Most programs look for and store Kerberos tickets in the default cred cache, commonly a file on disk. This has the following properties:

(1) If a process (intentionally or unintentionally) corrupts this file, it has far-reaching effects (2) Tickets are shared across processes. If one process gets a service ticket, another process will subsequently find that ticket in its credential cache.

In order to mitigate the effects of (1) without compromising the benefits of (2), we use a MEMORY cred cache (unless otherwise provided). In order to achieve sharing, we make sure to read/write service tickets from/to the default cred cache.

libkrb5 takes out POSIX locks when messing around with file caches, so it is safe for multiple processes to store tickets. See open_cache_file and close_cache_file in src/lib/krb5/ccache/cc_file.c.

If t was created using of_cred_cache, this just calls Internal.Cred_cache.get_credentials.

If t was created using in_memory, then we try to get a ticket in the following order:

(1) Check for a cached ticket in the memory cache (2) Call Internal.Cred_cache.get_credentials on the default cache. If this succeeds, store the ticket in the memory cache. (3) Get a ticket using the memory cache's TGT.

If cases (1) or (2) succeed, then both the memory cache and the default cache will have the ticket.

In case (3), only the memory cache will have the ticket.