Hello,

I have a server that sits between two LANs, and I wish to forward multicast traffic from one LAN to the other. I have multicast routing enabled in the kernel, and mrouted running, and if I watch /proc/net/ip_mr_cache, I can see it working properly. However, the traffic just simply doesn't get forwarded. These are the steps I am taking to troubleshoot the problem:

1. Launch VLC on a workstation on LAN A, and start multicast-streaming a video.
2. Examine ip_mr_cache on the server. I see the group show up in ip_mr_cache, with a proper Iif entry showing the interface connected to LAN A. The Oif column is blank.
3. Launch VLC on a workstation on LAN B, and try to connect to the multicast on the same group address.
4. Examine ip_mr_cache on the server. I now see an entry show up in the Oif column for the multicast group in question.
5. Check tcpdump on both interfaces on the server. Packets are not forwarding.

So, correct me if I'm wrong, but it seems that mrouted detects the multicast ok on LAN A, and also the receiver joining the group on LAN B. It seems to be properly configuring the multicast route in the kernel also. Running a tcpdump on the LAN A interface on the kernel shows a massive packet storm from the multicast stream. But running the same tcpdump on the LAN B interface shows that nothing is being forwarded. Also, the LAN B client doesn't start showing the video.

Oh, also, this server forwards unicast just fine. Is there a file like /proc/sys/net/ipv4/ip_forward that must be accessed to enable multicast forwarding? As far as I can tell it is all ready to go - the packets just aren't being forwarded.

Thanks for listening.