Apache Module mod_auth_ldap

Contents

• Operation
  • The Authentication Phase
  • The Authorization Phase
• The require Directives
  • require valid-user
  • require user
  • require group
  • require dn
• Examples
• Using TLS
• Using SSL
• Using Microsoft FrontPage with mod_auth_ldap
  • How It Works
  • Caveats

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

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:

• 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

mod_auth_ldap uses the following directives during the compare phase:

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:

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 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:

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

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:

Behavior of this directive is modified by the AuthLDAPCompareDNOnServer directive.

Examples

• 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

• The 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

• The next example is similar to the previous one, but is uses the common name instead of the UID. Note that 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.

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

• 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.
• 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.
• 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_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 mod_auth_ldap directives aren't in the same .htaccess file as the FrontPage directives, then the hack won't work, because mod_auth_ldap will never get a chance to process the .htaccess file, and won't be able to find the FrontPage-managed user file.