On Tue, Mar 05, 2013 at 05:35:10PM +0100, Samuel Thibault wrote: > The reason why IPv6 does not work when using -net socket,mcast=foo is > that since qemu explicitly sets IP_MULTICAST_LOOP to 1, it receives its > own frames. When the IPv6 stack performs duplicate addresse detection > (DAD) through a multicasted announce, it receives its own announcement, > and thus believes another machine has the same address.
The reason for IP_MULTICAST_LOOP is to allow QEMU processes running on the same host to communicate with each other. > AIUI, on a real physical network network boards do not receive the > multicasts they send, so the issue does not happen. Perhaps some boards > even filter out any frame with its own MAC as source, eliminating the > issue altogether. > > As a result, we should probably perform this kind of dropping, I'm just > wondering at which level that would be preferable. > > - We could do that in qemu_send_packet_async_with_flags, thus fixing the > issue for all kinds of frame transporters. > > - Or we could do that for the only case that matters, mcast, in > net_socket_send_dgram (which will thus do it for the unicast udp case > too). > > What do people think about it? We should fix the layer that introduces the problem. Therefore I think the fix needs to be net/socket.c. Unfortunately net/socket.c does not have the concept of a link-layer address, so we cannot easily filter out multicast packets coming from our NIC's address. Are you aware of a way to filter out just the packets sent by *this* process? Stefan
