We have an E device that sends a request multicast (239.1.2.3) UDP to port 17225 in network netE. The replier listens to the multicast address request, and replies with unicast UDP to the requester. When the replier is in netE, everything goes fine.
But when the replier is in netJ, something needs to be set up in the "router" device N. Device N has an interface to netE, and another interface to netJ. We want to apply NAT to the outgoing request. We want that N routes J's reply back to the requester, E. There can be multiple requesters (E1, E2...).
We are using Linux iptables for setting up this NAT "routing". We manage to NAT the request, and the packet goes in netJ to J, with dynamic src port #. J replies to the unicast address of N and the dst port # set to the received src port #.
The reply packet arrives to N, but it discards it saying "ICMP udp port # unreachable".
How should we set up iptables for correctly routing the reply? We are using pimd for multicast routing.
