Apache Web Server (auth_ldap)

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This section discusses how to enable authentication against a Directory Services using LDAP Module for Apache (auth_ldap).

Introduction

auth_ldap is an LDAP authentication module for Apache, the world's most popular Web server. auth_ldap has excellent performance, and supports Apache on both Unix and Windows NT. It also has support for LDAP over SSL, and a mode that lets Microsoft Frontpage clients manage their Web permissions while still using LDAP for authentication.

Support

In addition to the documentation, support for auth_ldap is available through the auth_ldap mailing list. You can also see old auth_ldap messages in the list archives.

Requirements

To compile auth_ldap, you will need either the Netscape Directory SDK, or the OpenLD AP SDK. For SSL, the Netscape SDK is required. auth_ldap has been tested against all versions of Apache 1.3, and a few of the later versions of Apache 1.2. It runs on Solaris, Linux, and Windows NT.

Documentation

• auth_ldap 1.4 Documentation
  
  
• auth_ldap 1.5 Developers Release Documentation
  
  
• auth_ldap FAQ
  
  
• auth_ldap 1.4 change log
  
  
• auth_ldap 1.5 developers' change log
  
  

Download

• Latest auth_ldap 1.5 Developers Release Source http://www.rudedog.org/auth_ldap/auth_ldap-1.5.0.tar.gz
  
  
• Latest auth_ldap 1.4 Source http://www.rudedog.org/auth_ldap/auth_ldap-1.4.3.tar.gz
  
  
• Other auth_ldap versions http://www.rudedog.org/auth_ldap/all_versions.xml
  
  

This is an authentication module for Apache that allows Apache to authenticate HTTP clients using user entries in an LDAP directory. The current stable version is 1.4.3, while the latest development release is 1.5.0. The latest version and the change log is available at http://www.rudedog.org/auth_ldap/.

auth_ldap supports the following features:

• Support for both Unix and Windows NT. As with Apache, Windows NT support is not as stable as Unix support.
  
  
• Supports the OpenLDAP SDK, the Mozilla Directory SDK, and Netscape's Directory SDK.
  
  
• Complex authorization policies can be implemented by representing the policy with LDAP filters.
  
  
• Support for Microsoft FrontPage allows FrontPage users to control access to their Web sites, while retaining LDAP for user authentication.
  
  
• Uses extensive caching of LDAP operations for increased performance.
  
  
• Support for LDAP over SSL (requires the Netscape SDK).
  
  

Building the Module

The module is compatible with Apache 1.3.x. To use the SSL extensions, you must use the Netscape SDK.

Note:

The Mozilla SDK is not the same as the Netscape SDK, and does not support SSL.

Under Unix, auth_ldap can be built as a Dynamic Shared Object (DSO) using Apache's apxs program. You should be able to build it using the old-style configure method, but I don't personally use that method, so I can't guarantee support for it. The directions in this section apply to building auth_ldap as a DSO.

Note:

A separate makefile is provided for building under Windows NT with Visual C++ 5.0.

1. Edit Makefile or Makefile.NT to customize it for your site. Make sure that you select the appropriate SDK, and turn on or off SSL. Remember, you need the Netscape SDK to use SSL. Run make (Unix) or make -m Makefile.NT (Windows NT), which should compile the DSO.
   
   
2. Under Unix, run make install to install the module with the rest of your Apache modules. Under Windows NT, you will have to copy the DLL by hand into the directory with the rest of your modules.
   
   
3. To load the module, add an appropriate LoadModule directive to your httpd.conf file, such as
   
   

      # For Unix
      LoadModule  auth_ldap_module  libexec/auth_ldap.so
      # For Windows NT
      LoadModule  auth_ldap_module  modules/AuthLDAP.dll

Note:

This module has not been extensively tested under Windows NT. Because Apache on NT is threaded, there are a lot of concurrency issues. I think that I have addressed most of the concurrency issues, although the locks are pretty coarse grained. If you have any experiences, good or bad, with the module under Windows NT, please let me know, so I can improve these instructions and the module. I'm especially open to receiving code improvements for NT.

Operation

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

Authentication Phase.  During the authentication phase, 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 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

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

• Grant access if there is a require valid-user directive.
  
  
• Grant access if there is a require user directive, and the username in the directive matches the username passed by the client.
  
  
• Grant access if there is a require dn directive, and the DN in the directive matches the DN fetched from the LDAP directory.
  
  
• Grant access if there is a require 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.
  
  

auth_ldap uses the following directives during the compare phase:

Auth_ldap Directives

This section contains the complete list of directives that are used by the auth_ldap module. For detailed information about the use of these directives, see: http://httpd.apache.org/docs/mod/directive-dict.html

AuthLDAPAuthoritative

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

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

AuthLDAPBindPassword

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.

AuthLDAPCacheSize

Specifies the maximum size of the primary LDAP cache. This cache contains successful search/binds. Set it to 0 to turn of search/bind caching. The default size is 1024 cached searches. See the section below on caching for complete information on caching LDAP operations in auth_ldap.

AuthLDAPCacheTTL

Specifies the time (in seconds) that an item in the search/bind cache remains valid. The default is 600 seconds (10 minutes). A value of 0 means that items in the cache never go stale.

AuthLDAPCertDBPath

Specifies in which directory auth_ldap should look for the certificate authorities database. There should be a file named cert7.db in that directory.

AuthLDAPCompareDNOnServer

When set, auth_ldap will use the LDAP server to compare the DNs. This is the only foolproof way to compare DNs. 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, auth_ldap simply does a string comparison. It is possible to get false negatives with this approach, but it is much faster. Note the auth_ldap cache can speed up DN comparison in most situations.

AuthLDAPDereferenceAliases

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

AuthLDAPFrontPageHack

See the section on using Using Microsoft FrontPage with auth_ldap.

AuthLDAPGroupAttribute

This directive specifies which LDAP attributes are used to check for group membership. Up to 10 attributes can be used by specifying this directive multiple times. If not specified, then auth_ldap uses the member and uniquemember attributes.

AuthLDAPGroupAttributeIsDN

When set, 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, say that the client send the username bjenson, which corresponds to the LDAP DN cn=Babs Jenson, o=Airius. If this directive is set, auth_ldap will check if the group has cn=Babs Jenson, o=Airius as a member. If this directive is not set, then auth_ldap will check if the group has bjenson as a member.

AuthLDAPOpCacheSize

This specifies the size of the cache auth_ldap uses to cache LDAP compare operations. The default is 1024 entries. Setting it to 0 disables operation caching.

AuthLDAPOpCacheTTL

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

AuthLDAPRemoteUserIsDN

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, which is consistent with the behavior of previous auth_ldap releases.

AuthLDAPUrl

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

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 below for examples of AuthLDAPURL URLs.

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, 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 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 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

The following are ways you can use the require directives:

• Grant access to anyone who exists in the LDAP directory, using their UID for searches.
  
  

   AuthLDAPURL ldap://ldap1.airius.com:389/ou=People,
   o=Airius?uid?sub?(objectClass=*)
   require valid-user

• This next example is the same as above; but with the fields that have useful defaults omitted. Also, note the use of a redundant LDAP server.
  
  

   AuthLDAPURL ldap://ldap1.airius.com ldap2.airius.com/ou=People,
   o=Airius
   require valid-user

• In this example we uses the common name instead of the UID. This could be problematical if multiple people in the directory share the same cn, because a search on cn must return exactly one entry. That's why this approach is not recommended: it's a better idea to choose an attribute that is guaranteed unique in your directory, such as uid.
  
  

   AuthLDAPURL ldap://ldap.airius.com/ou=People, o=Airius?cn
   require valid-user

• Grant access to anybody in the Administrators group. The users must authenticate using their UID.
  
  

   AuthLDAPURL ldap://ldap.airius.com/o=Airius?uid
   require group cn=Administrators, o=Airius

• The next example assumes that everyone at Airius who carries an alphanumeric pager will have an LDAP attribute of qpagePagerID. The example will grant access only to people (authenticated via their UID) who have alphanumeric pagers:
  
  

   AuthLDAPURL ldap://ldap.airius.com/o=Airius?uid??(qpagePagerID=*)
   require valid-user

• The next example demonstrates the power of using filters to accomplish complicated administrative requirements. Without filters, it would have been necessary to create a new LDAP group and ensure that the group's members remain synchronized with the pager users. This becomes trivial with filters. The goal is to grant access to anyone who has a filter, plus grant access to Joe Manager, who doesn't have a pager, but does need to access the same resource:
  
  

   AuthLDAPURL ldap://ldap.airius.com/o=Airius?uid??(|(qpagePagerID=*)(uid=jmanager))
   require valid-user

• This last may look confusing at first, so it helps to evaluate what the search filter will look like based on who connects, as shown below. The text in blue is the part that is filled in using the attribute specified in the URL. The text in red is the part that is filled in using the filter specified in the URL. The text in green is filled in using the information that is retrieved from the HTTP client. If Fred User connects as fuser, the filter would look like
  
  

   (&(|(qpagePagerID=*)(uid=jmanager))(uid=fuser)) 

• The above search will only succeed if fuser has a pager. When Joe Manager connects as jmanager, the filter looks like
  
  

   (&(|(qpagePagerID=*)(uid=jmanager))(uid=jmanager)) 

• The above search will succeed whether jmanager has a pager or not.
  
  

Caching in auth_ldap

For improved performance, auth_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 auth_ldap-protected pages. In addition, the load on the LDAP server will be significantly decreased.

auth_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.

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.

auth_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, auth_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, auth_ldap bypasses the search/bind phase.

The search and bind cache is controlled with the AuthLDAPCacheSize and AuthLDAPCacheTTL directives.

Operation Caches

During the compare phase, auth_ldap uses two operation caches to cache the compare operations that it does. The first compare cache is used to cache the results of compares done for the require user and require group directives. The second compare cache is used to cache the results of compares done for the require dn directive.

The behavior of both of these caches is controlled with the AuthLDAPOpCacheSize and AuthLDAPOpCacheTTL directives.

Monitoring the Cache

auth_ldap has a content handler that allows administrators to monitor the cache performance. The name of the content handler is auth-ldap-info, so the following directives could be used to access the auth_ldap cache information:

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

By fetching the URL http://servername/cache-info, the administrator can get a status report of every cache that is used by auth_ldap cache. Note that if Apache does not support MM-style 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.

Benchmarks

These benchmarks compare the performance for different cache configurations. They list the time to complete the benchmark, and the number of LDAP operations actually performed the LDAP server. All benchmarks were performed with Apache 1.3.4. Apache, the WWW client and the directory server all resided on the same system.

Note:

These benchmarks were performed using auth_ldap 1.4, which used a much different caching strategy, the meaning of some of the benchmarks will not be apparent. The latest version of auth_ldap uses a strategy that should be much more efficient in most situations, so the benchmarks are mostly of use only for historical reasons.

The first benchmark did 10,000 retrievals of the same page. The page was protected by a <Location> specification in access.conf that contained a single require user directive.

The second benchmark did 10,000 retrievals of the same page, but chose a random username from a pool of 1,000 names for each connection. The page was protected by a <Location> specification in access.conf that contained a require valid-user directive. Since this doesn't require a LDAP compare operation, there is no benchmark for the third level of caching (search, bind and compare).

These benchmarks are only included to give an idea about how caching can improve performance. They do not really simulate real world conditions, since both servers and the client were in a sandbox.

Using SSL

auth_ldap will not talk to any SSL server unless that server has a certificate signed by a known Certificate Authority. Once you've built auth_ldap with SSL support, auth_ldap still 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 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.

To use FrontPage with auth_ldap, ensure that auth_ldap is compiled with the -DAUTH_LDAP_FRONTPAGE_HACK. See the Makefile for more information.

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 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

• When choosing the LDAP URL, the attribute to use for authentication should be something that will also be valid for putting into a mod_auth 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 (although not as much confusion as if the entire FrontPage permissions model is broken).
  
  
• Apache must be compiled with mod_auth in order to use FrontPage support. This is because Apache will still use the mod_auth group file for determine the extent of a user's access to the FrontPage Web. Furthermore, auth_ldap must be called before mod_auth. This means that it must be loaded after mod_auth, because authentication modules are run in reverser order of how they were loaded. Order of modules is controlled by the LoadModule and AddModule directives.
  
  
• The directives must be put in the .htaccess files. Attempting to put them inside <Location> or <Directory> directives won't work. This is because auth_ldap has to be able to grab the AuthUserFile directive that is found in FrontPage .htaccess files so that it knows where to look for the valid user list. If the auth_ldap directives aren't in the same .htaccess file as the FrontPage directives, then the hack won't work, because auth_ldap will never get a chance to process the .htaccess file, and won't be able to find the FrontPage-managed user file.
  
  
• FrontPage support is still very experimental. On the whole, FrontPage is a huge fragile steaming pile of goo, and is subject to breakage and other lossage. While I am aware of people who have been using it with no apparent problems, I would not try to do anything fancy with the .htaccess files.
  
  

Acknowledgements

Thanks to: Andrew Cosgriff for suggesting the AuthLDAPRemoteUserIsDN directive. Kevin Hendrix for code and other advice with the caching functionality. Graham Leggett for goading me into doing SSL support as well as assistance with tracking down some thorny Solaris problems, bug reports, patches, and lots of other suggestions for improvements. Greg Muscarella for testing and critiquing of early versions. Edwin Shin for Win32 testing. Camiel Dobbelaar for various Makefile patches. Mikko Ahonen for patches needed to compile on AIX. Karel Zajicek for suggestions and patches for implementing the AuthLDAPMemberIsDN. Tony Lindgren for suggestions and patches for implementing the shared memory caching. Michael Smith for valuable bug reports.

Copyright

Copyright © 1998, 1999, Enbridge Pipelines Inc.
Copyright © 1999, 2000, Dave Carrigan
All rights reserved.

This module is free software; you can redistribute it and/or modify it under the same terms as Apache itself. This module is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose. The copyright holder of this module can not be held liable for any general, special, incidental or consequential damages arising out of the use of the module.