The purpose of the idmap backend parameter is to allow idmap to NOT use the local idmap tdb file to obtain SID to UID / GID mappings, but instead to obtain them from a common LDAP backend. This way all domain members and controllers will have the same UID and GID to SID mappings. This avoids the risk of UID / GID inconsistencies across UNIX / Linux systems that are sharing information over protocols other than SMB/CIFS (ie: NFS). ldap:ldap://ldapslave.example.com