mod_authnz_ldap Allows an LDAP directory to be used to store the database for HTTP Basic authentication. Extension mod_authnz_ldap.c authnz_ldap_module Available in version 2.1 and later

This module provides authentication front-ends such as mod_auth_basic to authenticate users through an ldap directory.

mod_authnz_ldap supports the following features:

When using mod_auth_basic, this module is invoked via the AuthBasicProvider directive with the ldap value.

 mod_ldap mod_auth_basic mod_authz_user mod_authz_groupfile
Contents
Operation

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

mod_authnz_ldap registers both an authn_ldap authentication provider and an anthz_ldap authorization handler. The authn_ldap authentication provider can be enabled through the AuthBasicProvider directive using the ldap value. The authz_ldap handler extends the Require directive's authorization types by adding ldap-user, ldap-dn and ldap-group values.

The Authentication Phase

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

  • Grant access if there is a require ldap-user directive, and the username in the directive matches the username passed by the client.
  • Grant access if there is a require ldap-dn directive, and the DN in the directive matches the DN fetched from the LDAP directory.
  • Grant access if there is a require ldap-group directive, and the DN fetched from the LDAP directory (or the username passed by the client) occurs in the LDAP group.
  • otherwise, deny or decline access

Other Require values may also be used which may require loading additional authorization modules.

  • Grant access if there is a require valid-user directive. (requires mod_authz_user)
  • Grant access if there is a require group directive, and mod_authz_groupfile has been loaded with the AuthGroupFile directive set.
  • others...

mod_authnz_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. mod_authnz_ldap extends the authorization types with ldap-user, ldap-dn and ldap-group. Other authorization types may also be used but may require that additional authorization modules be loaded.

require valid-user

If this directive exists, mod_authnz_ldap grants access to any user that has successfully authenticated during the search/bind phase. Requires that mod_authz_user be loaded and that the AuthLDAPAuthoritative directive be set to off.

require ldap-user

The require ldap-user directive specifies what usernames can access the resource. Once mod_authnz_ldap has retrieved a unique DN from the directory, it does an LDAP compare operation using the username specified in the require ldap-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 surrounded with double quotes. Multiple users can also be granted access by using multiple require ldap-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 ldap-user "Barbara Jenson"
require ldap-user "Fred User"
require ldap-user "Joe Manager"

Because of the way that mod_authnz_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 ldap-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 ldap-user bjenson fuser jmanager
require ldap-group

This directive specifies an LDAP group whose members are allowed access. It takes the distinguished name of the LDAP group. Note: Do not surround the group name with quotes. 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 ldap-group cn=Administrators, o=Airius

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

require ldap-dn

The require ldap-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 ldap-dn, then authorization is granted. Note: do not surround the distinguished name with quotes.

The following directive would grant access to a specific DN:

require ldap-dn cn=Barbara Jenson, o=Airius

Behavior of this directive is modified by the AuthLDAPCompareDNOnServer directive.

Examples
Using TLS

To use TLS, see the mod_ldap directives LDAPTrustedCA and LDAPTrustedCAType.

Using SSL

To use SSL, see the mod_ldap directives LDAPTrustedCA and LDAPTrustedCAType.

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

Using Microsoft FrontPage with mod_authnz_ldap

Normally, FrontPage uses FrontPage-web-specific user/group files (i.e., the mod_authn_file and mod_authz_groupfile modules) 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
AuthGroupFile mygroupfile
require group mygroupfile

AuthLDAPAuthoritative must be off to allow mod_authnz_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. 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. By substituting the ldap-group with group file authorization, Apache is allowed to consult the local user file (which is managed by FrontPage) - instead of LDAP - when handling authorizing the user.

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

Caveats
  • When choosing the LDAP URL, the attribute to use for authentication should be something that will also be valid for putting into a mod_authn_file user file. The user ID is ideal for this.
  • When adding users via FrontPage, FrontPage administrators should choose usernames that already exist in the LDAP directory (for obvious reasons). Also, the password that the administrator enters into the form is ignored, since Apache will actually be authenticating against the password in the LDAP database, and not against the password in the local user file. This could cause confusion for web administrators.
  • Apache must be compiled with mod_auth_basic, mod_authn_file and mod_authz_groupfile in order to use FrontPage support. This is because Apache will still use the mod_authz_groupfile group file for determine the extent of a user's access to the FrontPage web.
  • The directives must be put in the .htaccess files. Attempting to put them inside Location or Directory directives won't work. This is because mod_authnz_ldap has to be able to grab the AuthGroupFile directive that is found in FrontPage .htaccess files so that it knows where to look for the valid user list. If the mod_authnz_ldap directives aren't in the same .htaccess file as the FrontPage directives, then the hack won't work, because mod_authnz_ldap will never get a chance to process the .htaccess file, and won't be able to find the FrontPage-managed user file.
AuthLDAPAuthoritative Prevent other authentication modules from authenticating the user if this one fails AuthLDAPAuthoritative on|off AuthLDAPAuthoritative on directory.htaccess AuthConfig

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 Optional DN to use in binding to the LDAP server AuthLDAPBindDN distinguished-name directory.htaccess AuthConfig

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

 AuthLDAPBindPassword Password used in conjuction with the bind DN AuthLDAPBindPassword password directory.htaccess AuthConfig

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.

 AuthLDAPCharsetConfig Language to charset conversion configuration file AuthLDAPCharsetConfig file-path server config

The AuthLDAPCharsetConfig directive sets the location of the language to charset conversion configuration file. File-path is relative to the ServerRoot. This file specifies the list of language extensions to character sets. Most administrators use the provided charset.conv file, which associates common language extensions to character sets.

The file contains lines in the following format:

Language-Extension charset [Language-String] ...

The case of the extension does not matter. Blank lines, and lines beginning with a hash character (#) are ignored.

 AuthLDAPCompareDNOnServer Use the LDAP server to compare the DNs AuthLDAPCompareDNOnServer on|off AuthLDAPCompareDNOnServer on directory.htaccess AuthConfig

When set, mod_authnz_ldap will use the LDAP server to compare the DNs. This is the only foolproof way to compare DNs. mod_authnz_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_authnz_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 When will the module de-reference aliases AuthLDAPDereferenceAliases never|searching|finding|always AuthLDAPDereferenceAliases Always directory.htaccess AuthConfig

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

 AuthLDAPGroupAttribute LDAP attributes used to check for group membership AuthLDAPGroupAttribute attribute directory.htaccess AuthConfig

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_authnz_ldap uses the member and uniquemember attributes.

 AuthLDAPGroupAttributeIsDN Use the DN of the client username when checking for group membership AuthLDAPGroupAttributeIsDN on|off AuthLDAPGroupAttributeIsDN on directory.htaccess AuthConfig

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_authnz_ldap will check if the group has cn=Babs Jenson, o=Airius as a member. If this directive is not set, then mod_authnz_ldap will check if the group has bjenson as a member.

 AuthLDAPRemoteUserIsDN Use the DN of the client username to set the REMOTE_USER environment variable AuthLDAPRemoteUserIsDN on|off AuthLDAPRemoteUserIsDN off directory.htaccess AuthConfig

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.

 AuthLDAPUrl URL specifying the LDAP search parameters AuthLDAPUrl url directory.htaccess AuthConfig

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_authnz_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_authnz_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.