packet filtering vs. conserver

Hi,

Is there a way to make conserver listen on a limited number of ports
only (instead of opening random ports)? 

The manual page talks about the 'secondaryport' option but this seems to
do nothing at all, and I'm not at all sure that it's the option I need
anyway.

The reason I'm asking is that I have to devise a set of iptables (packet
filter) rules to let users in, as a policy.

thanks

Andras