Apache HTTP Server Version 2.4
|Description:||Manages a cache of authentication credentials to relieve the load on backends|
|Compatibility:||Version 2.3 and later|
Maintains a cache of authentication credentials, so that a new backend lookup is not required for every authenticated request.
Some users of more heavyweight authentication such as SQL database
mod_authn_dbd) have reported it putting an
unacceptable load on their authentication provider. A typical case
in point is where an HTML page contains hundreds of objects
(images, scripts, stylesheets, media, etc), and a request to the page
generates hundreds of effectively-immediate requests for authenticated
mod_authn_socache provides a solution to this problem by maintaining a cache of authentication credentials.
The authentication cache should be used where authentication
lookups impose a significant load on the server, or a backend or
network. Authentication by file (
or dbm (
mod_authn_dbm) are unlikely to benefit,
as these are fast and lightweight in their own right (though in some
cases, such as a network-mounted file, cacheing may be worthwhile).
Other providers such as SQL or LDAP based authentication are more
likely to benefit, particularly where there is an observed
performance issue. Amongst the standard modules,
mod_authnz_ldap manages its own cache, so only
mod_authn_dbd will usually benefit from this cache.
The basic rules to cache for a provider are:
A simple usage example to accelerate
using dbm as a cache engine:
#AuthnCacheSOCache is optional. If specified, it is server-wide AuthnCacheSOCache dbm <Directory "/usr/www/myhost/private"> AuthType Basic AuthName "Cached Authentication Example" AuthBasicProvider socache dbd AuthDBDUserPWQuery "SELECT password FROM authn WHERE user = %s" AuthnCacheProvideFor dbd Require valid-user #Optional AuthnCacheContext dbd-authn-example </Directory>
Module developers should note that their modules must be enabled for cacheing with mod_authn_socache. A single optional API function ap_authn_cache_store is provided to cache credentials a provider has just looked up or generated. Usage examples are available in r957072, in which three authn providers are enabled for cacheing.
|Description:||Specify a context string for use in the cache key|
This directive specifies a string to be used along with the supplied username (and realm in the case of Digest Authentication) in constructing a cache key. This serves to disambiguate identical usernames serving different authentication areas on the server.
Two special values for this are directory, which uses the directory context of the request as a string, and server which uses the virtual host name.
The default is directory, which is also the most
conservative setting. This is likely to be less than optimal, as it
(for example) causes $app-base, $app-base/images,
$app-base/scripts and $app-base/media each to
have its own separate cache key. A better policy is to name the
AuthnCacheContext for the password
provider: for example a htpasswd file or database table.
Contexts can be shared across different areas of a server, where credentials are shared. However, this has potential to become a vector for cross-site or cross-application security breaches, so this directive is not permitted in .htaccess contexts.
|Description:||Enable Authn caching configured anywhere|
This directive is not normally necessary: it is implied if authentication cacheing is enabled anywhere in httpd.conf. However, if it is not enabled anywhere in httpd.conf it will by default not be initialised, and is therefore not available in a .htaccess context. This directive ensures it is initialised so it can be used in .htaccess.
|Description:||Specify which authn provider(s) to cache for|
This directive specifies an authentication provider or providers to cache for. Credentials found by a provider not listed in an AuthnCacheProvideFor directive will not be cached.
For example, to cache credentials found by
or by a custom provider myprovider, but leave those looked
up by lightweight providers like file or dbm lookup alone:
AuthnCacheProvideFor dbd myprovider
|Description:||Select socache backend provider to use|
|Compatibility:||Optional provider arguments are available in Apache HTTP Server 2.4.7 and later|
This is a server-wide setting to select a provider for the shared object cache, followed by optional arguments for that provider. Some possible values for provider-name are "dbm", "dc", "memcache", or "shmcb", each subject to the appropriate module being loaded. If not set, your platform's default will be used.
|Description:||Set a timeout for cache entries|
Cacheing authentication data can be a security issue, though short-term cacheing is unlikely to be a problem. Typically a good solution is to cache credentials for as long as it takes to relieve the load on a backend, but no longer, though if changes to your users and passwords are infrequent then a longer timeout may suit you. The default 300 seconds (5 minutes) is both cautious and ample to keep the load on a backend such as dbd (SQL database queries) down.
This should not be confused with session timeout, which is an entirely separate issue. However, you may wish to check your session-management software for whether cached credentials can "accidentally" extend a session, and bear it in mind when setting your timeout.