Apply rules to 1 UDP port but port used as a socket: possible?

["zze-Proxy GRALL O ext FTRD/SVA/LAN" <ograll.ext@xxxxxxxxxxxxxxxxxxxx>]

Title: Apply rules to 1 UDP port but port used as a socket: possible?

Hello to IPTables users,

I'm trying to use a socket (on port 1727) to decode some proprietary UDP packets. When I've decoded them I want to apply a DNAT rule (and then a SNAT  too) to this port using a filter on @IP:port source for all following packets…

I use libiptc to manage iptables from my application.

My problem is that I can't apply rule to the packets. They all arrive on the socket. I think it may be due to connection tracking but I would like your points of view to be sure.

Is there any solution to do it ? For the moment I use v1.2.5a but I'm installing v1.2.9.

Thanx in advance,

Olivier GRALL.