multicast VPN breaks IPv6 Duplicate Address Detection
The multicast VPN facility in Qemu uses Multicast Loopback to make sure that other Qemu processes on the Host server receive the transmission. The side effect of this is that the process sending the packet also gets the packet back on its receive channel and currently this is not filtered but passed directly to the Virtual Machine.
You can see this effect by running tcpdump in the virtual machine. Every packet sent appears to be duplicated.
For IPv4 it doesn't appear to cause much of a problem, however with IPv6 the duplicate neighbor solicitation packet is precisely what the system uses to detect duplicate addresses. So IPv6 addresses fail to establish.
If you run 'ip addr' on a virtual Linux machine with IPv6 enabled you will see 'dadfailed' next to the link local address. 'ping6' will then not work.
Checked against 0.12.1.