Re: [Qemu-devel] Solution for qemu mcast / ipv6?

[Jamie Lokier <jamie@shareable.org>]

Paul Brook wrote:
> > > The bug is in the the mcast socket code. You need to fix that to stop
> > > looping back all the packets.
> >
> > Do you mean the mcast socket code in qemu or the way IP_MULTICAST_LOOP
> > works in the linux kernel?
> 
> I don't know. Hacking the generic vlan code is definitely the wrong solution 
> though.

I suspect there's no suitable kernel option, and this is for all
hosts, not just Linux.

One solution is filtering in the userspace code which talks to the
socket.

Filtering by MAC address is wrong as Paul has explained.

So the filtering which comes to mind is (a) keep _strong_ checksums of
all recently sent packets, (b) discard any received packets which
match a recently sent one, and (c) make sure to remove any checksum
when it matches in (b), because it's legitimate for the _same_ packet
to arrive from another host soon after, and that should not be
filtered.

Alternatively, presuming the idea is to tunnel raw ethernet packets
into a multicast IP packet, change the tunnelled format slightly to
include a unique sender identifier, and filter on that.  That's
unambiguous.

-- Jamie