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

[Mike Lovell <mike@dev-zero.net>]

On 03/08/2013 05:47 AM, Samuel Thibault wrote:
> Samuel Thibault, le Fri 08 Mar 2013 10:08:55 +0100, a écrit :
>> There does exist some unique address, which is returned by recvfrom,
>> I'll have a look at how to get access to it.
> Ah, no, it's not unique... It's just the host IP address and the same
> port as the multicast address, so it'll be the same for all qemus on the
> same host.  I've checked how Linux bounces the datagram, it's through
> the loopback interface, and thus dispatched over all listeners without
> distinction. I don't see any way to get the information that the packet
> comes from us, except using the ethernet content.

this is actually a problem that i dealt with when i was building the 
switched multicast backend i did last year. ( 
http://lists.nongnu.org/archive/html/qemu-devel/2012-06/msg04082.html )

one solution is to actually use two sockets. one that is bound to the 
multicast address, which receives the multicast packets, and another 
that is just bound to any ephemeral udp port, which is used for sending 
packets. when a packet is to be sent out to the multicast address, call 
sendto on the ephemeral socket with a destination of the multicast 
address. then, using recv_from on the multicast socket, packets being 
received can be compared to the local ephemeral address. if the address 
on the recv_from matches the address for the local ephemeral socket, the 
packet can just be dropped. no inspection of the packet being passed 
around is needed in this case.

if the group is interested is a solution like this, i can probably make 
some time over the next couple days to cook up a patch. thoughts?

mike