We have a HTTP, FTP, SMTP and a DNS server set up behand a 837 router with nat enabled. Everything except the DNS server is working ok. The configuration is in attachment
If I change the nat for port 53 to port 5353 local (and reconfigure the dns server to listen to port 5353) the dns server is working fine except if the computer sending the query uses port 53 UDP (and the answer has to be send back to port 53 UDP). The answer does not get through the router.