Configuring Kerberos with OpenLDAP back-end

See also

LDAP backend on Ubuntu 10.4 (lucid)

1. Set up SSL on the OpenLDAP server and client to ensure secure communication when the KDC service and LDAP server are on different machines. ldapi\:// can be used if the LDAP server and KDC service are running on the same machine.

   1. Setting up SSL on the OpenLDAP server:

      1. Get a CA certificate using OpenSSL tools

      2. Configure OpenLDAP server for using SSL/TLS

         For the latter, you need to specify the location of CA certificate location in slapd.conf file.

         Refer to the following link for more information: http://www.openldap.org/doc/admin23/tls.html

   2. Setting up SSL on OpenLDAP Client:

      1. For the KDC and Admin Server, you need to do the client-side configuration in ldap.conf. For example:

         TLS_CACERT /etc/openldap/certs/cacert.pem
         

2. Include the Kerberos schema file (kerberos.schema) in the configuration file (slapd.conf) on the LDAP Server, by providing the location where it is stored:

   include /etc/openldap/schema/kerberos.schema
   

3. Choose DNs for the krb5kdc and kadmind servers to bind to the LDAP server, and create them if necessary. These DNs will be specified with the ldap_kdc_dn and ldap_kadmind_dn directives in krb5.conf; their passwords can be stashed with “kdb5_ldap_util stashsrvpw” and the resulting file specified with the ldap_service_password_file directive.

4. Choose a DN for the global Kerberos container entry (but do not create the entry at this time). This DN will be specified with the ldap_kerberos_container_dn directive in krb5.conf. Realm container entries will be created underneath this DN. Principal entries may exist either underneath the realm container (the default) or in separate trees referenced from the realm container.

5. Configure the LDAP server ACLs to enable the KDC and kadmin server DNs to read and write the Kerberos data.

   Sample access control information:

    access to dn.base=""
        by * read
   
    access to dn.base="cn=Subschema"
        by * read
   
    access to attrs=userPassword,userPKCS12
        by self write
        by * auth
   
    access to attrs=shadowLastChange
        by self write
        by * read
   
    # Providing access to realm container
    access to dn.subtree= "cn=EXAMPLE.COM,cn=krbcontainer,dc=example,dc=com"
        by dn.exact="cn=kdc-service,dc=example,dc=com" read
        by dn.exact="cn=adm-service,dc=example,dc=com" write
        by * none
   
    # Providing access to principals, if not underneath realm container
    access to dn.subtree= "ou=users,dc=example,dc=com"
        by dn.exact="cn=kdc-service,dc=example,dc=com" read
        by dn.exact="cn=adm-service,dc=example,dc=com" write
        by * none
   
    access to *
        by * read
   
   If the locations of the container and principals or the DNs of
   the service objects for a realm are changed then this
   information should be updated.

6. Start the LDAP server as follows:

   slapd -h "ldapi:/// ldaps:///"

7. Modify the krb5.conf file to include LDAP specific items listed below:

   realms
       database_module
   
   dbmodules
       db_library
       db_module_dir
       ldap_kdc_dn
       ldap_kadmind_dn
       ldap_service_password_file
       ldap_servers
       ldap_conns_per_server

8. Create the realm using kdb5_ldap_util (see Creating a Kerberos realm):

   kdb5_ldap_util -D cn=admin,dc=example,dc=com create -subtrees ou=users,dc=example,dc=com -r EXAMPLE.COM -s

   Use the -subtrees option if the principals are to exist in a separate subtree from the realm container. Before executing the command, make sure that the subtree mentioned above (ou=users,dc=example,dc=com) exists. If the principals will exist underneath the realm container, omit the -subtrees option and do not worry about creating the principal subtree.

   For more information, refer to the section Operations on the LDAP database.

   The realm object is created under the ldap_kerberos_container_dn specified in the configuration file. This operation will also create the Kerberos container, if not present already. This will be used to store information related to all realms.

9. Stash the password of the service object used by the KDC and Administration service to bind to the LDAP server using the kdb5_ldap_util stashsrvpw command (see Stashing service object’s password). The object DN should be the same as ldap_kdc*_dn* and **ldap_kadmind_dn values specified in the krb5.conf file:

   kdb5_ldap_util -D cn=admin,dc=example,dc=com stashsrvpw -f /etc/kerberos/service.keyfile cn=krbadmin,dc=example,dc=com

10. Add krbPrincipalName to the indexes in slapd.conf to speed up the access.

With the LDAP back end it is possible to provide aliases for principal entries. Currently we provide no mechanism provided for creating aliases, so it must be done by direct manipulation of the LDAP entries.

An entry with aliases contains multiple values of the krbPrincipalName attribute. Since LDAP attribute values are not ordered, it is necessary to specify which principal name is canonical, by using the krbCanonicalName attribute. Therefore, to create aliases for an entry, first set the krbCanonicalName attribute of the entry to the canonical principal name (which should be identical to the pre-existing krbPrincipalName value), and then add additional krbPrincipalName attributes for the aliases.

Principal aliases are only returned by the KDC when the client requests canonicalization. Canonicalization is normally requested for service principals; for client principals, an explicit flag is often required (e.g. kinit -C) and canonicalization is only performed for initial ticket requests.