Apache Module mod_ldap
This module was created to improve the performance of
websites relying on backend connections to LDAP servers. In
addition to the functions provided by the standard LDAP
libraries, this module adds an LDAP connection pool and an LDAP
shared memory cache.
To enable this module, LDAP support must be compiled into
apr-util. This is achieved by adding the
flag to the
./configure script when building
SSL support requires that
mod_ldap be linked
with one of the following LDAP SDKs:
OpenLDAP SDK (both 1.x and 2.x),
Novell LDAP SDK or the
The following is an example configuration that uses
mod_ldap to increase the performance of HTTP Basic
authentication provided by
# Enable the LDAP connection pool and shared
# memory cache. Enable the LDAP cache status
# handler. Requires that mod_ldap and mod_auth_ldap
# be loaded. Change the "yourdomain.example.com" to
# match your domain.
Deny from all
Allow from yourdomain.example.com
LDAP connections are pooled from request to request. This
allows the LDAP server to remain connected and bound ready for
the next request, without the need to unbind/connect/rebind.
The performance advantages are similar to the effect of HTTP
On a busy server it is possible that many requests will try
and access the same LDAP server connection simultaneously.
Where an LDAP connection is in use, Apache will create a new
connection alongside the original one. This ensures that the
connection pool does not become a bottleneck.
There is no need to manually enable connection pooling in
the Apache configuration. Any module using this module for
access to LDAP services will share the connection pool.
For improved performance,
mod_ldap uses an aggressive
caching strategy to minimize the number of times that the LDAP
server must be contacted. Caching can easily double or triple
the throughput of Apache when it is serving pages protected
with mod_auth_ldap. In addition, the load on the LDAP server
will be significantly decreased.
mod_ldap supports two types of LDAP caching during
the search/bind phase with a search/bind cache and
during the compare phase with two operation
caches. Each LDAP URL that is used by the server has
its own set of these three caches.
The process of doing a search and then a bind is the
most time-consuming aspect of LDAP operation, especially if
the directory is large. The search/bind cache is used to
cache all searches that resulted in successful binds.
Negative results (i.e., unsuccessful searches, or searches
that did not result in a successful bind) are not cached.
The rationale behind this decision is that connections with
invalid credentials are only a tiny percentage of the total
number of connections, so by not caching invalid
credentials, the size of the cache is reduced.
mod_ldap stores the username, the DN
retrieved, the password used to bind, and the time of the bind
in the cache. Whenever a new connection is initiated with the
mod_ldap compares the password
of the new connection with the password in the cache. If the
passwords match, and if the cached entry is not too old,
mod_ldap bypasses the search/bind phase.
The search and bind cache is controlled with the
During attribute and distinguished name comparison
mod_ldap uses two operation caches
to cache the compare operations. The first compare cache is
used to cache the results of compares done to test for LDAP
group membership. The second compare cache is used to cache
the results of comparisons done between distinguished
The behavior of both of these caches is controlled with
mod_ldap has a content handler that allows
administrators to monitor the cache performance. The name of
the content handler is
ldap-status, so the
following directives could be used to access the
mod_ldap cache information:
By fetching the URL
the administrator can get a status report of every cache that is used
mod_ldap cache. Note that if Apache does not
support shared memory, then each
httpd instance has its
own cache, so reloading the URL will result in different
information each time, depending on which
instance processes the request.
The ability to create an SSL connections to an LDAP server
is defined by the directives
LDAPTrustedCAType. These directives specify the certificate
file or database and the certificate type. Whenever the LDAP url
mod_ldap will establish
a secure connection to the LDAP server.
# Establish an SSL LDAP connection. Requires that
# mod_ldap and mod_auth_ldap be loaded. Change the
# "yourdomain.example.com" to match your domain.
Deny from all
Allow from yourdomain.example.com
mod_ldap is linked against the
Netscape/iPlanet LDAP SDK, it will not talk to any SSL server
unless that server has a certificate signed by a known Certificate
Authority. As part of the configuration
mod_ldap needs to be told where it can find
a database containing the known CAs. This database is in the same
format as Netscape Communicator's
database. The easiest way to get this file is to start up a fresh
copy of Netscape, and grab the resulting
Specifies the maximum size of the primary LDAP cache. This
cache contains successful search/binds. Set it to 0 to turn off
search/bind caching. The default size is 1024 cached
Specifies the time (in seconds) that an item in the
search/bind cache remains valid. The default is 600 seconds (10
This specifies the number of entries
will use to cache LDAP compare operations. The default is 1024
entries. Setting it to 0 disables operation caching.
Specifies the time (in seconds) that entries in the
operation cache remain valid. The default is 600 seconds.
Specifies the directory path and file name of the shared memory
cache file. If not set, shared memory caching will not be used.
Specifies the number of bytes to allocate for the shared
memory cache. The default is 100kb. If set to 0, shared memory
caching will not be used.
It specifies the directory path and file name of the trusted CA
mod_ldap should use when establishing an SSL
connection to an LDAP server. If using the Netscape/iPlanet Directory
SDK, the file name should be
The following types are supported:
DER_FILE - file in binary DER format
BASE64_FILE - file in Base64 format
CERT7_DB_PATH - Netscape certificate database file ")