external auth filter - attribute cn doesn't work :-(

[kuda]

Hello, I have a big problem I am struggling with - I cannot get external authentication to work using (cn=%u) as filter value, when I ldapsearch zimbra&openldap db's both show equal (cn) results, zimbra's test work well, however one can see only "empty search" in logs when users try to login (without success of course) .. using the filter uid=%u (for test purposes) works perfectly but I can't use it since my uid are not equal accross the servers. I am not sure which ldap attributes can one use in ldapfilter ... thanks in advance for suggestions