Hi gurus! I have been playing with multicast in SER fow a while and it worked almost perfectly: SER is listening and sending wihout much difference as if it was a unicast address. Again thanks to SER developers!! There is a small detail which I hope you can help me to solve: multicast loopback. The problem is that SER receives all messages it sends to the multicast address and processes it as if it was received from the multicast address. Variables affecting the mcast loopback: SER config file mcast_loopback is set to no, and it is used in the opening of the mcast socket. So I guess the socket is effectively opened with the loopback disabled. The kernel MCAST_LOOPBACK is enabled, but I think that it does not affect the sockets opened with an explicit 0 in the MCAST_LOOPBACK setsockopt field. So I think it is not a matter of the multicast loopback parameters but a matter of the network topology. Please correct me if I am wrong!!!! Our host has a couple of listen= addresses, one for the unicast and another for the multicast. Guessings: Is it possible that SER uses the unicast interface to send the multicast message and therefore the multicast loopback check is not checked in the unicast sending socket???? But the same message, going deep into the TCP/UDP linux stack faces that there is an internal interface with the mcast address and it is therefore passed back????? I hope that I have been clear and that somebody can clarify me why is this happening (a solution is even better :) ) Thanks in advance, Samuel. Unclassified.