To request a custom matching rule on a specific attribute, a client application includes the matching rule OID in the search filter. LDAP v3 calls these search filters extensible match filters. An extensible match filter looks like the following:
This filter tells the server to search for Quentin in the common name (CN) of the entries by using matching rule 220.127.116.11. The matching rule happens to be a case exact match.
The case exact matching rule plug-in OID 18.104.22.168 enables Directory Server to perform the search correctly. Directory Server calls code in this matching rule plug-in to check for matches during a search. The server also uses the code to generate indexes that accelerate case exact searches. The indexes also help to sort entries found during such searches.