Apache Module mod_authnz_ldap

Contents

• Operation
  • The Authentication Phase
  • The Authorization Phase
• The Require Directives
  • Require valid-user
  • Require ldap-user
  • Require ldap-group
  • Require ldap-dn
  • Require ldap-attribute
  • Require ldap-filter
• Examples
• Using TLS
• Using SSL
• Exposing Login Information
• Using Microsoft FrontPage with mod_authnz_ldap
  • How It Works
  • Caveats

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 authz_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

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 or, potentially, in one of its sub-groups.
• Grant access if there is a Require ldap-attribute directive, and the attribute fetched from the LDAP directory matches the given value.
• Grant access if there is a Require ldap-filter directive, and the search filter successfully finds a single user object that matches the dn of the authenticated user.
• otherwise, deny or decline access

Other Require values may also be used which may require loading additional authorization modules. Note that if you use a Require value from another authorization module, you will need to ensure that AuthzLDAPAuthoritative is set to off to allow the authorization phase to fall back to the module providing the alternate Require value.

• 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:

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, ldap-group, ldap-attribute and ldap-filter. 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 AuthzLDAPAuthoritative 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:

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-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:

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

Members can also be found within sub-groups of a specified LDAP group if AuthLDAPMaxSubGroupDepth is set to a value greater than 0. For example, assume the following entries exist in the LDAP directory:

The following directives would allow access for Bob Ellis, Tom Jackson, Barbara Jensen, Fred User, Allan Jefferson, and Paul Tilley but would not allow access for Jim Swenson, or Elliot Rhodes (since they are at a sub-group depth of 2):

Behavior of this directive is modified by the AuthLDAPGroupAttribute, AuthLDAPGroupAttributeIsDN, AuthLDAPMaxSubGroupDepth, AuthLDAPSubGroupAttribute, and AuthLDAPSubGroupClass 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:

Behavior of this directive is modified by the AuthLDAPCompareDNOnServer directive.

Require ldap-attribute

The Require ldap-attribute directive allows the administrator to grant access based on attributes of the authenticated user in the LDAP directory. If the attribute in the directory matches the value given in the configuration, access is granted.

The following directive would grant access to anyone with the attribute employeeType = active

Multiple attribute/value pairs can be specified on the same line separated by spaces or they can be specified in multiple Require ldap-attribute directives. The effect of listing multiple attribute/values pairs is an OR operation. Access will be granted if any of the listed attribute values match the value of the corresponding attribute in the user object. If the value of the attribute contains a space, only the value must be within double quotes.

The following directive would grant access to anyone with the city attribute equal to "San Jose" or status equal to "Active"

Require ldap-filter

The Require ldap-filter directive allows the administrator to grant access based on a complex LDAP search filter. If the dn returned by the filter search matches the authenticated user dn, access is granted.

The following directive would grant access to anyone having a cell phone and is in the marketing department

The difference between the Require ldap-filter directive and the Require ldap-attribute directive is that ldap-filter performs a search operation on the LDAP directory using the specified search filter rather than a simple attribute comparison. If a simple attribute comparison is all that is required, the comparison operation performed by ldap-attribute will be faster than the search operation used by ldap-filter especially within a large directory.

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 it 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 ldap-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 pager, 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. 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 SSL

To use SSL, see the mod_ldap directives LDAPTrustedClientCert, LDAPTrustedGlobalCert and LDAPTrustedMode.

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

Exposing Login Information

Whenever a query is made to the LDAP server, all LDAP attributes returned by the query are placed in the environment, using environment variables with the prefix "AUTHENTICATE_".

If an LDAP query for example returned the username, common name and telephone number of a user, a CGI program will have access to this information without the need to make a second independent LDAP query to gather this additional information.

This has the potential to dramatically simplify the coding and configuration required in some web applications.

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"
AuthzLDAPAuthoritative off
AuthGroupFile mygroupfile
Require group mygroupfile

AuthzLDAPAuthoritative 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.