Caching of authentication results
=================================

Dovecot supports caching the results of password and user database lookups. The
following rules apply to using the authentication cache:

 * Data is used from the cache if it's not expired ('auth_cache_ttl' setting)
    * If authentication fails this time, but it didn't fail last time, it's
      assumed that the password has changed and a database lookup is done.
 * If a database lookup fails because of some internal error, but data still
   exists in the cache (even if expired), the cached data is used. This allows
   Dovecot to log in some users even if the database is temporarily down.

The authentication cache can be flushed by sending a SIGHUP to dovecot-auth.

Sending SIGUSR2 to dovecot-auth makes it log the number of cache hits and
misses. You can use that information for tuning the cache size and TTL.

Settings
--------

The settings related to the authentication cache are:

 * 'auth_cache_size': Authentication cache size in kilobytes, 0 disables
   caching (default). A typical passdb cache entry is around 50 bytes and a
   typical userdb cache entry is around 100-200 bytes, depending on the amount
   of information your user and password database lookups return.
 * 'auth_cache_ttl': Time to live in seconds for cache entries. A cache entry
   is no longer used (except for internal failures) if it was created more than
   this many seconds ago. Entries are removed from the cache only when the
   cache is full and a new entry is to be added.
 * 'auth_cache_negative_ttl': (v1.1+ only) If a passdb or userdb lookup didn't
   return any data (i.e. the user doesn't exist), it's also stored in the cache
   as a negative entry. This setting allows you to give negative entries a
   different TTL. 0 disables negative caching completely.

It should be pretty safe to set very high TTLs, because the only field that
usually can change is the user's password, and Dovecot attempts to catch those
cases (see the rules above).

Cache keys
----------

Usually only the username uniquely identifies a user, but in some setups you
may need something more, for example the remote IP address. For SQL and LDAP
lookups Dovecot figures this out automatically by using all the
used<%variables> [Variables.txt] as the cache key. For example if your SQL
query contains %s, %u and %r the cache entry is used only if all of them
(service name, username and remote IP) match for the new lookup.

With other databases Dovecot doesn't know what could affect caching, so you
have to tell Dovecot manually. The following databases require specifying the
cache key:

 * vpopmail
 * pam
 * bsdauth

For example if the PAM lookup depends on username and service, you can use:

---%<-------------------------------------------------------------------------
passdb pam {
  args = cache_key=%s%u *
}
---%<-------------------------------------------------------------------------

(This file was created from the wiki on 2008-06-20 04:42)
