Comment 9 for bug 761469

I found this bug too.

Simple way how to reproduce:

Run kvm guest 1 and 2, with net something like this:
g1:-net nic,vlan=0,model=virtio,macaddr=52:54:00:00:01:01 -net socket,vlan=0,mcast=239.255.0.1:4097

g2: -net nic,vlan=0,model=virtio,macaddr=52:54:00:00:02:01 -net socket,vlan=0,mcast=239.255.0.1:4097

This should give you two running vm on same net.

Now in each guest configure ip addresses:
g1: ifconfig eth0 192.168.1.1
g2: ifconfig eth0 192.168.1.2

now on g2 ping 192.168.1.1

and on g1 run tcpdump -i eth0 icmp
and every second you will get:
time ip 192.168.1.2 ... ICMP echo request
time ip 192.168.1.1 ... ICMP echo reply
time ip 192.168.1.1 ... ICMP echo reply

There shouldn't be two echo replies, but only one.

Another effect is, that Duplicate Address Detection (for ipv6) simply doesn't work (this is what original bug is talking about).

Of course, solution is not to disable loopback for mcast socket, because there must be chance to run more then one VM on same host on same mcast address:port.

Solution may be to detect sending socket ip:port pair and simply drop packets received from this ip:port.