kerberos¶
DESCRIPTION¶
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 or certificates to those programs.
If you receive the following response from kinit:
kinit: 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
ssh service running on the machine ABC (ssh/ABC@REALM), which is
different from the ssh service running on the machine XYZ
(ssh/XYZ@REALM). The third part of a Kerberos name is the realm.
The realm corresponds to the Kerberos service providing authentication
for the principal. Realms are conventionally all-uppercase, and often
match the end of hostnames in the realm (for instance, host01.example.com
might be in realm EXAMPLE.COM).
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
cbrown/root@FUBAR.ORG
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 ssh. The ticket transactions are done transparently, so you don’t have to worry about their management.
Note, however, that tickets expire. Administrators may configure more
privileged tickets, such as those with service or instance of root
or admin
, to expire in a few minutes, while tickets that carry
more ordinary privileges may be good for several hours or a day. If
your login session extends beyond the time limit, you will have to
re-authenticate yourself to Kerberos to get new tickets using the
kinit command.
Some tickets are renewable beyond their initial lifetime. This
means that kinit -R
can extend their lifetime without requiring
you to re-authenticate.
If you wish to delete your local tickets, use the kdestroy command.
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. This can be useful for, e.g., running kinit on your local machine and then sshing into another to do work. Note that this should not be done on untrusted machines since they will then have your tickets.
ENVIRONMENT VARIABLES¶
Several environment variables affect the operation of Kerberos-enabled programs. These include:
- KRB5CCNAME
Default name for the credentials cache file, in the form TYPE:residual. The type of the default cache may determine the availability of a cache collection.
FILE
is not a collection type;KEYRING
,DIR
, andKCM
are.If not set, the value of default_ccache_name from configuration files (see KRB5_CONFIG) will be used. If that is also not set, the default type is
FILE
, and the residual is the path /tmp/krb5cc_*uid*, where uid is the decimal user ID of the user.- KRB5_KTNAME
- Specifies the location of the default 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. If unset, DEFKTNAME will be used.
- KRB5_CONFIG
- Specifies the location of the Kerberos configuration file. The
default is SYSCONFDIR
/krb5.conf
. Multiple filenames can be specified, separated by a colon; all files which are present will be read. - KRB5_KDC_PROFILE
- 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 LOCALSTATEDIR
/krb5kdc
/kdc.conf
. - KRB5RCACHETYPE
- Specifies the default type of replay cache to use for servers.
Valid types include
dfl
for the normal file type andnone
for no replay cache. The default isdfl
. - 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. - KRB5_TRACE
- Specifies a filename to write trace log output to. Trace logs can
help illuminate decisions made internally by the Kerberos
libraries. For example,
env KRB5_TRACE=/dev/stderr kinit
would send tracing information for kinit to/dev/stderr
. The default is not to write trace log output anywhere. - KRB5_CLIENT_KTNAME
- Default client keytab file name. If unset, DEFCKTNAME will be used).
- KPROP_PORT
- kprop port to use. Defaults to 754.
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.
SEE ALSO¶
kdestroy, kinit, klist, kswitch, kpasswd, ksu, krb5.conf, kdc.conf, kadmin, kadmind, kdb5_util, krb5kdc
BUGS¶
AUTHORS¶
HISTORY¶
The MIT Kerberos 5 implementation was developed at MIT, with contributions from many outside parties. It is currently maintained by the MIT Kerberos Consortium.
RESTRICTIONS¶
Copyright 1985, 1986, 1989-1996, 2002, 2011, 2018 Masachusetts Institute of Technology