Chapter 6. Network Authentication with Kerberos

Contents

6.1. Kerberos Terminology
6.2. How Kerberos Works
6.3. Users' View of Kerberos
6.4. Installing and Administering Kerberos
6.5. For More Information

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:

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

6.1. Kerberos Terminology

The following glossary defines some Kerberos terminology.

credential

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.

ticket

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.

authenticator

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.

principal

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

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

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

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

mutual authentication

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

session key

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

replay

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

server or service

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

6.2. How Kerberos Works

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

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

6.2.1. First Contact

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

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

  • The current time

  • A lifetime assigned to this ticket

  • The client's IP address

  • The newly-generated session key

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

6.2.2. Requesting a Service

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

  • The client's principal

  • The client's IP address

  • The current time

  • A checksum (chosen by the client)

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

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

6.2.3. Mutual Authentication

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

6.2.4. Ticket Granting—Contacting All Servers

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

  • The requested principal

  • The ticket-granting ticket

  • An authenticator

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

  • The client's principal

  • The server's principal

  • The current time

  • The client's IP address

  • The newly-generated session key

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

6.2.5. Compatibility to Windows 2000

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

6.3. Users' View of Kerberos

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

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

  • telnet, telnetd

  • rlogin

  • rsh, rcp, rshd

  • ftp, ftpd

  • ksu

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

6.4. Installing and Administering Kerberos

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

General Preparations

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

Basic Configuration

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

Enabling Kerberos Authentication

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

6.4.1. Kerberos Network Topology

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

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

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

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

  • Configure the client machines to use Kerberos authentication.

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

Figure 6.1. Kerberos Network Topology

Kerberos Network Topology

[Tip]Configuring Subnet Routing

For a setup similar to the one in Figure 6.1, “Kerberos Network Topology”, configure routing between the two subnets (192.168.1.0/24 and 192.168.2.0/24). Refer to Section “Configuring Routing” (Chapter 13, Basic Networking, ↑Reference) for more information on configuring routing with YaST.

6.4.2. Choosing the Kerberos Realms

The domain of a Kerberos installation is called a realm and is identified by a name, such as EXAMPLE.COM or simply ACCOUNTING. Kerberos is case-sensitive, so 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. You cannot set up a realm named EXAMPLE.COM, have two subrealms named DEVELOPMENT and ACCOUNTING underneath it, and expect the two subordinate realms to somehow inherit principals from EXAMPLE.COM. Instead, you would have three separate realms for which you would have to configure crossrealm authentication for users from one realm to interact with servers or other users from another realm.

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

6.4.3. Setting Up the KDC Hardware

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

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

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

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

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

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

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

    passwd:         files 
    group:          files

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

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

6.4.4. Configuring Time Synchronization

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

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

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

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

It is also possible to adjust the maximum deviation Kerberos allows when checking time stamps. This value (called clock skew) can be set in the krb5.conf file as described in Section 6.4.6.2.3, “Adjusting the Clock Skew”.

6.4.5. Configuring the KDC

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

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

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

  3. Create the Kerberos Database.  Kerberos keeps a database of all principal identifiers and the secret keys of all principals that need to be authenticated. Refer to Section 6.4.5.1, “Setting Up the Database” for details.

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

  5. Adjust the Kerberos Database: Add Administrators.  You need at least one administrative principal to run and administer Kerberos. This principal must be added before starting the KDC. Refer to Section 6.4.5.2, “Creating a Principal” for details.

  6. Start the Kerberos Daemon.  Once the KDC software is installed and properly configured, start the Kerberos daemon to provide Kerberos service for your realm. Refer to Section 6.4.5.3, “Starting the KDC” for details.

  7. Create a Principal for Yourself.  You need a principal for yourself. Refer to Section 6.4.5.2, “Creating a Principal” for details.

6.4.5.1. Setting Up the Database

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

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

To create the stash file and the database, run:

kdb5_util create -r EXAMPLE.COM -s

You will see the following output:

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

1

Type the master password.

2

Type the password again.

To verify, use the list command:

kadmin.local

kadmin> 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

6.4.5.2. Creating a Principal

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

kadmin.local

kadmin> ank geeko

You will see the following output:

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

1

Type geeko's password.

1

Type geeko's password again.

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

6.4.5.3. Starting the KDC

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

6.4.6. Configuring Kerberos Clients

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

6.4.6.1. Configuring a Kerberos Client with YaST

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

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

  2. Select Use Kerberos.

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

    1. DNS-Based Static Kerberos Client

      [Note]Using DNS Support

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

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

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

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

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

Figure 6.2. YaST: Basic Configuration of a Kerberos Client

YaST: Basic Configuration of a Kerberos Client

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

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

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

  • Enable the transfer of certain tickets by selecting Proxiable.

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

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

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

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

Figure 6.3. YaST: Advanced Configuration of a Kerberos Client

YaST: Advanced Configuration of a Kerberos Client

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

6.4.6.2. Manually Configuring Kerberos Clients

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

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

6.4.6.2.1. Static Configuration

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

To configure your Kerberos clients, add the following stanza to krb5.conf (where kdc.example.com is the hostname 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, just add additional statements to the [realms] section.

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

[domain_realm]
        .example.com = EXAMPLE.COM
        www.foobar.com = 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.foobar.com should also be considered a member of the EXAMPLE.COM realm.

6.4.6.2.2. DNS-Based Configuration

DNS-based Kerberos configuration makes heavy use of SRV records. See (RFC2052) A DNS RR for specifying the location of services at 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 hostname. The priority defines the order in which hosts should be tried (lower values indicate a higher priority). The weight value is there to support some sort of load balancing among servers of equal priority. You probably do not need any of this, so it is okay to set these to zero.

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

_kerberos

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.
_kerberos-adm

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.hostname, as shown here:

_kerberos.www.foobar.com.  IN TXT "EXAMPLE.COM"
6.4.6.2.3. Adjusting the Clock Skew

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

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

[libdefaults]
        clockskew = 60

6.4.7. Configuring Remote Kerberos Administration

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

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

geeko/admin              *

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

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

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

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 joe
Principal: geeko@EXAMPLE.COM
Expiration date: [never]
Last password change: Wed Jan 12 17:28:46 CET 2005
Password expiration date: [none]
Maximum ticket life: 0 days 08:00:00
Maximum renewable life: 7 days 00:00:00
Last modified: Wed Jan 12 17:59:49 CET 2005 (geeko/admin@EXAMPLE.COM)
Last successful authentication: [never]
Last failed authentication: [never]
Failed password attempts: 0
Number of keys: 2
Key: vno 1, Triple DES cbc mode with HMAC/sha1, no salt
Key: vno 1, DES cbc mode with CRC-32, no salt
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, refer to http://web.mit.edu/kerberos/www/krb5-1.8/krb5-1.8.3/doc/krb5-admin.html#Kadmin%20Options, or the man8 kadmin manual page.

6.4.8. Creating Kerberos Service Principals

So far, only user credentials have been discussed. However, Kerberos-compatible services usually need to authenticate themselves to the client user, too. Therefore, special service principals must be present in the Kerberos database for each service offered in the realm. For example, if 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 hostname.

Valid service descriptors are:

Service Descriptor

Service

host

Telnet, RSH, SSH

nfs

NFSv4 (with Kerberos support)

HTTP

HTTP (with Kerberos authentication)

imap

IMAP

pop

POP3

ldap

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, he needs to type his password so Kerberos can decrypt the ticket. It would be quite inconvenient for the system administrator if he had to obtain new tickets for the SSH daemon every eight hours or so.

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

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

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

6.4.9. Enabling PAM Support for Kerberos

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

pam-config --add --krb5

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

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

6.4.10. Configuring SSH for Kerberos Authentication

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

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

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

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

Then restart your SSH daemon using rcsshd restart.

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

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

[Tip]Additional Information

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

6.4.11. Using LDAP and Kerberos

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

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

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

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

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

To make the keytab file readable by group LDAP, execute

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

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

ktadd -k /etc/openldap/ldap.keytab ldap/ldap.example.com@EXAMPLE.COM

Then in the shell run:

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

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

OPENLDAP_KRB5_KEYTAB="/etc/openldap/ldap.keytab"

Finally, restart the LDAP server using rcldap restart.

6.4.11.1. Using Kerberos Authentication with LDAP

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

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

SASL/GSSAPI authentication started
SASL SSF: 56
SASL installing layers
[...]

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

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

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

6.4.11.2. Kerberos Authentication and LDAP Access Control

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

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

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

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

authz-regexp
   uid=(.*),cn=GSSAPI,cn=auth 
   uid=$1,ou=people,dc=example,dc=com

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

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

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

6.5. For More Information

The official site of the MIT Kerberos is http://web.mit.edu/kerberos. There, find links to any other relevant resource concerning Kerberos, including Kerberos installation, user, and administration guides.

The paper at ftp://athena-dist.mit.edu/pub/kerberos/doc/usenix.PS gives quite an extensive insight to the basic principles of Kerberos, without being too difficult to read. It also provides a lot of opportunities for further investigation and reading about Kerberos.

The official Kerberos FAQ is available at http://www.nrl.navy.mil/CCS/people/kenh/kerberos-faq.html. The book Kerberos—A Network Authentication System by Brian Tung (ISBN 0-201-37924-4) offers extensive information.


openSUSE Security Guide 12.2