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[Serge Hallyn <1218959@xxxxxxxxxxxxxxxxxx>]

There has also been some concern about the propriety of the
proposed fix (when a newer kernel with the upstream fixes should
appear), in addition to lack of verifiaction - so please go ahead
and supersede this if you haven't already.

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https://bugs.launchpad.net/bugs/1218959

Title:
  KVM virbr# no longer forwards multicast traffic by default (U12.04)

Status in “libvirt” package in Ubuntu:
  Fix Released
Status in “linux” package in Ubuntu:
  Fix Released
Status in “libvirt” source package in Precise:
  Fix Committed
Status in “linux” source package in Precise:
  Won't Fix

Bug description:
  A recent kernel update (Apr 2013) has made it's way to U12.04.2 LTS
  (approx June-Aug 2013) and has stopped the (default) behaviour of
  automatically forwarding multicast traffic over virbr#.  Some updates
  the bridge subsystem now, by default, disable multicast traffic
  without IGMP Querier being enabled on that bridge.

  The corresponding Fedora/RHEL bug tracks the progress/updates of this specific change in relation to Fedora.  
  https://bugzilla.redhat.com/show_bug.cgi?id=880035

  (I have yet to find a similar bug report in Ubuntu so I have created
  this bug to help the Ubuntu community identify multicast issues that
  may arise since April 2013 in U12.04 LTS and presumably other Ubuntu
  releases as backports are made and break regression testing.)

  Using the latest patches in U12.04.2 LTS this following addition, with some modifications, can be made to the udev rules will enable multicast on virbr# bridge:
  https://bugzilla.redhat.com/show_bug.cgi?id=880035#c38

  While this is an improvement/correction to KVM bridge networking it
  has broken existing functionality in U12.04.2 LTS and broke regressed
  functionality that once worked.

  IMPACT: multicast is broken over libvirt bridges
  FIX: set a (new, introduced by a new kernel) toggle on the bridges
  TEST CASE: cat /sys/devices/virtual/net/virbr0/bridge/multicast_querier - if 0, then forwarding multicast will be broken.
  REGRESSION POTENTIAL: should be none. older kernels do not have the toggle, and failure to set it will be ignored

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