UDP multicast client does not see UDP multicast traffic generated by tcpreplay

Question

I have two programs:

• server ... it generates UDP traffic on a chosen multicast
• listener ... it prints UDP traffic on a chosen multicast (it sub

Answer 1

This is just a theory, but it might be that the packets are discarded by the receiving side due to their checksums being wrong.

That could happen if the machine where you run tcpdump has IP or UDP checksum offloading enabled. That means the packages you capture locally haven't their checksums calculated yet, which the hardware does before sending them out. When you then tcpreplay those packets, the checksums are not calculated, as tcpreplay works on a lower level than the socket API you used to generate the packets.

In order to verify the correctness of the checksums (both those of the dump file as well as those of the packets spit out by the subsequent tcpreplay), tcpdump -v ... will warn you about wrong checksums. wireshark also colors wrongly checksummed frames differently (unless turned off in the wireshark settings).

Did you try to tcpdump the packets only on the sending host, or also on the receiving host? The latter would remove the checksum errors, if that is indeed your problem.