If you receive the following response from kinit(1):
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(1) 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(1) 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.
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.
Steve Miller, MIT Project Athena/Digital Equipment Corporation Clifford Neuman, MIT Project Athena Greg Hudson, MIT Kerberos Consortium Robbie Harwood, Red Hat, Inc.