This option is undocumented, but it present in ejabberd_auth_ldap.erl
consists of two parts - DNFilter, DNFilterAttrs
DNFilterAttrs is used in some queries as "attributes" parameter, but this option is empty by default. Queries with empty attributes return ALL existing attributes, this causes a lot of traffic.
Added ticket
I've added this as a ticket:Document ldap_dn_filter in the Guide