KERBEROS(1) General Commands Manual KERBEROS(1)
kerberos - introduction to the Kerberos system
The Kerberos system authenticates individual users in a network environment. After authenticating yourself to Kerberos, you can use Ker-
beros-enabled programs without having to present passwords.
If you enter your username and kinit responds with this message:
kinit(v5): Client not found in Kerberos database while getting initial credentials
you haven't been registered as a Kerberos user. See your system administrator.
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, however, 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; i.e. 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:
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 trans-
parently, so you don't have to worry about their management.
Note, however, that tickets expire. Privileged tickets, such as those with the instance ``root'', expire in a few minutes, while tickets
that carry more ordinary privileges may be good for several hours or a day, depending on the installation's policy. If your login session
extends beyond the time limit, you will have to re-authenticate yourself to Kerberos to get new tickets. Use the kinit command to re-
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 the kinit(1) and kdestroy(1) manual pages.
Kerberos tickets can be forwarded. In order to forward tickets, you must request forwardable tickets when you kinit. Once you have for-
wardable tickets, most Kerberos programs have a command line option to forward them to the remote host.
Several environment variables affect the operation of Kerberos-enabled programs. These include:
Specifies the location of the credential cache, in the form TYPE:residual. If no type prefix is present, the FILE type is assumed
and residual is the pathname of the cache file. A collection of multiple caches may be used by specifying the DIR type and the
pathname of a private directory (which must already exist). The default cache file is /tmp/krb5cc_uid where uid is the decimal user
ID of the user.
Specifies the location of the keytab file, in the form TYPE:residual. If no type is present, the FILE type is assumed and residual
is the pathname of the keytab file. The default keytab file is /etc/krb5.keytab.
Specifies the location of the Kerberos configuration file. The default is /etc/krb5.conf.
Specifies the location of the KDC configuration file, which contains additional configuration directives for the Key Distribution
Center daemon and associated programs. The default is /usr/local/var/krb5kdc/kdc.conf.
Specifies the default type of replay cache to use for servers. Valid types include "dfl" for the normal file type and "none" for no
replay cache. KRB5RCACHEDIR Specifies the default directory for replay caches used by servers. The default is the value of the
TMPDIR environment variable, or /var/tmp if TMPDIR is not set.
Specifies a filename to write trace log output to. Trace logs can help illuminate decisions made internally by the Kerberos
libraries. The default is not to write trace log output anywhere.
Most environment variables are disabled for certain programs, such as login system programs and setuid programs, which are designed to be
secure when run within an untrusted process environment.
kdestroy(1), kinit(1), klist(1), kswitch(1), kpasswd(1), ksu(1), krb5.conf(5), kdc.conf(5), kadmin(1), kadmind(8), kdb5_util(8), krb5kdc(8)
Steve Miller, MIT Project Athena/Digital Equipment Corporation
Clifford Neuman, MIT Project Athena
Greg Hudson, MIT Kerberos Consortium
The MIT Kerberos 5 implementation was developed at MIT, with contributions from many outside parties. It is currently maintained by the
MIT Kerberos Consortium.
Copyright 1985,1986,1989-1996,2002,2011 Massachusetts Institute of Technology