap device to receive packets sent by your operating system. 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:
Routing is an easy solution to set up. It is more efficient and scales better than a bridged VPN. Furthermore, it allows the user to tune MTU (Maximum Transfer Unit) to raise efficiency. However, in a heterogeneous environment, if you do not have a Samba server on the gateway, NetBIOS broadcasts do not work. If you need IPv6, the drivers for the tun devices on both ends must support this protocol explicitly. This scenario is depicted in Figure 26.1, “Routed 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 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 scenario is depicted in the following figures.
The major difference between bridging and routing is that a routed VPN cannot IP-broadcast while a bridged VPN can.
In the following example, we will create a point-to-point VPN tunnel. The
example 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 IP_OF_SERVER
and your client will use the IP address
IP_OF_CLIENT
.
Make sure you select addresses which do not conflict with other IP addresses.
This following scenario is provided as an example meant for familiarizing yourself with VPN technology. Do not use this as a real world scenario, as it can compromise the security and safety of your IT infrastructure!
To simplify working with OpenVPN configuration files, we recommend the following:
Place your OpenVPN configuration files in the directory
/etc/openvpn
.
Name your configuration files
MY_CONFIGURATION.conf
.
If there are multiple files that belong to the same configuration, place
them in a subdirectory like
/etc/openvpn/MY_CONFIGURATION
.
To configure a VPN server, proceed as follows:
Install the package openvpn
on the machine that will later become your VPN server.
Open a shell, become root
and create the VPN secret key:
root #
openvpn --genkey --secret /etc/openvpn/secret.key
Copy the secret key to your client:
root #
scp /etc/openvpn/secret.key root@IP_OF_CLIENT:/etc/openvpn/
Create the file /etc/openvpn/server.conf
with the
following content:
dev tun ifconfig IP_OF_SERVER IP_OF_CLIENT secret secret.key
Set up a tun device configuration by creating a file called
/etc/sysconfig/network/ifcfg-tun0
with the following
content:
STARTMODE='manual' BOOTPROTO='static' TUNNEL='tun' TUNNEL_SET_OWNER='nobody' TUNNEL_SET_GROUP='nobody' LINK_REQUIRED=no PRE_UP_SCRIPT='systemd:openvpn@server' PRE_DOWN_SCRIPT='systemd:openvpn@service'
The notation openvpn@server
points to the OpenVPN
server configuration file located at
/etc/openvpn/server.conf
. For more information, see
/usr/share/doc/packages/openvpn/README.SUSE
.
If you use a firewall, start YaST and open UDP port 1194 (
› › ).
Start the OpenVPN server service by setting the tun device to
up
:
tux >
sudo
wicked ifup tun0
You should see the confirmation:
tun0 up
To configure the VPN client, do the following:
Install the package openvpn
on your client VPN machine.
Create /etc/openvpn/client.conf
with the
following content:
remote DOMAIN_OR_PUBLIC_IP_OF_SERVER dev tun ifconfig IP_OF_CLIENT IP_OF_SERVER secret secret.key
Replace the placeholder IP_OF_CLIENT in the first line with either the domain name, or the public IP address of your server.
Set up a tun device configuration by creating a file called
/etc/sysconfig/network/ifcfg-tun0
with the following
content:
STARTMODE='manual' BOOTPROTO='static' TUNNEL='tun' TUNNEL_SET_OWNER='nobody' TUNNEL_SET_GROUP='nobody' LINK_REQUIRED=no PRE_UP_SCRIPT='systemd:openvpn@client' PRE_DOWN_SCRIPT='systemd:openvpn@client'
If you use a firewall, start YaST and open UDP port 1194 as described in Step 6 of Procedure 26.1, “VPN Server Configuration”.
Start the OpenVPN server service by setting the tun device to
up
:
tux >
sudo
wicked ifup tun0
You should see the confirmation:
tun0 up
After OpenVPN has successfully started, test the availability of the tun device with the following command:
ip addr show tun0
To verify the VPN connection, use ping
on both client
and server side to see if they can reach each other. Ping the server
from the client:
ping -I tun0 IP_OF_SERVER
Ping the client from the server:
ping -I tun0 IP_OF_CLIENT
The example in Section 26.2 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.
This setup involves the following basic steps:
Before a VPN connection can be established, the client must authenticate the server certificate. Conversely, the server must also authenticate the client certificate. This is called mutual authentication.
Creating certificates is not supported on openSUSE Leap. The following assumes you have created a CA certificate, a server certificate, and a client certificate on another system.
The server certificate is required in the PEM and unencrypted
key in PEM formats. Copy the PEM version to
/etc/openvpn/server_crt.pem
on the VPN server. The
unencrypted version needs to go to
/etc/openvpn/server_key.pem
.
Client certificates need to be of the format PKCS12 (preferred) or PEM. The
certificate in PKCS12 format needs to contain the CA chain and needs to be
copied to
/etc/openvpn/CLIENT.p12
. In
case you have client certificates in PEM format containing the CA chain,
copy them to
/etc/openvpn/CLIENT.pem
. In
case you have split the PEM certificates into client certificate
(*.ca
), client key (*.key
), and
the CA certificate (*.ca
), copy these files to
/etc/openvpn/
on each client.
The CA certificate needs to be copied to
/etc/openvpn/vpn_ca.pem
on the server and each client.
If you split client certificates into client certificate, client key, and the CA certificate, you need to provide the respective file names in the OpenVPN configuration file on the respective clients (see Example 26.1, “VPN Server Configuration File”).
As the basis of your configuration file, copy
/usr/share/doc/packages/openvpn/sample-config-files/server.conf
to /etc/openvpn/
. Then customize it to your needs.
# /etc/openvpn/server.conf port 1194 1 proto udp 2 dev tun0 3 # Security 4 ca vpn_ca.pem cert server_crt.pem key server_key.pem # ns-cert-type server remote-cert-tls client 5 dh server/dh2048.pem 6 server 192.168.1.0 255.255.255.0 7 ifconfig-pool-persist /var/run/openvpn/ipp.txt 8 # Privileges 9 user nobody group nobody # Other configuration 10 keepalive 10 120 comp-lzo persist-key persist-tun # status /var/log/openvpn-status.tun0.log 11 # log-append /var/log/openvpn-server.log 12 verb 4
The TCP/UDP port on which OpenVPN listens. You need to open the port in the firewall, see Chapter 25, Masquerading and Firewalls. The standard port for VPN is 1194, so you can usually leave that as it is. | |
The protocol, either UDP or TCP. | |
The tun or tap device. For the difference between these, see Section 26.1.1, “Terminology”. | |
The following lines contain the relative or absolute path to the root
server CA certificate ( | |
Require that peer certificates have been signed with an explicit key usage and extended key usage based on RFC3280 TLS rules. | |
The Diffie-Hellman parameters. Create the required file with the following command: openssl dhparam -out /etc/openvpn/dh2048.pem 2048 | |
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, run the OpenVPN daemon with reduced privileges. To
do so, specify that it should use the group and user
| |
Several configuration options—see the comment in the
example configuration file:
| |
Enable this option to write short status updates with statistical data (“operational status dump”) to the named file. By default, this is not enabled.
All output is written to the system journal, which can be displayed with
| |
By default, log messages go to syslog. Overwrite this behavior by
removing the hash character. In that case, all messages go to
|
After having completed this configuration, you can see log messages of
your OpenVPN server under /var/log/openvpn.log
.
After having started it for the first time, it should finish with:
... Initialization Sequence Completed
If you do not see this message, check the log carefully for any hints of what is wrong in your configuration file.
As the basis of your configuration file, copy
/usr/share/doc/packages/openvpn/sample-config-files/client.conf
to /etc/openvpn/
. Then customize it to your needs.
# /etc/openvpn/client.conf client 1 dev tun 2 proto udp 3 remote IP_OR_HOST_NAME 1194 4 resolv-retry infinite nobind remote-cert-tls server 5 # Privileges 6 user nobody group nobody # Try to preserve some state across restarts. persist-key persist-tun # Security 7 pkcs12 client1.p12 comp-lzo 8
Specifies 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. | |
This is security option for clients which ensures that the host they connect to is a designated server. | |
Replace the placeholder IP_OR_HOST_NAME
with the respective host name or IP address of your VPN server. After
the host name, the port of the server is given. You can have multiple
lines of | |
For security reasons, run the OpenVPN daemon with reduced privileges. To
do so, specify that it should use the group and user
| |
Contains the client files. For security reasons, use a separate pair of files for each client. | |
Turn on compression. Only use this parameter if compression is enabled on the server as well. |
You can also use YaST to set up a VPN server. However, the YaST module does not support OpenVPN. Instead, it provides support for the IPsec protocol (as implemented in the software StrongSwan). Like OpenVPN, IPsec is a widely supported VPN scheme.
To start the YaST VPN module, select
› .Under
, activate .To create a new VPN, click
, then enter a name for the connection.Under
, select .Then choose the scenario:
The scenarios
and are best suited to Linux client setups.The scenario
sets up a configuration that is natively supported by modern versions of Android, iOS, and macOS. It is based on a pre-shared key setup with an additional user name and password authentication.The scenario
is a configuration that is natively supported by Windows and BlackBerry devices. It is based on a certificate setup with an additional user name and password authentication.For this example, choose
.To specify the key, click
. Activate , then type the secret key. Confirm with .Choose whether and how to limit access within your VPN under https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing.
. To enable only certain IP ranges, specify these in CIDR format, separated by commas in . For more information about the CIDR format, seeUnder
, specify the format of IP addresses your VPN should provide to its clients.To finish, click
. The YaST VPN module will now automatically add and enable firewall rules to allow clients to connect to the new VPN.
To view the connection status,
in the following confirmation window, click systemctl status
for your VPN, which allows you to check
if the VPN is running and configured correctly.
For more information on setting up a VPN connection using NetworkManager, see Book “Reference”, Chapter 28 “Using NetworkManager”, Section 28.3.4 “NetworkManager and VPN”.
For more information about VPN in general, see:
http://www.openvpn.net: the OpenVPN home page
man
openvpn
/usr/share/doc/packages/openvpn/sample-config-files/
:
example configuration files for different scenarios.
/usr/src/linux/Documentation/networking/tuntap.txt
,
to install the kernel-source
package.
Many security vulnerabilities result from bugs in trusted programs. A trusted program runs with privileges that attackers want to possess. The program fails to keep that trust if there is a bug in the program that allows the attacker to acquire said privilege.
Prepare a successful deployment of AppArmor on your system by carefully considering the following items:
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 wor…
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 polic…
AppArmor ships with a set of profiles enabled by default. These are created by the AppArmor developers, and are stored in /etc/apparmor.d. In addition to these profiles, openSUSE Leap ships profiles for individual applications together with the relevant application. These profiles are not enabled by…
YaST provides a basic way to build profiles and manage AppArmor® profiles. 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 enviro…
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.
An 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, distincti…
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 authen…
After creating profiles and immunizing your applications, openSUSE® Leap 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 d…
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 sol…
See profile foundation classes below.
Many security vulnerabilities result from bugs in trusted programs. A trusted program runs with privileges that attackers want to possess. 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. A security profile is 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).
The AppArmor-related kernel code and associated control scripts to enforce AppArmor policies on your openSUSE® Leap system.
For more information about the science and security of AppArmor, refer to the following papers:
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.
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.
This document tries to convey a better understanding of the technical details of AppArmor. It is available at http://en.opensuse.org/SDB:AppArmor_geeks.
Prepare a successful deployment of AppArmor on your system by carefully considering the following items:
Determine the applications to profile. Read more on this in Section 28.3, “Choosing Applications to Profile”.
Build the needed profiles as roughly outlined in Section 28.4, “Building and Modifying Profiles”. Check the results and adjust the profiles when necessary.
Update your profiles whenever your environment changes or you need to react to security events logged by the reporting tool of AppArmor. Refer to Section 28.5, “Updating Your Profiles”.
AppArmor is installed and running on any installation of openSUSE® Leap by default, regardless of what patterns are installed. The packages listed below are needed for a fully-functional instance of AppArmor:
apparmor-docs
apparmor-parser
apparmor-profiles
apparmor-utils
audit
libapparmor1
perl-libapparmor
yast2-apparmor
If AppArmor is not installed on your system, install the pattern
apparmor
for a complete
AppArmor installation. Either use the YaST Software Management
module for installation, or use Zypper on the command line:
tux >
sudo
zypper in -t pattern apparmor
AppArmor is configured to run by default on any fresh installation of openSUSE Leap. There are two ways of toggling the status of AppArmor:
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.
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:
Start YaST.
Select
› .
Mark apparmor
by clicking its row in the list of
services, then click in the lower
part of the window. Check that changed to
in the apparmor
row.
Confirm with
.AppArmor will not be initialized on reboot, and stays inactive until you re-enable it. Re-enabling a service using the YaST tool is similar to disabling it.
Toggle the status of AppArmor in a running system by using the AppArmor Configuration window. These changes take effect when you apply them and survive a reboot of the system. To toggle the status of AppArmor, proceed as follows:
Start YaST, select
, and click in the main window.Enable AppArmor by checking or disable AppArmor by deselecting it.
Click
in the window.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
.
aa-unconfined
#19848 /usr/sbin/cupsd not confined 19887 /usr/sbin/sshd not confined 19947 /usr/lib/postfix/master not confined 1328 /usr/sbin/smbd confined by '/usr/sbin/smbd (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.
For more information about choosing the right applications to profile, refer to Section 29.2, “Determining Programs to Immunize”.
AppArmor on openSUSE Leap 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. The main difference is that YaST supports only basic functionality for AppArmor profiles, while the command line tools let you update/tune the profiles in a more fine-grained way.
For each application, perform the following steps to create a profile:
As root
, let AppArmor create a rough outline of the
application's profile by running aa-genprof
PROGRAM_NAME.
or
Outline the basic profile by running
› › › and specifying the complete path to the application you want to profile.A new basic profile is outlined and put into learning mode, which means that it logs any activity of the program you are executing, but does not yet restrict it.
Run the full range of the application's actions to let AppArmor get a very specific picture of its activities.
Let AppArmor analyze the log files generated in Step 2 by typing S in aa-genprof.
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.
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.
When you finish aa-genprof
, your profile is set to
enforce mode. The profile is applied and AppArmor restricts the
application according to it.
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 33.7.3.2, “aa-complain—Entering Complain or Learning Mode”
and
Section 33.7.3.6, “aa-enforce—Entering Enforce Mode”.
Test your profile settings by performing every task you need with the application you confined. Normally, the confined program runs smoothly and you do not notice AppArmor activities. 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:
/var/log/audit/audit.log
|
The command journalctl | grep -i apparmor
|
The command dmesg -T
|
To adjust the profile, analyze the log messages relating to this application again as described in Section 33.7.3.9, “aa-logprof—Scanning the System Log”. Determine the access rights or restrictions when prompted.
For more information about profile building and modification, refer to Chapter 30, Profile Components and Syntax, Chapter 32, Building and Managing Profiles with YaST, and Chapter 33, Building Profiles from the Command Line.
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 be addressed by
aa-logprof
. For more information, see
Section 33.7.3.9, “aa-logprof—Scanning the System Log”.
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 called 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. 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.
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 30, Profile Components and Syntax, Chapter 32, Building and Managing Profiles with YaST, or Chapter 33, 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.
To get a more in-depth overview of AppArmor and the overall concept behind it, refer to Section 27.2, “Background Information on AppArmor Profiling”.
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 30.1, “Breaking an 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-status
aa-status
reports various aspects of the current
state of the running AppArmor confinement.
aa-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 33.7.3.12, “aa-unconfined—Identifying Unprotected Processes”
for detailed information about this tool.
aa-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 33.7.3.1, “aa-autodep—Creating Approximate Profiles”
for detailed information about this tool.
aa-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 33.7.3.8, “aa-genprof—Generating Profiles”
for detailed information about this tool.
aa-logprof
aa-logprof
interactively scans and reviews the log
entries generated by an application that is confined by an AppArmor
profile in both complain and enforced modes. It assists you in
generating new entries in the profile concerned. Refer to
Section 33.7.3.9, “aa-logprof—Scanning the System Log”
for detailed information about this tool.
aa-easyprof
aa-easyprof
provides an easy-to-use interface for
AppArmor profile generation. aa-easyprof
supports
the use of templates and policy groups to quickly profile an
application. Note that while this tool can help with policy
generation, its utility is dependent on the quality of the templates,
policy groups and abstractions used. aa-easyprof
may create a profile that is less restricted than creating the profile
with aa-genprof
and aa-logprof
.
aa-complain
aa-complain
toggles the mode of an AppArmor profile
from enforce to complain. Violations to rules set in a profile are
logged, but the profile is not enforced. Refer to
Section 33.7.3.2, “aa-complain—Entering Complain or Learning Mode”
for detailed information about this tool.
aa-enforce
aa-enforce
toggles the mode of an AppArmor profile
from complain to enforce. Violations to rules set in a profile are
logged and not permitted—the profile is enforced. Refer to
Section 33.7.3.6, “aa-enforce—Entering Enforce Mode”
for detailed information about this tool.
aa-disable
aa-disable
disables the enforcement mode for one or
more AppArmor profiles. This command will unload the profile from the
kernel and prevent it from being loaded on AppArmor start-up. The
aa-enforce
and aa-complain
utilities may be used to change this behavior.
aa-exec
aa-exec
launches a program confined by the
specified AppArmor profile and/or namespace. If both a profile and
namespace are specified, the command will be confined by the profile
in the new policy namespace. If only a namespace is specified, the
profile name of the current confinement will be used. If neither a
profile or namespace is specified, the command will be run using
standard profile attachment—as if run without
aa-exec
.
aa-notify
aa-notify
is a handy utility that displays AppArmor
notifications in your desktop environment. You can also configure it
to display a summary of notifications for the specified number of
recent days. For more information, see
Section 33.7.3.13, “aa-notify”.
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 29.3, “Immunizing cron
Jobs”.
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 29.4.1, “Immunizing Web Applications”.
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 29.4.2, “Immunizing Network Agents”.
Conversely, unprivileged programs do not need to be profiled. For
example, a shell script might invoke the cp
program to copy a file. Because cp
does not by
default have its own profile or subprofile, it inherits the profile
of the parent shell script. Thus cp
can copy any
files that the parent shell script's profile can read and write.
cron
Jobs #Edit source
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:
/etc/crontab /etc/cron.d/* /etc/cron.daily/* /etc/cron.hourly/* /etc/cron.monthly/* /etc/cron.weekly/*
The crontab
command lists/edits the current user's
crontab. To manipulate root
's
cron
jobs, first become
root
, and then edit the tasks with crontab -e
or list them with crontab -l
.
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 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:
37021 /usr/sbin/sshd2 confined by '/usr/sbin/sshd3 (enforce)' 4040 /usr/sbin/smbd confined by '/usr/sbin/smbd (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)' 646 /usr/lib/wicked/bin/wickedd-dhcp4 not confined 647 /usr/lib/wicked/bin/wickedd-dhcp6 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 <apparmor@lists.ubuntu.com> 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 Leap. We cannot guarantee that every profile will be included, but we make a sincere effort to include as much as possible.
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 Leap, 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
aa-genprof
and aa-logprof
tools to
create or update their AppArmor profiles.
Because CGI programs are executed by the Apache Web server, the profile
for Apache itself, usr.sbin.httpd2-prefork
for
Apache2 on openSUSE Leap, must be modified to add execute permissions
to each of these programs. For example, adding the line
/srv/www/cgi-bin/my_hit_counter.pl rPx
grants Apache
permission to execute the Perl script
my_hit_counter.pl
and requires that there be a
dedicated profile for my_hit_counter.pl
. If
my_hit_counter.pl
does not have a dedicated profile
associated with it, the rule should say
/srv/www/cgi-bin/my_hit_counter.pl rix
to cause
my_hit_counter.pl
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 example,
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 CGI
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 34.2, “Managing ChangeHat-Aware Applications”.
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 the log file when AppArmor complains about access violations. See Chapter 36, 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 to the
local system time and locale files:
/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
sufficient to display a Web page. The
DEFAULT_URI
hat that AppArmor provides by
default is the following:
^DEFAULT_URI { /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. For more information on
confining Web applications with Apache, see
Chapter 34, Profiling Your Web Applications Using ChangeHat.
To find network server daemons and network clients (such as
fetchmail
or Firefox) that need to be profiled,
you should inspect the open ports on your machine. Also 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 an 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 as root
.
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.
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 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 are include statements that contain chunks of profiles that are specific to program suites.
Capability entries are profile entries for any of the POSIX.1e http://en.wikipedia.org/wiki/POSIX#POSIX.1 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 mediate network access based on the address type and family.
Local variables define shortcuts for paths.
File Access Control Entries specify the set of files an application can access.
rlimit entries set and control an application's resource limits.
For help determining the programs to profile, refer to Section 29.2, “Determining Programs to Immunize”. To start building AppArmor profiles with YaST, proceed to Chapter 32, Building and Managing Profiles with YaST. To build profiles using the AppArmor command line interface, proceed to Chapter 33, Building Profiles from the Command Line.
For more details about creating AppArmor profiles, see
man 5 apparmor
.
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 /usr/bin/foo2 {3 #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/ld.so.cache r, /etc/foo/* r, /lib/ld-*.so* mr, /lib/lib*.so* mr, /proc/[0-9]** r, /usr/lib/** mr, /tmp/ r,9 /tmp/foo.pid 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 ( | |
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 30.5, “Network Access Control”. | |
A link pair rule specifying the source and the target of a link. See Section 30.7.6, “Link Pair” for more information. | |
The curly 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 | |
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 30.7.8, “Owner Conditional Rules” for more information. | |
This entry defines a transition to the local profile
| |
A named profile transition to the profile bin_generic located in the global scope. See Section 30.12.7, “Named Profile Transitions” for details. | |
The local profile | |
This section references a “hat” subprofile of the application. For more details on AppArmor's ChangeHat feature, refer to Chapter 34, 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 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
in /var/log/messages
, or only in
journalctl
if no traditional syslog is
installed. Often 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.
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.
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
.
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 30.12.7, “Named Profile Transitions”) or with the
change_profile
rule (see
Section 30.2.5, “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.
Local profiles provide a convenient way to provide specialized
confinement for utility programs launched by a confined application.
They are specified like standard profiles, except that 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 30.12.2, “Discrete Local Profile Execute Mode (Cx)”) or a named
profile transition (see
Section 30.12.7, “Named Profile Transitions”).
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 34, Profiling Your Web Applications Using ChangeHat
for a detailed description.
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 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 (for example
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
might not be seen by the kernel and hence not restricted by the new
profile.
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 process's memory mappings,
nor do they close resources that have already been opened.
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.
Include statements normally begin with a hash (#
)
sign. This is confusing because the same hash sign is used for comments
inside profile files. Because of this, #include
is
treated as an include only if there is no preceding #
(##include
is a comment) and there is no whitespace
between #
and include
(#
include
is a comment).
You can also use include
without the leading
#
.
include "/etc/apparmor.d/abstractions/foo"
is the same as using
#include "/etc/apparmor.d/abstractions/foo"
Note that because includes follow the C pre-processor syntax, they do not have a trailing ',' like most AppArmor rules.
By slight changes in syntax, you can modify the behavior of
include
. If you use ""
around the
including path, you instruct the parser to do an absolute or relative
path lookup.
include "/etc/apparmor.d/abstractions/foo" # absolute path include "abstractions/foo" # relative path to the directory of current file
Note that when using relative path includes, when the file is included,
it is considered the new current file for its includes. For example,
suppose you are in the /etc/apparmor.d/bar
file,
then
include "abstractions/foo"
includes the file /etc/apparmor.d/abstractions/foo
.
If then there is
include "example"
inside the /etc/apparmor.d/abstractions/foo
file, it
includes /etc/apparmor.d/abstractions/example
.
The use of <>
specifies to try the include
path (specified by -I
, defaults to the
/etc/apparmor.d
directory) in an ordered way. So
assuming the include path is
-I /etc/apparmor.d/ -I /usr/share/apparmor/
then the include statement
include <abstractions/foo>
will try /etc/apparmor.d/abstractions/foo
, and if
that file does not exist, the next try is
/usr/share/apparmor/abstractions/foo
.
The default include path can be overridden manually by passing
-I
to the apparmor_parser
, or by
setting the include paths in
/etc/apparmor/parser.conf
:
Include /usr/share/apparmor/ Include /etc/apparmor.d/
Multiple entries are allowed, and they are taken in the same order as
when they are when using -I
or
--Include
from the apparmor_parser
command line.
If an include ends with '/', this is considered a directory include, and all files within the directory are included.
To assist you in profiling your applications, AppArmor provides three classes of includes: abstractions, program chunks and tunables.
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 also require
other files listed in the abstraction file (depending on the local
configuration and the specific requirements of the program). Find
abstractions in /etc/apparmor.d/abstractions
.
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.
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
.
Capability rules are simply the word capability
followed by the name of the POSIX.1e capability as defined in the
capabilities(7)
man page. You can list multiple
capabilities in a single rule, or grant all implemented capabilities with
the bare keyword capability
.
capability dac_override sys_admin, # multiple capabilities capability, # grant all capabilities
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: | |
Supported types: | |
Supported protocols: |
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.
network1, network inet2, network inet63, network inet stream4, network inet tcp5, network tcp6,
Allow all networking. No restrictions applied with regard 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. |
A profile is usually attached to a program by specifying a full path to the program's executable. For example in the case of a standard profile (see Section 30.2.1, “Standard Profiles”), the profile is defined by
/usr/bin/foo { ... }
The following sections describe several useful techniques that can be applied when naming a profile or putting a profile in the context of other existing ones, or specifying file paths.
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
(but not /some/ itself).
/some/random/example/** r
Give read access to files and directories under
/some/random/example
(but not
/some/random/example/ itself).
/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
Example: a rule that matches |
|
Substitutes for the single character |
|
Expands to one rule to match
Example: a rule that matches |
|
Substitutes for any character except |
Profile flags control the behavior of the related profile. You can add profile flags to the profile definition by editing it manually, see the following syntax:
/path/to/profiled/binary flags=(list_of_flags) { [...] }
You can use multiple flags separated by a comma ',' or space ' '. There are three basic types of profile flags: mode, relative, and attach flags.
Mode flag is complain
(illegal
accesses are allowed and logged). If it is omitted, the profile is in
enforce
mode (enforces the policy).
A more flexible way of setting the whole profile into complain mode is
to create a symbolic link from the profile file inside the
/etc/apparmor.d/force-complain/
directory.
ln -s /etc/apparmor.d/bin.ping /etc/apparmor.d/force-complain/bin.ping
Relative flags are
chroot_relative
(states that the profile is relative
to the chroot instead of namespace) or
namespace_relative
(the default, with the path being
relative to outside the chroot). They are mutually exclusive.
Attach flags consist of two pairs of mutually
exclusive flags: attach_disconnected
or
no_attach_disconnected
(determine if path names
resolved to be outside of the namespace are attached to the root, which
means they have the '/' character at the beginning), and
chroot_attach
or chroot_no_attach
(control path name generation when in a chroot environment while a file
is accessed that is external to the chroot but within the namespace).
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 need 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:
@{CHROOT_BASE}=/tmp/foo /sbin/rsyslogd { ... # chrooted applications @{CHROOT_BASE}/var/lib/*/dev/log w, @{CHROOT_BASE}/var/log/** w, ... }
In the following example, while @{HOMEDIRS} lists where all the user home directories are stored, @{HOME} is a space-separated list of home directories. Later on, @{HOMEDIRS} is expanded by two new specific places where user home directories are stored.
@{HOMEDIRS}=/home/ @{HOME}=@{HOMEDIRS}/*/ /root/ [...] @{HOMEDIRS}+=/srv/nfs/home/ /mnt/home/
With the current AppArmor tools, variables can only be used when manually editing and maintaining a profile.
Profile names can contain globbing expressions allowing the profile to match against multiple binaries.
The following example is valid for systems where the
foo
binary resides either in
/usr/bin
or /bin
.
/{usr/,}bin/foo { ... }
In the following example, when matching against the executable
/bin/foo
, the /bin/foo
profile
is an exact match so it is chosen. For the executable
/bin/fat
, the profile /bin/foo
does not match, and because the /bin/f*
profile is
more specific (less general) than /bin/**
, the
/bin/f*
profile is chosen.
/bin/foo { ... } /bin/f* { ... } /bin/** { ... }
For more information on profile name globbing examples, see the man page
of AppArmor, man 5 apparmor.d,
, section
Globbing
.
Namespaces are used to provide different profiles sets. Say one for the
system, another for a chroot environment or container. Namespaces are
hierarchical—a namespace can see its children but a child
cannot see its parent. Namespace names start with a colon
:
followed by an alphanumeric string, a trailing
colon :
and an optional double slash
//
, such as
:childNameSpace://
Profiles loaded to a child namespace will be prefixed with their namespace name (viewed from a parent's perspective):
:childNameSpace://apache
Namespaces can be entered via the change_profile
API,
or named profile transitions:
/path/to/executable px -> :childNameSpace://apache
Profiles can have a name, and an attachment specification. This allows for profiles with a logical name that can be more meaningful to users/administrators than a profile name that contains pattern matching (see Section 30.6.3, “Pattern Matching”). For example, the default profile
/** { ... }
can be named
profile default /** { ... }
Also, a profile with pattern matching can be named. For example:
/usr/lib64/firefox*/firefox-*bin { ... }
can be named
profile firefox /usr/lib64/firefox*/firefox-*bin { ... }
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. The alias rule says to treat rules that have the same source prefix as if the rules are at target prefix.
alias /home/ -> /usr/home/
All the rules that have a prefix match to /home/
will provide access to /usr/home/
. For example
/home/username/** r,
allows as well access to
/usr/home/username/** r,
Aliases provide a quick way of remapping rules without the need to
rewrite them. They keep the source path still accessible—in our
example, the alias rule keeps the paths under
/home/
still accessible.
With the alias
rule, you can point to multiple
targets at the same time.
alias /home/ -> /usr/home/ alias /home/ -> /mnt/home/
With the current AppArmor tools, alias rules can only be used when manually editing and maintaining a profile.
Insert global alias definitions in the file
/etc/apparmor.d/tunables/alias
.
File permission access modes consist of combinations of the following modes:
|
Read mode |
|
Write mode (mutually exclusive to |
|
Append mode (mutually exclusive to |
|
File locking mode |
|
Link mode |
|
Link pair rule (cannot be combined with other access modes) |
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.
Allows the program to have write access to the resource. Files must have this permission if they are to be unlinked (removed).
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 a subset of the permissions
associated with the write mode, the w
and
a
permission flags cannot be used together and are
mutually exclusive.
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.
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 (but the destination does not need link access).
The link mode grants permission to link to arbitrary files, provided the link has a subset of the permissions granted by the target (subset permission test).
/srv/www/htdocs/index.html rl,
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.
link /srv/www/htdocs/index.html -> /var/www/index.html
To force the rule to require the test, the subset
keyword is used. The following rules are equivalent:
/var/www/index.html l, link subset /var/www/index.html -> /**,
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.
allow
and file
Rules #Edit source
The allow
prefix is optional, and it is idiomatically
implied if not specified and the deny
(see
Section 30.7.9, “Deny Rules”) keyword is not used.
allow file /example r, allow /example r, allow network,
You can also use the optional file
keyword. If you
omit it and there are no other rule types that start with a keyword,
such as network
or mount
, it is
automatically implied.
file /example/rule r,
is equivalent to
/example/rule r,
The following rule grants access to all files:
file,
which is equal to
/** rwmlk,
File rules can use leading or trailing permissions. The permissions should not be specified as a trailing permission, but rather used at the start of the rule. This is important in that it makes file rules behave like any other rule types.
/path rw, # old style rw /path, # leading permission file rw /path, # with explicit 'file' keyword allow file rw /path, # optional 'allow' keyword added
The file rules can be extended so that they can be conditional upon
the user being the owner of the file (the fsuid needs to match the
file's uid). For this purpose the owner
keyword
is put in front of the rule. Owner conditional rules accumulate like
regular file rules do.
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.
Owner conditional rules are considered a subset of regular file rules. If a regular file rule overlaps with an owner conditional file rule, the rules are merged. Consider the following example.
/foo r, owner /foo rw, # or w,
The rules are merged—it results in r
for
everybody, and w
for the owner only.
To address everybody but the owner of the file,
use the keyword other
.
owner /foo rw, other /foo r,
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, put the keyword audit
in front of the
deny entry.
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, owner /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 use 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.
AppArmor can limit mount and unmount operations, including file system
types and mount flags. The rule syntax is based on the
mount
command syntax and starts with one of the
keywords mount
, remount
, or
umount
. Conditions are optional and unspecified
conditionals are assumed to match all entries. For example, not
specifying a file system means that all file systems are matched.
Conditionals can be specified by specifying conditionals with
options=
or options in
.
options=
specifies conditionals that have to be met
exactly. The rule
mount options=ro /dev/foo -E /mnt/,
matches
root #
mount -o ro /dev/foo /mnt
but does not match
root #
mount -o ro,atime /dev/foo /mnt
root #
mount -o rw /dev/foo /mnt
options in
requires that at least one of the
listed mount options is used. The rule
mount options in (ro,atime) /dev/foo -> /mnt/,
matches
root #
mount -o ro /dev/foo /mnt
root #
mount -o ro,atime /dev/foo /mnt
root #
mount -o atime /dev/foo /mnt
but does not match
root #
mount -o ro,sync /dev/foo /mnt
root #
mount -o ro,atime,sync /dev/foo /mnt
root #
mount -o rw /dev/foo /mnt
root #
mount -o rw,noatime /dev/foo /mnt
root #
mount /dev/foo /mnt
With multiple conditionals, the rule grants permission for each set of options. The rule
mount options=ro options=atime
matches
root #
mount -o ro /dev/foo /mnt
root #
mount -o atime /dev/foo /mnt
but does not match
root #
mount -o ro,atime /dev/foo /mnt
Separate mount rules are distinct and the options do not accumulate. The rules
mount options=ro, mount options=atime,
are not equivalent to
mount options=(ro,atime), mount options in (ro,atime),
The following rule allows mounting /dev/foo
on
/mnt/
read only and using inode access times or
allows mounting /dev/foo
on
/mnt/
with some combination of 'nodev' and
'user'.
mount options=(ro, atime) options in (nodev, user) /dev/foo -> /mnt/,
allows
root #
mount -o ro,atime /dev/foo /mnt
root #
mount -o nodev /dev/foo /mnt
root #
mount -o user /dev/foo /mnt
root #
mount -o nodev,user /dev/foo /mnt
AppArmor can limit changing the root file system. The syntax is
pivot_root [oldroot=OLD_ROOT] NEW_ROOT
The paths specified in 'pivot_root' rules must end with '/' since they are directories.
# Allow any pivot pivot_root, # Allow pivoting to any new root directory and putting the old root # directory at /mnt/root/old/ pivot_root oldroot=/mnt/root/old/, # Allow pivoting the root directory to /mnt/root/ pivot_root /mnt/root/, # Allow pivoting to /mnt/root/ and putting the old root directory at # /mnt/root/old/ pivot_root oldroot=/mnt/root/old/ /mnt/root/, # Allow pivoting to /mnt/root/, putting the old root directory at # /mnt/root/old/ and transition to the /mnt/root/sbin/init profile pivot_root oldroot=/mnt/root/old/ /mnt/root/ -> /mnt/root/sbin/init,
AppArmor supports limiting ptrace system calls. ptrace rules are accumulated so that the granted ptrace permissions are the union of all the listed ptrace rule permissions. If a rule does not specify an access list, permissions are implicitly granted.
The trace
and tracedby
permissions control ptrace(2); read
and
readby
control proc(5) file system access,
kcmp(2), futexes (get_robust_list(2)) and perf trace events.
For a ptrace operation to be allowed, the tracing and traced
processes need the correct permissions. This means that the tracing
process needs the trace
permission and the traced
process needs the tracedby
permission.
Example AppArmor PTrace rules:
# Allow all PTrace access ptrace, # Explicitly allow all PTrace access, ptrace (read, readby, trace, tracedby), # Explicitly deny use of ptrace(2) deny ptrace (trace), # Allow unconfined processes (eg, a debugger) to ptrace us ptrace (readby, tracedby) peer=unconfined, # Allow ptrace of a process running under the /usr/bin/foo profile ptrace (trace) peer=/usr/bin/foo,
AppArmor supports limiting inter-process signals. AppArmor signal rules are accumulated so that the granted signal permissions are the union of all the listed signal rule permissions. AppArmor signal permissions are implied when a rule does not explicitly state an access list.
The sending and receiving process must both have the correct permissions.
Example signal rules:
# Allow all signal access signal, # Explicitly deny sending the HUP and INT signals deny signal (send) set=(hup, int), # Allow unconfined processes to send us signals signal (receive) peer=unconfined, # Allow sending of signals to a process running under the /usr/bin/foo # profile signal (send) peer=/usr/bin/foo, # Allow checking for PID existence signal (receive, send) set=("exists"), # Allow us to signal ourselves using the built-in @{profile_name} variable signal peer=@{profile_name}, # Allow two real-time signals signal set=(rtmin+0 rtmin+32),
Execute modes, also named profile transitions, consist of the following modes:
|
Discrete profile execute mode |
|
Discrete local profile execute mode |
|
Unconfined execute mode |
|
Inherit execute mode |
|
Allow |
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.
Incompatible with Ux
, ux
,
px
, and ix
.
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.
Currently, Cx transitions are limited to top level profiles and cannot be used in hats and children profiles. This restriction will be removed in the future.
Incompatible with Ux
, ux
,
Px
, px
, cx
, and
ix
.
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 unconfined execution rights, it is possible to
bypass the mandatory constraints imposed on all confined processes.
Allowing a root process to go unconfined means it can change AppArmor
policy itself. For more information about what is constrained, see the
apparmor(7)
man page.
This mode is incompatible with ux
,
px
, Px
, and ix
.
Use the lowercase versions of exec modes—px
,
cx
, ux
—only in very
special cases. They do 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 these modes only
if the child absolutely must be run unconfined and
LD_PRELOAD
must be used. Any profile using such modes
provides negligible security. Use at your own risk.
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
.