[APACHE DOCUMENTATION]

Apache HTTP Server Version 2.0

Apache Module mod_auth_ldap

Description: Allows an LDAP directory to be used to store the database for HTTP Basic authentication.
Status: experimental
Module Identifier: auth_ldap_module
Compatibility: Available in version 2.0.41 and later

Summary

mod_auth_ldap supports the following features:

Directives

See also

Contents

Operation

There are two phases in granting access to a user. The first phase is authentication, in which mod_auth_ldap verifies that the user's credentials are valid. This also called the search/bind phase. The second phase is authorization, in which mod_auth_ldap determines if the authenticated user is allowed access to the resource in question. This is also known as the compare phase.

The Authentication Phase

During the authentication phase, mod_auth_ldap searches for an entry in the directory that matches the username that the HTTP client passes. If a single unique match is found, then mod_auth_ldap attempts to bind to the directory server using the DN of the entry plus the password provided by the HTTP client. Because it does a search, then a bind, it is often referred to as the search/bind phase. Here are the steps taken during the search/bind phase.

  1. Generate a search filter by combining the attribute and filter provided in the AuthLDAPURL directive with the username passed by the HTTP client.
  2. Search the directory using the generated filter. If the search does not return exactly one entry, deny or decline access.
  3. Fetch the distinguished name of the entry retrieved from the search and attempt to bind to the LDAP server using the DN and the password passed by the HTTP client. If the bind is unsuccessful, deny or decline access.

The following directives are used during the search/bind phase

AuthLDAPURL Specifies the LDAP server, the base DN, the attribute to use in the search, as well as the extra search filter to use.
AuthLDAPBindDN An optional DN to bind with during the search phase.
AuthLDAPBindPassword An optional password to bind with during the search phase.

The Authorization Phase

During the authorization phase, mod_auth_ldap attempts to determine if the user is authorized to access the resource. Many of these checks require mod_auth_ldap to do a compare operation on the LDAP server. This is why this phase is often referred to as the compare phase. mod_auth_ldap accepts the following Require directives to determine if the credentials are acceptable:

mod_auth_ldap uses the following directives during the compare phase:

AuthLDAPURL The attribute specified in the URL is used in compare operations for the require user operation.
AuthLDAPCompareDNOnServer Determines the behavior of the require dn directive.
AuthLDAPGroupAttribute Determines the attribute to use for comparisons in the require group directive.
AuthLDAPGroupAttributeIsDN Specifies whether to use the user DN or the username when doing comparisons for the require group directive.

The require Directives

Apache's Require directives are used during the authorization phase to ensure that a user is allowed to access a resource.

require valid-user

If this directive exists, mod_auth_ldap grants access to any user that has successfully authenticated during the search/bind phase.

require user

The require user directive specifies what usernames can access the resource. Once mod_auth_ldap has retrieved a unique DN from the directory, it does an LDAP compare operation using the username specified in the require user to see if that username is part of the just-fetched LDAP entry. Multiple users can be granted access by putting multiple usernames on the line, separated with spaces. If a username has a space in it, then it must be the only user on the line. In this case, multiple users can be granted access by using multiple require user directives, with one user per line. For example, with a AuthLDAPURL of ldap://ldap/o=Airius?cn (i.e., cn is used for searches), the following require directives could be used to restrict access:

require user Barbara Jenson
require user Fred User
require user Joe Manager

Because of the way that mod_auth_ldap handles this directive, Barbara Jenson could sign on as Barbara Jenson, Babs Jenson or any other cn that she has in her LDAP entry. Only the single require user line is needed to support all values of the attribute in the user's entry.

If the uid attribute was used instead of the cn attribute in the URL above, the above three lines could be condensed to

require user bjenson fuser jmanager

require group

This directive specifies an LDAP group whose members are allowed access. It takes the distinguished name of the LDAP group. For example, assume that the following entry existed in the LDAP directory:

dn: cn=Administrators, o=Airius
objectClass: groupOfUniqueNames
uniqueMember: cn=Barbara Jenson, o=Airius
uniqueMember: cn=Fred User, o=Airius

The following directive would grant access to both Fred and Barbara:

require group cn=Administrators, o=Airius

Behavior of this directive is modified by the AuthLDAPGroupAttribute and AuthLDAPGroupAttributeIsDN directives.

require dn

The require dn directive allows the administrator to grant access based on distinguished names. It specifies a DN that must match for access to be granted. If the distinguished name that was retrieved from the directory server matches the distinguished name in the require dn, then authorization is granted.

The following directive would grant access to a specific DN:

require dn cn=Barbara Jenson, o=Airius

Behavior of this directive is modified by the AuthLDAPCompareDNOnServer directive.

Examples

Using TLS

To use TLS, simply set the AuthLDAPStartTLS to on. Nothing else needs to be done (other than ensure that your LDAP server is configured for TLS).

Using SSL

If mod_auth_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_auth_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.

To specify a secure LDAP server, use ldaps:// in the AuthLDAPURL directive, instead of ldap://.

Using Microsoft FrontPage with mod_auth_ldap

Normally, FrontPage uses FrontPage-web-specific user/group files (i.e., the mod_auth module) to handle all authentication. Unfortunately, it is not possible to just change to LDAP authentication by adding the proper directives, because it will break the Permissions forms in the FrontPage client, which attempt to modify the standard text-based authorization files.

Once a FrontPage web has been created, adding LDAP authentication to it is a matter of adding the following directives to every .htaccess file that gets created in the web

AuthLDAPURL            the url
AuthLDAPAuthoritative  off
AuthLDAPFrontPageHack  on

AuthLDAPAuthoritative must be off to allow mod_auth_ldap to decline group authentication so that Apache will fall back to file authentication for checking group membership. This allows the FrontPage-managed group file to be used.

How It Works

FrontPage restricts access to a web by adding the require valid-user directive to the .htaccess files. If AuthLDAPFrontPageHack is not on, the require valid-user directive will succeed for any user who is valid as far as LDAP is concerned. This means that anybody who has an entry in the LDAP directory is considered a valid user, whereas FrontPage considers only those people in the local user file to be valid. The purpose of the hack is to force Apache to consult the local user file (which is managed by FrontPage) - instead of LDAP - when handling the require valid-user directive.

Once directives have been added as specified above, FrontPage users will be able to perform all management operations from the FrontPage client.

Caveats

AuthLDAPAuthoritative Directive

Description:
Syntax: AuthLDAPAuthoritative on|off
Default: AuthLDAPAuthoritative on
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

Set to off if this module should let other authentication modules attempt to authenticate the user, should authentication with this module fail. Control is only passed on to lower modules if there is no DN or rule that matches the supplied user name (as passed by the client).

AuthLDAPBindDN Directive

Description:
Syntax: AuthLDAPBindDN distinguished-name
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

An optional DN used to bind to the server when searching for entries. If not provided, mod_auth_ldap will use an anonymous bind.

AuthLDAPBindPassword Directive

Description:
Syntax: AuthLDAPBindPassword password
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

A bind password to use in conjunction with the bind DN. Note that the bind password is probably sensitive data, and should be properly protected. You should only use the AuthLDAPBindDN and AuthLDAPBindPassword if you absolutely need them to search the directory.

AuthLDAPCompareDNOnServer Directive

Description:
Syntax: AuthLDAPCompareDNOnServer on|off
Default: AuthLDAPCompareDNOnServer on
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

When set, mod_auth_ldap will use the LDAP server to compare the DNs. This is the only foolproof way to compare DNs. mod_auth_ldap will search the directory for the DN specified with the require dn directive, then, retrieve the DN and compare it with the DN retrieved from the user entry. If this directive is not set, mod_auth_ldap simply does a string comparison. It is possible to get false negatives with this approach, but it is much faster. Note the mod_ldap cache can speed up DN comparison in most situations.

AuthLDAPDereferenceAliases Directive

Description:
Syntax: AuthLDAPDereferenceAliases never|searching|finding|always
Default: AuthLDAPDereferenceAliases Always
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

This directive specifies when mod_auth_ldap will de-reference aliases during LDAP operations. The default is always.

AuthLDAPEnabled Directive

Description:
Syntax: AuthLDAPEnabled on|off
Default: AuthLDAPEnabled on
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

Set to off to disable mod_auth_ldap in certain directories. This is useful if you have mod_auth_ldap enabled at or near the top of your tree, but want to disable it completely in certain locations.

AuthLDAPFrontPageHack Directive

Description:
Syntax: AuthLDAPFrontPageHack on|off
Default: AuthLDAPFronPageHack off
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

See the section on using Microsoft FrontPage with mod_auth_ldap.

AuthLDAPGroupAttribute Directive

Description:
Syntax: AuthLDAPGroupAttribute attribute
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

This directive specifies which LDAP attributes are used to check for group membership. Multiple attributes can be used by specifying this directive multiple times. If not specified, then mod_auth_ldap uses the member and uniquemember attributes.

AuthLDAPGroupAttributeIsDN Directive

Description:
Syntax: AuthLDAPGroupAttributeIsDN on|off
Default: AuthLDAPGroupAttributeIsDN on
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

When set on, this directive says to use the distinguished name of the client username when checking for group membership. Otherwise, the username will be used. For example, assume that the client sent the username bjenson, which corresponds to the LDAP DN cn=Babs Jenson, o=Airius. If this directive is set, mod_auth_ldap will check if the group has cn=Babs Jenson, o=Airius as a member. If this directive is not set, then mod_auth_ldap will check if the group has bjenson as a member.

AuthLDAPRemoteUserIsDN Directive

Description:
Syntax: AuthLDAPRemoteUserIsDN on|off
Default: AuthLDAPUserIsDN off
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

If this directive is set to on, the value of the REMOTE_USER environment variable will be set to the full distinguished name of the authenticated user, rather than just the username that was passed by the client. It is turned off by default.

AuthLDAPStartTLS directive Directive

Description:
Syntax: AuthLDAPStartTLS on|off
Default: AuthLDAPStartTLS off
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

If this directive is set to on, mod_auth_ldap will start a secure TLS session after connecting to the LDAP server. This requires your LDAP server to support TLS.

AuthLDAPUrl Directive

Description:
Syntax: AuthLDAPUrl url
Context: directory, .htaccess
Override: AuthConfig
Status: experimental
Module: mod_auth_ldap

An RFC 2255 URL which specifies the LDAP search parameters to use. The syntax of the URL is

ldap://host:port/basedn?attribute?scope?filter
ldap
For regular ldap, use the string ldap. For secure LDAP, use ldaps instead. Secure LDAP is only available if Apache was linked to an LDAP library with SSL support.
host:port

The name/port of the ldap server (defaults to localhost:389 for ldap, and localhost:636 for ldaps). To specify multiple, redundant LDAP servers, just list all servers, separated by spaces. mod_auth_ldap will try connecting to each server in turn, until it makes a successful connection.

Once a connection has been made to a server, that connection remains active for the life of the httpd process, or until the LDAP server goes down.

If the LDAP server goes down and breaks an existing connection, mod_auth_ldap will attempt to re-connect, starting with the primary server, and trying each redundant server in turn. Note that this is different than a true round-robin search.

basedn
The DN of the branch of the directory where all searches should start from. At the very least, this must be the top of your directory tree, but could also specify a subtree in the directory.
attribute
The attribute to search for. Although RFC 2255 allows a comma-separated list of attributes, only the first attribute will be used, no matter how many are provided. If no attributes are provided, the default is to use uid. It's a good idea to choose an attribute that will be unique across all entries in the subtree you will be using.
scope
The scope of the search. Can be either one or sub. Note that a scope of base is also supported by RFC 2255, but is not supported by this module. If the scope is not provided, or if base scope is specified, the default is to use a scope of sub.
filter
A valid LDAP search filter. If not provided, defaults to (objectClass=*), which will search for all objects in the tree. Filters are limited to approximately 8000 characters (the definition of MAX_STRING_LEN in the Apache source code). This should be than sufficient for any application.

When doing searches, the attribute, filter and username passed by the HTTP client are combined to create a search filter that looks like (&(filter)(attribute=username)).

For example, consider an URL of ldap://ldap.airius.com/o=Airius?cn?sub?(posixid=*). When a client attempts to connect using a username of Babs Jenson, the resulting search filter will be (&(posixid=*)(cn=Babs Jenson)).

See above for examples of AuthLDAPURL URLs.

Apache HTTP Server Version 2.0

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