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The Apache HTTP Server Reference Manual
by Apache Software Foundation
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3.49  Apache Module mod_ldap



Description:

LDAP connection pooling and result caching services for use by other LDAP modules

Status:

Extension

Module Identifier:

ldap_module

Source File:

util_ldap.c

Compatibility:

Available in version 2.0.41 and later



Summary

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/TLS support is dependant on which LDAP toolkit has been linked to APR. As of this writing, APR-util supports: OpenLDAP SDK42 (2.x or later), Novell LDAP SDK43 , Mozilla LDAP SDK44 , native Solaris LDAP SDK (Mozilla based), native Microsoft LDAP SDK, or the iPlanet (Netscape)45 SDK. See the APR46 website for details.

Directives:

LDAPCacheEntries

LDAPCacheTTL

LDAPConnectionTimeout

LDAPOpCacheEntries

LDAPOpCacheTTL

LDAPSharedCacheFile

LDAPSharedCacheSize

LDAPTrustedClientCert

LDAPTrustedGlobalCert

LDAPTrustedMode

LDAPVerifyServerCert

3.49.1  Example Configuration

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

# Enable the LDAP connection pool and shared
# memory cache. Enable the LDAP cache status
# handler. Requires that mod_ldap and mod_authnz_ldap
# be loaded. Change the "yourdomain.example.com" to
# match your domain.
LDAPSharedCacheSize 500000
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
AuthLDAPURL ldap://127.0.0.1/dc=example,dc=com?uid?one
AuthzLDAPAuthoritative off
Require valid-user

</Location>

3.49.2  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.

3.49.3  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_authnz_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.

3.49.4  Using SSL/TLS

The ability to create SSL and TLS connections to an LDAP server is defined by the directives LDAPTrustedGlobalCert, LDAPTrustedClientCert and LDAPTrustedMode. These directives specify the CA and optional client certificates to be used, as well as the type of encryption to be used on the connection (none, SSL or TLS/STARTTLS).

# Establish an SSL LDAP connection on port 636. Requires that
# mod_ldap and mod_authnz_ldap be loaded. Change the
# "yourdomain.example.com" to match your domain.
LDAPTrustedGlobalCert CA_DER /certs/certfile.der
<Location /ldap-status>

SetHandler ldap-status
Order deny,allow
Deny from all
Allow from yourdomain.example.com
AuthLDAPURL ldaps://127.0.0.1/dc=example,dc=com?uid?one
AuthzLDAPAuthoritative off
Require valid-user

</Location>

# Establish a TLS LDAP connection on port 389. Requires that
# mod_ldap and mod_authnz_ldap be loaded. Change the
# "yourdomain.example.com" to match your domain.
LDAPTrustedGlobalCert CA_DER /certs/certfile.der
<Location /ldap-status>

SetHandler ldap-status
Order deny,allow
Deny from all
Allow from yourdomain.example.com
AuthLDAPURL ldap://127.0.0.1/dc=example,dc=com?uid?one TLS
AuthzLDAPAuthoritative off
Require valid-user

</Location>

3.49.5  SSL/TLS Certificates

The different LDAP SDKs have widely different methods of setting and handling both CA and client side certificates.

If you intend to use SSL or TLS, read this section CAREFULLY so as to understand the differences between configurations on the different LDAP toolkits supported.

Netscape/Mozilla/iPlanet SDK

CA certificates are specified within a file called cert7.db. The SDK will not talk to any LDAP server whose certificate was not signed by a CA specified in this file. If client certificates are required, an optional key3.db file may be specified with an optional password. The secmod file can be specified if required. These files are in the same format as used by the Netscape Communicator or Mozilla web browsers. The easiest way to obtain these files is to grab them from your browser installation.

Client certificates are specified per connection using the LDAPTrustedClientCert directive by referring to the certificate "nickname". An optional password may be specified to unlock the certificate’s private key.

The SDK supports SSL only. An attempt to use STARTTLS will cause an error when an attempt is made to contact the LDAP server at runtime.

# Specify a Netscape CA certificate file
LDAPTrustedGlobalCert CA_CERT7_DB /certs/cert7.db
# Specify an optional key3.db file for client certificate support
LDAPTrustedGlobalCert CERT_KEY3_DB /certs/key3.db
# Specify the secmod file if required
LDAPTrustedGlobalCert CA_SECMOD /certs/secmod
<Location /ldap-status>

SetHandler ldap-status
Order deny,allow
Deny from all
Allow from yourdomain.example.com
LDAPTrustedClientCert CERT_NICKNAME <nickname> [password]
AuthLDAPURL ldaps://127.0.0.1/dc=example,dc=com?uid?one
AuthzLDAPAuthoritative off
Require valid-user

</Location>

Novell SDK

One or more CA certificates must be specified for the Novell SDK to work correctly. These certificates can be specified as binary DER or Base64 (PEM) encoded files.

Note: Client certificates are specified globally rather than per connection, and so must be specified with the LDAPTrustedGlobalCert directive as below. Trying to set client certificates via the LDAPTrustedClientCert directive will cause an error to be logged when an attempt is made to connect to the LDAP server..

The SDK supports both SSL and STARTTLS, set using the LDAPTrustedMode parameter. If an ldaps:// URL is specified, SSL mode is forced, override this directive.

# Specify two CA certificate files
LDAPTrustedGlobalCert CA_DER /certs/cacert1.der
LDAPTrustedGlobalCert CA_BASE64 /certs/cacert2.pem
# Specify a client certificate file and key
LDAPTrustedGlobalCert CERT_BASE64 /certs/cert1.pem
LDAPTrustedGlobalCert KEY_BASE64 /certs/key1.pem [password]
# Do not use this directive, as it will throw an error
#LDAPTrustedClientCert CERT_BASE64 /certs/cert1.pem

OpenLDAP SDK

One or more CA certificates must be specified for the OpenLDAP SDK to work correctly. These certificates can be specified as binary DER or Base64 (PEM) encoded files.

Client certificates are specified per connection using the LDAPTrustedClientCert directive.

The documentation for the SDK claims to support both SSL and STARTTLS, however STARTTLS does not seem to work on all versions of the SDK. The SSL/TLS mode can be set using the LDAPTrustedMode parameter. If an ldaps:// URL is specified, SSL mode is forced. The OpenLDAP documentation notes that SSL (ldaps://) support has been deprecated to be replaced with TLS, although the SSL functionality still works.

# Specify two CA certificate files
LDAPTrustedGlobalCert CA_DER /certs/cacert1.der
LDAPTrustedGlobalCert CA_BASE64 /certs/cacert2.pem
<Location /ldap-status>

SetHandler ldap-status
Order deny,allow
Deny from all
Allow from yourdomain.example.com
LDAPTrustedClientCert CERT_BASE64 /certs/cert1.pem
LDAPTrustedClientCert KEY_BASE64 /certs/key1.pem
AuthLDAPURL ldaps://127.0.0.1/dc=example,dc=com?uid?one
AuthzLDAPAuthoritative off
Require valid-user

</Location>

Solaris SDK

SSL/TLS for the native Solaris LDAP libraries is not yet supported. If required, install and use the OpenLDAP libraries instead.

Microsoft SDK

SSL/TLS certificate configuration for the native Microsoft LDAP libraries is done inside the system registry, and no configuration directives are required.

Both SSL and TLS are supported by using the ldaps:// URL format, or by using the LDAPTrustedMode directive accordingly.

Note: The status of support for client certificates is not yet known for this toolkit.

LDAPCacheEntries Directive

Description:

Maximum number of entries in the primary LDAP cache

Syntax:

LDAPCacheEntries number

Default:

LDAPCacheEntries 1024

Context:

server config

Status:

Extension

Module:

mod_ldap

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 Directive

Description:

Time that cached items remain valid

Syntax:

LDAPCacheTTL seconds

Default:

LDAPCacheTTL 600

Context:

server config

Status:

Extension

Module:

mod_ldap

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

LDAPConnectionTimeout Directive

Description:

Specifies the socket connection timeout in seconds

Syntax:

LDAPConnectionTimeout seconds

Context:

server config

Status:

Extension

Module:

mod_ldap

This directive configures the LDAP_OPT_NETWORK_TIMEOUT option in the underlying LDAP client library, when available. This value typically controls how long the LDAP client library will wait for the TCP connection to the LDAP server to complete.

If a connection is not successful with the timeout period, either an error will be returned or the LDAP client library will attempt to connect to a secondary LDAP server if one is specified (via a space-separated list of hostnames in the AuthLDAPURL).

The default is 10 seconds, if the LDAP client library linked with the server supports the LDAP_OPT_NETWORK_TIMEOUT option.

LDAPConnectionTimeout is only available when the LDAP client library linked with the server supports the LDAP_OPT_NETWORK_TIMEOUT option, and the ultimate behavior is dictated entirely by the LDAP client library.

LDAPOpCacheEntries Directive

Description:

Number of entries used to cache LDAP compare operations

Syntax:

LDAPOpCacheEntries number

Default:

LDAPOpCacheEntries 1024

Context:

server config

Status:

Extension

Module:

mod_ldap

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 Directive

Description:

Time that entries in the operation cache remain valid

Syntax:

LDAPOpCacheTTL seconds

Default:

LDAPOpCacheTTL 600

Context:

server config

Status:

Extension

Module:

mod_ldap

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

LDAPSharedCacheFile Directive

Description:

Sets the shared memory cache file

Syntax:

LDAPSharedCacheFile directory-path/filename

Context:

server config

Status:

Extension

Module:

mod_ldap

Specifies the directory path and file name of the shared memory cache file. If not set, anonymous shared memory will be used if the platform supports it.

LDAPSharedCacheSize Directive

Description:

Size in bytes of the shared-memory cache

Syntax:

LDAPSharedCacheSize bytes

Default:

LDAPSharedCacheSize 500000

Context:

server config

Status:

Extension

Module:

mod_ldap

Specifies the number of bytes to allocate for the shared memory cache. The default is 500kb. If set to 0, shared memory caching will not be used.

LDAPTrustedClientCert Directive

Description:

Sets the file containing or nickname referring to a per connection client certificate. Not all LDAP toolkits support per connection client certificates.

Syntax:

LDAPTrustedClientCert type directory-path/filename/nickname [password]

Context:

server config, virtual host, directory, .htaccess

Status:

Extension

Module:

mod_ldap

It specifies the directory path, file name or nickname of a per connection client certificate used when establishing an SSL or TLS connection to an LDAP server. Different locations or directories may have their own independent client certificate settings. Some LDAP toolkits (notably Novell) do not support per connection client certificates, and will throw an error on LDAP server connection if you try to use this directive (Use the LDAPTrustedGlobalCert directive instead for Novell client certificates - See the SSL/TLS certificate guide above for details). The type specifies the kind of certificate parameter being set, depending on the LDAP toolkit being used. Supported types are:

  • CERT_DER - binary DER encoded client certificate
  • CERT_BASE64 - PEM encoded client certificate
  • CERT_NICKNAME - Client certificate "nickname" (Netscape SDK)
  • KEY_DER - binary DER encoded private key
  • KEY_BASE64 - PEM encoded private key

LDAPTrustedGlobalCert Directive

Description:

Sets the file or database containing global trusted Certificate Authority or global client certificates

Syntax:

LDAPTrustedGlobalCert type directory-path/filename [password]

Context:

server config

Status:

Extension

Module:

mod_ldap

It specifies the directory path and file name of the trusted CA certificates and/or system wide client certificates mod_ldap should use when establishing an SSL or TLS connection to an LDAP server. Note that all certificate information specified using this directive is applied globally to the entire server installation. Some LDAP toolkits (notably Novell) require all client certificates to be set globally using this directive. Most other toolkits require clients certificates to be set per Directory or per Location using LDAPTrustedClientCert. If you get this wrong, an error may be logged when an attempt is made to contact the LDAP server, or the connection may silently fail (See the SSL/TLS certificate guide above for details). The type specifies the kind of certificate parameter being set, depending on the LDAP toolkit being used. Supported types are:

  • CA_DER - binary DER encoded CA certificate
  • CA_BASE64 - PEM encoded CA certificate
  • CA_CERT7_DB - Netscape cert7.db CA certificate database file
  • CA_SECMOD - Netscape secmod database file
  • CERT_DER - binary DER encoded client certificate
  • CERT_BASE64 - PEM encoded client certificate
  • CERT_KEY3_DB - Netscape key3.db client certificate database file
  • CERT_NICKNAME - Client certificate "nickname" (Netscape SDK)
  • CERT_PFX - PKCS#12 encoded client certificate (Novell SDK)
  • KEY_DER - binary DER encoded private key
  • KEY_BASE64 - PEM encoded private key
  • KEY_PFX - PKCS#12 encoded private key (Novell SDK)

LDAPTrustedMode Directive

Description:

Specifies the SSL/TLS mode to be used when connecting to an LDAP server.

Syntax:

LDAPTrustedMode type

Context:

server config, virtual host

Status:

Extension

Module:

mod_ldap

The following modes are supported:

  • NONE - no encryption
  • SSL - ldaps:// encryption on default port 636
  • TLS - STARTTLS encryption on default port 389

Not all LDAP toolkits support all the above modes. An error message will be logged at runtime if a mode is not supported, and the connection to the LDAP server will fail.

If an ldaps:// URL is specified, the mode becomes SSL and the setting of LDAPTrustedMode is ignored.

LDAPVerifyServerCert Directive

Description:

Force server certificate verification

Syntax:

LDAPVerifyServerCert On|Off

Default:

LDAPVerifyServerCert On

Context:

server config

Status:

Extension

Module:

mod_ldap

Specifies whether to force the verification of a server certificate when establishing an SSL connection to the LDAP server.

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