Apache Module mod_ldap

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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

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 dependent on which LDAP toolkit has been linked to APR. As of this writing, APR-util supports: OpenLDAP SDK (2.x or later), Novell LDAP SDK, Mozilla LDAP SDK, native Solaris LDAP SDK (Mozilla based) or the native Microsoft LDAP SDK. See the APR website for details.

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

    Require host yourdomain.example.com

    Satisfy any
    AuthType Basic
    AuthName "LDAP Protected"
    AuthBasicProvider ldap
    AuthLDAPURL "ldap://127.0.0.1/dc=example,dc=com?uid?one"
    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 connections can keep track of the ldap client credentials used when binding to an LDAP server. These credentials can be provided to LDAP servers that do not allow anonymous binds during referral chasing. To control this feature, see the LDAPReferrals and LDAPReferralHopLimit directives. By default, this feature is enabled.

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.

Note that, when group membership is being checked, any sub-group comparison results are cached to speed future sub-group comparisons.

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 https://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/TLS

The ability to create an 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

    Require host yourdomain.example.com

    Satisfy any
    AuthType Basic
    AuthName "LDAP Protected"
    AuthBasicProvider ldap
    AuthLDAPURL "ldaps://127.0.0.1/dc=example,dc=com?uid?one"
    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

    Require host yourdomain.example.com

    Satisfy any
    AuthType Basic
    AuthName "LDAP Protected"
    AuthBasicProvider ldap
    AuthLDAPURL "ldap://127.0.0.1/dc=example,dc=com?uid?one" TLS
    Require valid-user
</Location>

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

    Require host yourdomain.example.com

    Satisfy any
    AuthType Basic
    AuthName "LDAP Protected"
    AuthBasicProvider ldap
    LDAPTrustedClientCert CERT_NICKNAME <nickname> [password]
    AuthLDAPURL "ldaps://127.0.0.1/dc=example,dc=com?uid?one"
    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.

Both CA and client certificates may be specified globally (LDAPTrustedGlobalCert) or per-connection (LDAPTrustedClientCert). When any settings are specified per-connection, the global settings are superceded.

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

    Require host yourdomain.example.com

    LDAPTrustedClientCert CERT_BASE64 "/certs/cert1.pem"
    LDAPTrustedClientCert KEY_BASE64 "/certs/key1.pem"
    # CA certs respecified due to per-directory client certs
    LDAPTrustedClientCert CA_DER "/certs/cacert1.der"
    LDAPTrustedClientCert CA_BASE64 "/certs/cacert2.pem"
    Satisfy any
    AuthType Basic
    AuthName "LDAP Protected"
    AuthBasicProvider ldap
    AuthLDAPURL "ldaps://127.0.0.1/dc=example,dc=com?uid?one"
    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).

LDAPConnectionPoolTTL Directive

Description:	Discard backend connections that have been sitting in the connection pool too long
Syntax:	`LDAPConnectionPoolTTL n`
Default:	`LDAPConnectionPoolTTL -1`
Context:	server config, virtual host
Status:	Extension
Module:	mod_ldap
Compatibility:	Apache HTTP Server 2.3.12 and later

Specifies the maximum age, in seconds, that a pooled LDAP connection can remain idle and still be available for use. Connections are cleaned up when they are next needed, not asynchronously.

A setting of 0 causes connections to never be saved in the backend connection pool. The default value of -1, and any other negative value, allows connections of any age to be reused.

For performance reasons, the reference time used by this directive is based on when the LDAP connection is returned to the pool, not the time of the last successful I/O with the LDAP server.

Since 2.4.10, new measures are in place to avoid the reference time from being inflated by cache hits or slow requests. First, the reference time is not updated if no backend LDAP conncetions were needed. Second, the reference time uses the time the HTTP request was received instead of the time the request is completed.

This timeout defaults to units of seconds, but accepts suffixes for milliseconds (ms), minutes (min), and hours (h).

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 (or LDAP_OPT_CONNECT_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 (or LDAP_OPT_CONNECT_TIMEOUT) option, and the ultimate behavior is dictated entirely by the LDAP client library.

LDAPLibraryDebug Directive

Description:	Enable debugging in the LDAP SDK
Syntax:	`LDAPLibraryDebug 7`
Default:	`disabled`
Context:	server config
Status:	Extension
Module:	mod_ldap

Turns on SDK-specific LDAP debug options that generally cause the LDAP SDK to log verbose trace information to the main Apache error log. The trace messages from the LDAP SDK provide gory details that can be useful during debugging of connectivity problems with backend LDAP servers

This option is only configurable when Apache HTTP Server is linked with an LDAP SDK that implements LDAP_OPT_DEBUG or LDAP_OPT_DEBUG_LEVEL, such as OpenLDAP (a value of 7 is verbose) or Tivoli Directory Server (a value of 65535 is verbose).

The logged information will likely contain plaintext credentials being used or validated by LDAP authentication, so care should be taken in protecting and purging the error log when this directive is used.

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.

LDAPReferralHopLimit Directive

Description:	The maximum number of referral hops to chase before terminating an LDAP query.
Syntax:	`LDAPReferralHopLimit number`
Default:	`SDK dependent, typically between 5 and 10`
Context:	directory, .htaccess
Override:	AuthConfig
Status:	Extension
Module:	mod_ldap

This directive, if enabled by the LDAPReferrals directive, limits the number of referral hops that are followed before terminating an LDAP query.

Support for this tunable is uncommon in LDAP SDKs.

LDAPReferrals Directive

Description:	Enable referral chasing during queries to the LDAP server.
Syntax:	`LDAPReferrals On\|Off\|default`
Default:	`LDAPReferrals On`
Context:	directory, .htaccess
Override:	AuthConfig
Status:	Extension
Module:	mod_ldap
Compatibility:	The `default` parameter is available in Apache 2.4.7 and later

Some LDAP servers divide their directory among multiple domains and use referrals to direct a client when a domain boundary is crossed. This is similar to a HTTP redirect. LDAP client libraries may or may not chase referrals by default. This directive explicitly configures the referral chasing in the underlying SDK.

LDAPReferrals takes the following values:

"on": When set to "on", the underlying SDK's referral chasing state is enabled, LDAPReferralHopLimit is used to override the SDK's hop limit, and an LDAP rebind callback is registered.
"off": When set to "off", the underlying SDK's referral chasing state is disabled completely.
"default": When set to "default", the underlying SDK's referral chasing state is not changed, LDAPReferralHopLimit is not used to overide the SDK's hop limit, and no LDAP rebind callback is registered.

The directive LDAPReferralHopLimit works in conjunction with this directive to limit the number of referral hops to follow before terminating the LDAP query. When referral processing is enabled by a value of "On", client credentials will be provided, via a rebind callback, for any LDAP server requiring them.

LDAPRetries Directive

Description:	Configures the number of LDAP server retries.
Syntax:	`LDAPRetries number-of-retries`
Default:	`LDAPRetries 3`
Context:	server config
Status:	Extension
Module:	mod_ldap

The server will retry failed LDAP requests up to LDAPRetries times. Setting this directive to 0 disables retries.

LDAP errors such as timeouts and refused connections are retryable.

LDAPRetryDelay Directive

Description:	Configures the delay between LDAP server retries.
Syntax:	`LDAPRetryDelay seconds`
Default:	`LDAPRetryDelay 0`
Context:	server config
Status:	Extension
Module:	mod_ldap

If LDAPRetryDelay is set to a non-zero value, the server will delay retrying an LDAP request for the specified amount of time. Setting this directive to 0 will result in any retry to occur without delay.

LDAP errors such as timeouts and refused connections are retryable.

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 and every HTTPD process will create its own cache.

LDAPTimeout Directive

Description:	Specifies the timeout for LDAP search and bind operations, in seconds
Syntax:	`LDAPTimeout seconds`
Default:	`LDAPTimeout 60`
Context:	server config
Status:	Extension
Module:	mod_ldap
Compatibility:	Apache HTTP Server 2.3.5 and later

This directive configures the timeout for bind and search operations, as well as the LDAP_OPT_TIMEOUT option in the underlying LDAP client library, when available.

If the timeout expires, httpd will retry in case an existing connection has been silently dropped by a firewall. However, performance will be much better if the firewall is configured to send TCP RST packets instead of silently dropping packets.

Timeouts for ldap compare operations requires an SDK with LDAP_OPT_TIMEOUT, such as OpenLDAP >= 2.4.4.

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:	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:

CA_DER - binary DER encoded CA certificate
CA_BASE64 - PEM encoded CA certificate
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.