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, 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': 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 {
  driver = pam
  args = cache_key=%s%u *
}
---%<-------------------------------------------------------------------------

Password changing scenarios
---------------------------

Normal scenario:

 1. User logs in with password X. The password X is added to cache and login
    succeeds.
 2. Password is changed to Y.
 3. User logs in with password Y. The cached password X doesn't match Y, but
    since the previous authentication was successful Dovecot does another
    backend passdb lookup to see if the password changed. It did, so the
    password Y is cached and login succeeds.

Using old cached password scenario:

 1. User logs in with password X. The password X is added to cache and login
    succeeds.
 2. Password is changed to Y.
 3. User logs in with password X. The cached password X matches X, so login
    succeeds.

Early change scenario:

 1. User logs in with password X. The password X is added to cache and login
    succeeds.
 2. User logs in with password Y. The cached password X doesn't match Y, but
    since the previous authentication was successful Dovecot does another
    backend passdb lookup to see if the password changed. It didn't, so the
    login fails.
 3. Password is changed to Y.
 4. User logs in with password Y. The cached password X doesn't match Y and the
    previous authentication was unsuccessful, so Dovecot doesn't bother doing
    another backend passdb lookup (until cache TTL expires). The login fails.

(This file was created from the wiki on 2017-10-10 04:42)