15.7 Kerberos5

Contributed by Tillman Hodgson. Based on a contribution by Mark Murray.

Kerberos is a network add-on system/protocol that allows users to authenticate themselves through the services of a secure server. Services such as remote login, remote copy, secure inter-system file copying and other high-risk tasks are made considerably safer and more controllable.

Kerberos can be described as an identity-verifying proxy system. It can also be described as a trusted third-party authentication system. Kerberos provides only one function — the secure authentication of users on the network. It does not provide authorization functions (what users are allowed to do) or auditing functions (what those users did). After a client and server have used Kerberos to prove their identity, they can also encrypt all of their communications to assure privacy and data integrity as they go about their business.

Therefore it is highly recommended that Kerberos be used with other security methods which provide authorization and audit services.

The following instructions can be used as a guide on how to set up Kerberos as distributed for FreeBSD. However, you should refer to the relevant manual pages for a complete description.

For purposes of demonstrating a Kerberos installation, the various name spaces will be handled as follows:

Note: Please use real domain names when setting up Kerberos even if you intend to run it internally. This avoids DNS problems and assures inter-operation with other Kerberos realms.

15.7.1 History

Kerberos was created by MIT as a solution to network security problems. The Kerberos protocol uses strong cryptography so that a client can prove its identity to a server (and vice versa) across an insecure network connection.

Kerberos is both the name of a network authentication protocol and an adjective to describe programs that implement the program (Kerberos telnet, for example). The current version of the protocol is version 5, described in RFC 1510.

Several free implementations of this protocol are available, covering a wide range of operating systems. The Massachusetts Institute of Technology (MIT), where Kerberos was originally developed, continues to develop their Kerberos package. It is commonly used in the US as a cryptography product, as such it has historically been affected by US export regulations. The MIT Kerberos is available as a port (security/krb5). Heimdal Kerberos is another version 5 implementation, and was explicitly developed outside of the US to avoid export regulations (and is thus often included in non-commercial UNIX® variants). The Heimdal Kerberos distribution is available as a port (security/heimdal), and a minimal installation of it is included in the base FreeBSD install.

In order to reach the widest audience, these instructions assume the use of the Heimdal distribution included in FreeBSD.

15.7.2 Setting up a Heimdal KDC

The Key Distribution Center (KDC) is the centralized authentication service that Kerberos provides — it is the computer that issues Kerberos tickets. The KDC is considered “trusted” by all other computers in the Kerberos realm, and thus has heightened security concerns.

Note that while running the Kerberos server requires very few computing resources, a dedicated machine acting only as a KDC is recommended for security reasons.

To begin setting up a KDC, ensure that your /etc/rc.conf file contains the correct settings to act as a KDC (you may need to adjust paths to reflect your own system):


Next we will set up your Kerberos config file, /etc/krb5.conf:

    default_realm = EXAMPLE.ORG
        kdc = kerberos.example.org
        admin_server = kerberos.example.org
    .example.org = EXAMPLE.ORG

Note that this /etc/krb5.conf file implies that your KDC will have the fully-qualified hostname of kerberos.example.org. You will need to add a CNAME (alias) entry to your zone file to accomplish this if your KDC has a different hostname.

Note: For large networks with a properly configured BIND DNS server, the above example could be trimmed to:

      default_realm = EXAMPLE.ORG

With the following lines being appended to the example.org zonefile:

_kerberos._udp      IN  SRV     01 00 88 kerberos.example.org.
_kerberos._tcp      IN  SRV     01 00 88 kerberos.example.org.
_kpasswd._udp       IN  SRV     01 00 464 kerberos.example.org.
_kerberos-adm._tcp  IN  SRV     01 00 749 kerberos.example.org.
_kerberos           IN  TXT     EXAMPLE.ORG

Note: For clients to be able to find the Kerberos services, you must have either a fully configured /etc/krb5.conf or a minimally configured /etc/krb5.conf and a properly configured DNS server.

Next we will create the Kerberos database. This database contains the keys of all principals encrypted with a master password. You are not required to remember this password, it will be stored in a file (/var/heimdal/m-key). To create the master key, run kstash and enter a password.

Once the master key has been created, you can initialize the database using the kadmin program with the -l option (standing for “local”). This option instructs kadmin to modify the database files directly rather than going through the kadmind network service. This handles the chicken-and-egg problem of trying to connect to the database before it is created. Once you have the kadmin prompt, use the init command to create your realms initial database.

Lastly, while still in kadmin, create your first principal using the add command. Stick to the defaults options for the principal for now, you can always change them later with the modify command. Note that you can use the ? command at any prompt to see the available options.

A sample database creation session is shown below:

# kstash
Master key: xxxxxxxx
Verifying password - Master key: xxxxxxxx

# kadmin -l
kadmin> init EXAMPLE.ORG
Realm max ticket life [unlimited]:
kadmin> add tillman
Max ticket life [unlimited]:
Max renewable life [unlimited]:
Attributes []:
Password: xxxxxxxx
Verifying password - Password: xxxxxxxx

Now it is time to start up the KDC services. Run service kerberos start and service kadmind start to bring up the services. Note that you will not have any kerberized daemons running at this point but you should be able to confirm that the KDC is functioning by obtaining and listing a ticket for the principal (user) that you just created from the command-line of the KDC itself:

% kinit tillman
tillman@EXAMPLE.ORG's Password:

% klist
Credentials cache: FILE:/tmp/krb5cc_500
	Principal: tillman@EXAMPLE.ORG

  Issued           Expires          Principal
Aug 27 15:37:58  Aug 28 01:37:58  krbtgt/EXAMPLE.ORG@EXAMPLE.ORG

The ticket can then be revoked when you have finished:

% kdestroy

15.7.3 Kerberos Enabling a Server with Heimdal Services

First, we need a copy of the Kerberos configuration file, /etc/krb5.conf. To do so, simply copy it over to the client computer from the KDC in a secure fashion (using network utilities, such as scp(1), or physically via a floppy disk).

Next you need a /etc/krb5.keytab file. This is the major difference between a server providing Kerberos enabled daemons and a workstation — the server must have a keytab file. This file contains the server's host key, which allows it and the KDC to verify each others identity. It must be transmitted to the server in a secure fashion, as the security of the server can be broken if the key is made public. This explicitly means that transferring it via a clear text channel, such as FTP, is a very bad idea.

Typically, you transfer the keytab to the server using the kadmin program. This is handy because you also need to create the host principal (the KDC end of the krb5.keytab) using kadmin.

Note that you must have already obtained a ticket and that this ticket must be allowed to use the kadmin interface in the kadmind.acl. See the section titled “Remote administration” in the Heimdal info pages (info heimdal) for details on designing access control lists. If you do not want to enable remote kadmin access, you can simply securely connect to the KDC (via local console, ssh(1) or Kerberos telnet(1)) and perform administration locally using kadmin -l.

After installing the /etc/krb5.conf file, you can use kadmin from the Kerberos server. The add --random-key command will let you add the server's host principal, and the ext command will allow you to extract the server's host principal to its own keytab. For example:

# kadmin
kadmin> add --random-key host/myserver.example.org
Max ticket life [unlimited]:
Max renewable life [unlimited]:
Attributes []:
kadmin> ext host/myserver.example.org
kadmin> exit

Note that the ext command (short for “extract”) stores the extracted key in /etc/krb5.keytab by default.

If you do not have kadmind running on the KDC (possibly for security reasons) and thus do not have access to kadmin remotely, you can add the host principal (host/myserver.EXAMPLE.ORG) directly on the KDC and then extract it to a temporary file (to avoid over-writing the /etc/krb5.keytab on the KDC) using something like this:

# kadmin
kadmin> ext --keytab=/tmp/example.keytab host/myserver.example.org
kadmin> exit

You can then securely copy the keytab to the server computer (using scp or a floppy, for example). Be sure to specify a non-default keytab name to avoid over-writing the keytab on the KDC.

At this point your server can communicate with the KDC (due to its krb5.conf file) and it can prove its own identity (due to the krb5.keytab file). It is now ready for you to enable some Kerberos services. For this example we will enable the telnet service by putting a line like this into your /etc/inetd.conf and then restarting the inetd(8) service with service inetd restart:

telnet    stream  tcp     nowait  root    /usr/libexec/telnetd  telnetd -a user

The critical bit is that the -a (for authentication) type is set to user. Consult the telnetd(8) manual page for more details.

15.7.4 Kerberos Enabling a Client with Heimdal

Setting up a client computer is almost trivially easy. As far as Kerberos configuration goes, you only need the Kerberos configuration file, located at /etc/krb5.conf. Simply securely copy it over to the client computer from the KDC.

Test your client computer by attempting to use kinit, klist, and kdestroy from the client to obtain, show, and then delete a ticket for the principal you created above. You should also be able to use Kerberos applications to connect to Kerberos enabled servers, though if that does not work and obtaining a ticket does the problem is likely with the server and not with the client or the KDC.

When testing an application like telnet, try using a packet sniffer (such as tcpdump(1)) to confirm that your password is not sent in the clear. Try using telnet with the -x option, which encrypts the entire data stream (similar to ssh).

Various non-core Kerberos client applications are also installed by default. This is where the “minimal” nature of the base Heimdal installation is felt: telnet is the only Kerberos enabled service.

The Heimdal port adds some of the missing client applications: Kerberos enabled versions of ftp, rsh, rcp, rlogin, and a few other less common programs. The MIT port also contains a full suite of Kerberos client applications.

15.7.5 User Configuration Files: .k5login and .k5users

Users within a realm typically have their Kerberos principal (such as tillman@EXAMPLE.ORG) mapped to a local user account (such as a local account named tillman). Client applications such as telnet usually do not require a user name or a principal.

Occasionally, however, you want to grant access to a local user account to someone who does not have a matching Kerberos principal. For example, tillman@EXAMPLE.ORG may need access to the local user account webdevelopers. Other principals may also need access to that local account.

The .k5login and .k5users files, placed in a users home directory, can be used similar to a powerful combination of .hosts and .rhosts, solving this problem. For example, if a .k5login with the following contents:


Were to be placed into the home directory of the local user webdevelopers then both principals listed would have access to that account without requiring a shared password.

Reading the manual pages for these commands is recommended. Note that the ksu manual page covers .k5users.

15.7.6 Kerberos Tips, Tricks, and Troubleshooting

15.7.7 Differences with the MIT Port

The major difference between the MIT and Heimdal installs relates to the kadmin program which has a different (but equivalent) set of commands and uses a different protocol. This has a large implications if your KDC is MIT as you will not be able to use the Heimdal kadmin program to administer your KDC remotely (or vice versa, for that matter).

The client applications may also take slightly different command line options to accomplish the same tasks. Following the instructions on the MIT Kerberos web site (http://web.mit.edu/Kerberos/www/) is recommended. Be careful of path issues: the MIT port installs into /usr/local/ by default, and the “normal” system applications may be run instead of MIT if your PATH environment variable lists the system directories first.

Note: With the MIT security/krb5 port that is provided by FreeBSD, be sure to read the /usr/local/share/doc/krb5/README.FreeBSD file installed by the port if you want to understand why logins via telnetd and klogind behave somewhat oddly. Most importantly, correcting the “incorrect permissions on cache file” behavior requires that the login.krb5 binary be used for authentication so that it can properly change ownership for the forwarded credentials.

The rc.conf must also be modified to contain the following configuration:


This is done because the applications for MIT kerberos installs binaries in the /usr/local hierarchy.

15.7.8 Mitigating Limitations Found in Kerberos Kerberos is an all-or-nothing approach

Every service enabled on the network must be modified to work with Kerberos (or be otherwise secured against network attacks) or else the users credentials could be stolen and re-used. An example of this would be Kerberos enabling all remote shells (via rsh and telnet, for example) but not converting the POP3 mail server which sends passwords in plain text. Kerberos is Intended for Single-User Workstations

In a multi-user environment, Kerberos is less secure. This is because it stores the tickets in the /tmp directory, which is readable by all users. If a user is sharing a computer with several other people simultaneously (i.e. multi-user), it is possible that the user's tickets can be stolen (copied) by another user.

This can be overcome with the -c filename command-line option or (preferably) the KRB5CCNAME environment variable, but this is rarely done. In principal, storing the ticket in the users home directory and using simple file permissions can mitigate this problem. The KDC is a Single Point of Failure

By design, the KDC must be as secure as the master password database is contained on it. The KDC should have absolutely no other services running on it and should be physically secured. The danger is high because Kerberos stores all passwords encrypted with the same key (the “master” key), which in turn is stored as a file on the KDC.

As a side note, a compromised master key is not quite as bad as one might normally fear. The master key is only used to encrypt the Kerberos database and as a seed for the random number generator. As long as access to your KDC is secure, an attacker cannot do much with the master key.

Additionally, if the KDC is unavailable (perhaps due to a denial of service attack or network problems) the network services are unusable as authentication can not be performed, a recipe for a denial-of-service attack. This can alleviated with multiple KDCs (a single master and one or more slaves) and with careful implementation of secondary or fall-back authentication (PAM is excellent for this). Kerberos Shortcomings

Kerberos allows users, hosts and services to authenticate between themselves. It does not have a mechanism to authenticate the KDC to the users, hosts or services. This means that a trojanned kinit (for example) could record all user names and passwords. Something like security/tripwire or other file system integrity checking tools can alleviate this.

15.7.9 Resources and further information