[Qemu-devel] [Bug 761469] Re: multicast VPN breaks IPv6 Duplicate Address Detection

[Neil Wilson]

** Bug watch added: Red Hat Bugzilla #557188
   https://bugzilla.redhat.com/show_bug.cgi?id=557188

** Also affects: fedora via
   https://bugzilla.redhat.com/show_bug.cgi?id=557188
   Importance: Unknown
       Status: Unknown

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https://bugs.launchpad.net/bugs/761469

Title:
  multicast VPN breaks IPv6 Duplicate Address Detection

Status in QEMU:
  New
Status in Fedora:
  Unknown

Bug description:
  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.