6.9.7. About Authorization and Group Loading with LDAP

An LDAP directory contains entries for user accounts and groups, cross referenced by attributes. Depending on the LDAP server configuration, a user entity may map the groups the user belongs to through memberOf attributes; a group entity may map which users belong to it through uniqueMember attributes; or both mappings may be maintained by the LDAP server.
Users generally authenticate against the server using a simple user name. When searching for group membership information, depending on the directory server in use, searches could be performed using this simple name or using the distinguished name of the user's entry in the directory.
The authentication step of a user connecting to the server always happens first. Once the user is successfully authenticated the server loads the user's groups. The authentication step and the authorization step each require a connection to the LDAP server. The realm optimizes this process by reusing the authentication connection for the group loading step. As will be shown within the configuration steps below it is possible to define rules within the authorization section to convert a user's simple user name to their distinguished name. The result of a "user name to distinguished name mapping" search during authentication is cached and reused during the authorization query when the force attribute is set to "false". When force is true, the search is performed again during authorization (while loading groups). This is typically done when different servers perform authentication and authorization. 
<authorization>
    <ldap connection="...">
    	<!-- OPTIONAL -->
       <username-to-dn force="true"> 
           <!-- Only one of the following. -->
           <username-is-dn />
           <username-filter base-dn="..." recursive="..." user-dn-attribute="..." attribute="..." />
           <advanced-filter base-dn="..." recursive="..." user-dn-attribute="..." filter="..." />
        </username-to-dn>
        
       <group-search group-name="..." iterative="..." group-dn-attribute="..." group-name-attribute="..." >
           <!-- One of the following -->
           <group-to-principal base-dn="..." recursive="..." search-by="...">
               <membership-filter principal-attribute="..." />
           </group-to-principal>
           <principal-to-group group-attribute="..." />
       </group-search>
    </ldap>
</authorization>

Important

These examples specify some attributes with their default values. This is done for demonstration. Attributes that specify their default values are removed from the configuration when it is persisted by the server. The exception is the force attribute. It is required, even when set to the default value of false.

username-to-dn

The username-to-dn element specifies how to map the user name to the distinguished name of their entry in the LDAP directory. This element is only required when both of the following are true:
  • The authentication and authorization steps are against different LDAP servers.
  • The group search uses the distinguished name.
1:1 username-to-dn

This specifies that the user name entered by the remote user is the user's distinguished name. 
<username-to-dn force="false">
   <username-is-dn />
</username-to-dn>

This defines a 1:1 mapping and there is no additional configuration.
username-filter

The next option is very similar to the simple option described above for the authentication step. A specified attribute is searched for a match against the supplied user name. 
<username-to-dn force="true">
    <username-filter base-dn="dc=people,dc=harold,dc=example,dc=com" recursive="false" attribute="sn" user-dn-attribute="dn" />
</username-to-dn>

The attributes that can be set here are:

  • base-dn: The distinguished name of the context to begin the search.

  • recursive: Whether the search will extend to sub contexts. Defaults to false.

  • attribute: The attribute of the users entry to try and match against the supplied user name. Defaults to uid.

  • user-dn-attribute: The attribute to read to obtain the users distinguished name. Defaults to dn.
advanced-filter

The final option is to specify an advanced filter, as in the authentication section this is an opportunity to use a custom filter to locate the users distinguished name. 
<username-to-dn force="true">
    <advanced-filter base-dn="dc=people,dc=harold,dc=example,dc=com" recursive="false" filter="sAMAccountName={0}" user-dn-attribute="dn" />
</username-to-dn>

For the attributes that match those in the username-filter example, the meaning and default values are the same. There is one new attribute:

  • filter: Custom filter used to search for a user's entry where the user name will be substituted in the {0} place holder.

Important

The XML must remain valid after the filter is defined so if any special characters are used such as & ensure the proper form is used. For example &amp; for the & character.

The Group Search

There are two different styles that can be used when searching for group membership information. The first style is where the user's entry contains an attribute that references the groups the user is a member of. The second style is where the group contains an attribute referencing the users entry.

When there is a choice of which style to use Red Hat recommends that the configuration for a user's entry referencing the group is used. This is because with this method group information can be loaded by reading attributes of known distinguished names without having to perform any searches. The other approach requires extensive searches to identify the groups that reference the user.

Before describing the configuration here are some LDIF examples to illustrate this.

Example 6.1. Principal to Group - LDIF example.

This example illustrates where we have a user TestUserOne who is a member of GroupOne, GroupOne is in turn a member of GroupFive. The group membership is shown by the use of a memberOf attribute which is set to the distinguished name of the group of which the user (or group) is a member.

It is not shown here but a user could potentially have multiple memberOf attributes set, one for each group of which the user is directly a member. 
dn: uid=TestUserOne,ou=users,dc=principal-to-group,dc=example,dc=org
objectClass: extensibleObject
objectClass: top
objectClass: groupMember
objectClass: inetOrgPerson
objectClass: uidObject
objectClass: person
objectClass: organizationalPerson
cn: Test User One
sn: Test User One
uid: TestUserOne
distinguishedName: uid=TestUserOne,ou=users,dc=principal-to-group,dc=example,dc=org
memberOf: uid=GroupOne,ou=groups,dc=principal-to-group,dc=example,dc=org
memberOf: uid=Slashy/Group,ou=groups,dc=principal-to-group,dc=example,dc=org
userPassword:: e1NTSEF9WFpURzhLVjc4WVZBQUJNbEI3Ym96UVAva0RTNlFNWUpLOTdTMUE9PQ==

dn: uid=GroupOne,ou=groups,dc=principal-to-group,dc=example,dc=org
objectClass: extensibleObject
objectClass: top
objectClass: groupMember
objectClass: group
objectClass: uidObject
uid: GroupOne
distinguishedName: uid=GroupOne,ou=groups,dc=principal-to-group,dc=example,dc=org
memberOf: uid=GroupFive,ou=subgroups,ou=groups,dc=principal-to-group,dc=example,dc=org

dn: uid=GroupFive,ou=subgroups,ou=groups,dc=principal-to-group,dc=example,dc=org
objectClass: extensibleObject
objectClass: top
objectClass: groupMember
objectClass: group
objectClass: uidObject
uid: GroupFive
distinguishedName: uid=GroupFive,ou=subgroups,ou=groups,dc=principal-to-group,dc=example,dc=org

Example 6.2. Group to Principal - LDIF Example

This example shows the same user TestUserOne who is a member of GroupOne which is in turn a member of GroupFive - however in this case it is an attribute uniqueMember from the group to the user being used for the cross reference.

Again the attribute used for the group membership cross reference can be repeated, if you look at GroupFive there is also a reference to another user TestUserFive which is not shown here. 
dn: uid=TestUserOne,ou=users,dc=group-to-principal,dc=example,dc=org
objectClass: top
objectClass: inetOrgPerson
objectClass: uidObject
objectClass: person
objectClass: organizationalPerson
cn: Test User One
sn: Test User One
uid: TestUserOne
userPassword:: e1NTSEF9SjR0OTRDR1ltaHc1VVZQOEJvbXhUYjl1dkFVd1lQTmRLSEdzaWc9PQ==

dn: uid=GroupOne,ou=groups,dc=group-to-principal,dc=example,dc=org
objectClass: top
objectClass: groupOfUniqueNames
objectClass: uidObject
cn: Group One
uid: GroupOne
uniqueMember: uid=TestUserOne,ou=users,dc=group-to-principal,dc=example,dc=org

dn: uid=GroupFive,ou=subgroups,ou=groups,dc=group-to-principal,dc=example,dc=org
objectClass: top
objectClass: groupOfUniqueNames
objectClass: uidObject
cn: Group Five
uid: GroupFive
uniqueMember: uid=TestUserFive,ou=users,dc=group-to-principal,dc=example,dc=org
uniqueMember: uid=GroupOne,ou=groups,dc=group-to-principal,dc=example,dc=org

General Group Searching

Before looking at the examples for the two approaches shown above we first need to define the attributes common to both of these. 
<group-search group-name="..." iterative="..." group-dn-attribute="..." group-name-attribute="..." >
    ...
</group-search>

  • group-name: This attribute is used to specify the form that should be used for the group name returned as the list of groups of which the user is a member. This can either be the simple form of the group name or the group's distinguished name. If the distinguished name is required this attribute can be set to DISTINGUISHED_NAME. Defaults to SIMPLE.

  • iterative: This attribute is used to indicate if, after identifying the groups a user is a member of, we should also iteratively search based on the groups to identify which groups the groups are a member of. If iterative searching is enabled we keep going until either we reach a group that is not a member if any other groups or a cycle is detected. Defaults to false.

Cyclic group membership is not a problem. A record of each search is kept to prevent groups that have already been searched from being searched again.

Important

For iterative searching to work the group entries need to look the same as user entries. The same approach used to identify the groups a user is a member of is then used to identify the groups of which the group is a member. This would not be possible if for group to group membership the name of the attribute used for the cross reference changes or if the direction of the reference changes.

  • group-dn-attribute: On an entry for a group which attribute is its distinguished name. Defaults to dn.

  • group-name-attribute: On an entry for a group which attribute is its simple name. Defaults to uid.

Example 6.3. Principal to Group Example Configuration

Based on the example LDIF from above here is an example configuration iteratively loading a user's groups where the attribute used to cross reference is the memberOf attribute on the user. 
<authorization>
    <ldap connection="LocalLdap">
        <username-to-dn>
            <username-filter base-dn="ou=users,dc=principal-to-group,dc=example,dc=org" recursive="false" attribute="uid" user-dn-attribute="dn" />
        </username-to-dn>
        <group-search group-name="SIMPLE" iterative="true" group-dn-attribute="dn" group-name-attribute="uid">
            <principal-to-group group-attribute="memberOf" />
        </group-search>
    </ldap>
</authorization>

The most important aspect of this configuration is that the principal-to-group element has been added with a single attribute.

  • group-attribute: The name of the attribute on the user entry that matches the distinguished name of the group the user is a member of. Defaults to memberOf.

Example 6.4. Group to Principal Example Configuration

This example shows an iterative search for the group to principal LDIF example shown above. 
<authorization>
      <ldap connection="LocalLdap">
          <username-to-dn>
              <username-filter base-dn="ou=users,dc=group-to-principal,dc=example,dc=org" recursive="false" attribute="uid" user-dn-attribute="dn" />
          </username-to-dn>
          <group-search group-name="SIMPLE" iterative="true" group-dn-attribute="dn" group-name-attribute="uid">
              <group-to-principal base-dn="ou=groups,dc=group-to-principal,dc=example,dc=org" recursive="true" search-by="DISTINGUISHED_NAME">
                  <membership-filter principal-attribute="uniqueMember" />
              </group-to-principal>
          </group-search>
      </ldap>
  </authorization>

Here an element group-to-principal is added. This element is used to define how searches for groups that reference the user entry will be performed. The following attributes are set:

  • base-dn: The distinguished name of the context to use to begin the search.

  • recursive: Whether sub-contexts also be searched. Defaults to false.

  • search-by: The form of the role name used in searches. Valid values are SIMPLE and DISTINGUISHED_NAME. Defaults to DISTINGUISHED_NAME.

Within the group-to-principal element there is a membership-filter element to define the cross reference.

  • principal-attribute: The name of the attribute on the group entry that references the user entry. Defaults to member.
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