[Serusers] rtpproxy ip

Hi serusers, after reading lots of docs and mailing lists I still have some doubts about nathelper+rtpproxy and the error message I usually get from it is

rtpproxy: can't bind to the IPv4 port 35000: Cannot assign requested address

what is the correct way to start rtpproxy? do I have to provide nat and public ip?

In the following situation: machine behind nat with private IP, running ser and reachable from the outside via port forwarding on router's public ip (forwarding port 5060 for ser and ports 35000-.... for rtpproxy range, both udp/tcp), can I use rtpproxy? how should i start it? (in the above situation ser registration from the outside works!)

thanks in advance