An optional base DN, search scope and LDAP filter to restrict LDAP searches for this attribute type. syntax: search_base[?scope?[filter][?search_base?scope?[filter]]*] The scope can be one of "base", "onelevel" or "subtree". The filter must be a valid LDAP search filter as specified by http://www.ietf.org/rfc/rfc2254.txt For examples of this syntax, please refer to the ldap_search_base examples section. Default: the value of ldap_search_base