mod_ldap
LDAP connection pooling and result caching services for use by other LDAP modules
Experimental
util_ldap.c
ldap_module
Available in version 2.0.41 and later

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 `--with-ldap` flag to the `./configure` script when building Apache.

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 iPlanet(Netscape) SDK.

# Example Configuration

The following is an example configuration that uses mod_ldap to increase the performance of HTTP Basic authentication provided by mod_auth_ldap.

# 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.
LDAPSharedCacheSize 200000
LDAPCacheEntries 1024
LDAPCacheTTL 600
LDAPOpCacheEntries 1024
LDAPOpCacheTTL 600
<Location /ldap-status>
SetHandler ldap-status
Order deny,allow
Deny from all
Allow from yourdomain.example.com
AuthLDAPEnabled on
AuthLDAPURL ldap://127.0.0.1/dc=example,dc=com?uid?one
AuthLDAPAuthoritative on
require valid-user
</Location>

# LDAP Connection Pool

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 keepalives.

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.

# LDAP Cache

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 Search/Bind Cache

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 same username, 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 LDAPCacheEntries and LDAPCacheTTL directives.

## Operation Caches

During attribute and distinguished name comparison functions, 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 names.

The behavior of both of these caches is controlled with the LDAPOpCacheEntries and LDAPOpCacheTTL directives.

## Monitoring the Cache

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:

<Location /server/cache-info>
SetHandler ldap-status
</Location>

By fetching the URL `http://servername/cache-info`, the administrator can get a status report of every cache that is used by 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 `httpd` instance processes the request.

# Using SSL

The ability to create an SSL connections to an LDAP server is defined by the directives LDAPTrustedCA and LDAPTrustedCAType. These directives specify the certificate file or database and the certificate type. Whenever the LDAP url includes ldaps://, 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.
LDAPTrustedCA /certs/certfile.der
LDAPTrustedCAType DER_FILE
<Location /ldap-status>
SetHandler ldap-status
Order deny,allow
Deny from all
Allow from yourdomain.example.com
AuthLDAPEnabled on
AuthLDAPURL ldaps://127.0.0.1/dc=example,dc=com?uid?one
AuthLDAPAuthoritative on
require valid-user
</Location>

If 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 `cert7.db` database. The easiest way to get this file is to start up a fresh copy of Netscape, and grab the resulting `$HOME/.netscape/cert7.db` file.

LDAPSharedCacheSize
Size in bytes of the shared-memory cache
LDAPSharedCacheSize bytes
LDAPSharedCacheSize 102400
server config

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.

LDAPSharedCacheFile
Sets the shared memory chache file
LDAPSharedCacheFile directory-path/filename
server config

Specifies the directory path and file name of the shared memory cache file. If not set, shared memory caching will not be used.

LDAPCacheEntries
Maximum number of entries in the primary LDAP cache
LDAPCacheEntries number
LDAPCacheEntries 1024
server config

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 searches.

LDAPCacheTTL
Time that cached items remain valid
LDAPCacheTTL seconds
LDAPCacheTTL 600
server config

Specifies the time (in seconds) that an item in the search/bind cache remains valid. The default is 600 seconds (10 minutes).

LDAPOpCacheEntries
Number of entries used to cache LDAP compare operations
LDAPOpCacheEntries number
LDAPOpCacheEntries 1024
server config

This specifies the number of entries mod_ldap will use to cache LDAP compare operations. The default is 1024 entries. Setting it to 0 disables operation caching.

LDAPOpCacheTTL
Time that entries in the operation cache remain valid
LDAPOpCacheTTL seconds
LDAPOpCacheTTL 600
server config

Specifies the time (in seconds) that entries in the operation cache remain valid. The default is 600 seconds.

LDAPTrustedCA
Sets the file containing the trusted Certificate Authority certificate or database
LDAPTrustedCA directory-path/filename
server config

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 `cert7.db`.

LDAPTrustedCAType
Specifies the type of the Certificate Authority file
LDAPTrustedCAType type
server config

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 ")