The .k5login file, which resides in a user's home directory, contains a list of the Kerberos principals. Anyone with valid tickets for a principal in the file is allowed host access with the UID of the user in whose home directory the file resides. One common use is to place a .k5login file in root's home directory, thereby granting system administrators remote root access to the host via Kerberos.
Suppose the user alice had a .k5login file in her home directory containing just the following line:
This would allow bob to use Kerberos network applications, such as ssh(1), to access alice's account, using bob's Kerberos tickets. In a default configuration (with k5login_authoritative set to true in krb5.conf), this .k5login file would not let alice use those network applications to access her account, since she is not listed! With no .k5login file, or with k5login_authoritative set to false, a default rule would permit the principal alice in the machine's default realm to access the alice account.
Let us further suppose that alice is a system administrator. Alice and the other system administrators would have their principals in root's .k5login file on each host:
This would allow either system administrator to log in to these hosts using their Kerberos tickets instead of having to type the root password. Note that because bob retains the Kerberos tickets for his own principal, bob@FOOBAR.ORG, he would not have any of the privileges that require alice's tickets, such as root access to any of the site's hosts, or the ability to change alice's password.
The man page .k5login(5) is an alias of k5login(5).