Re: [Qemu-devel] [PER] Re: socket, mcast looping back frames -> IPv6 broken

[Samuel Thibault]

Stefan Hajnoczi, le Wed 06 Mar 2013 13:29:37 +0100, a écrit :
> 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.

Sure, I've seen the comment, I wasn't suggesting to drop that :)

> > 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.

Ok.

> 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?

I haven't seen any in the Linux source code.  One thing that should
work, however, is to use recvfrom, and drop whatever comes from our
sockname.

Samuel