Currently the default ccache directory is hardcoded to be /tmp on Unix-
like platforms:

    snprintf(name_buf, name_size, "FILE:/tmp/krb5cc_%ld", (long) 
getuid());

Zaar Hai has requested that this variable be configurable via krb5.conf.  
An example use case would be to choose a directory which is mounted on a 
memory filesystem (if for some reason /tmp needs to be on real disk), in 
order to ensure that ccaches do not survive a reboot.

Mailing list thread at:

http://mailman.mit.edu/pipermail/kerberos/2010-October/016634.html