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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
Kerberos-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
you haven't been registered as a Kerberos user. See your system
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
transparently, 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-authenticate
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
forwardable 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
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
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),
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