[Exim] LDAP query

Sorry if this might be slightly OT
but I didn't get it from the manual.

How is correct syntax of this query
in url form?

ldapsearch -b 'dc=dff,dc=local' 'gecos=Torsten.Curdt' uid -h kurn

I tried

ldap_default_servers = kurn::389
query = "ldap:///gecos=$local_part,dc=dff,dc=local?uid?"

But this doesn't produce the same query
as the above as I can see in the ldap
log file.
--
Torsten