• Authentication
• Caching

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 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 *
}