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HP-UX 11i Version 3: February 2007

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Kerberos — introduction to the Kerberos system


The Kerberos system authenticates individual users in a network environment. After authenticating yourself to Kerberos, you can use network utilities such as rlogin, rcp, and remsh without having to present passwords to the remote hosts and without having to edit and use .rhosts files. Note that these utilities will work without passwords only if the remote machines you deal with support the Kerberos system.

If you enter your username and the remote machine is not a Kerberos system, you will get the following message:

Principal unknown (Kerberos) you haven't been registered as a Kerberos user.

You will have to see your system administrator when the above message is encountered.

A Kerberos name usually contains three parts. The first is the primary, which is usually a user's or service's name. The second is the instance, which in the case of a user is usually null. Some users may have privileged instances, such as "root" or "admin". In the case of a service, the instance is the fully qualified name of the machine on which it runs; that is, there can be an rlogin service running on the machine ABC, which is different from the rlogin service running on the machine XYZ. The third part of a Kerberos name is the realm. The realm corresponds to the Kerberos service providing authentication for the principal.

When writing a Kerberos name, the principal name is separated from the instance (if not null) by a slash (/), and the realm (if not the local realm) follows, preceded by an @ sign. The following are examples of valid Kerberos names:

  • david

  • jennifer/admin

  • joeuser@BLEEP.COM


When you authenticate yourself with Kerberos, you get an initial Kerberos ticket. A Kerberos ticket is an encrypted protocol message that provides authentication. Kerberos uses this ticket for network utilities such as rlogin and rcp. The ticket transactions are done transparently, so you do not have to worry about their management.

Note, however, that tickets will expire. Privileged tickets, such as those with the instance "root", expire within a few minutes, while tickets that carry more ordinary privileges may be valid for several hours or a day, depending on the Kerberos server configuration. If your login session extends beyond the lifetime limit, you will have to re-authenticate yourself to Kerberos to get new tickets. Use the kinit command to re-authenticate yourself.

If you use the kinit command to get your tickets, make sure you use the kdestroy command to destroy your tickets before you end your login session. You should put the kdestroy command in your .logout file so that your tickets will be destroyed automatically when you logout. For more information about the kinit and kdestroy commands, see kinit(1) and kdestroy(1).

Kerberos tickets can be forwarded. In order to forward tickets, you must request forwardable tickets when you use the kinit command. Once you have forwardable tickets, most Kerberos programs have a command line option to forward them to the remote host.

Currently, Kerberos support is available for the following network services: rlogin, remsh, rcp, telnet, ftp, and ssh.


Kerberos was developed by the Massachusetts Institute of Technology by Steve Miller, MIT Project Athena/Digital Equipment Corporation, and Clifford Neuman, MIT Project Athena.

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