Re: Bug#625914: linux-image-2.6.38-2-amd64: bridging is not interacting well with multicast in 2.6.38-4

On Tue, 10 May 2011 03:38:44 +0100
Ben Hutchings [off-list ref] wrote:

On Fri, 2011-05-06 at 13:12 -0700, Noah Meyerhans wrote:

quoted

Package: linux-2.6
Version: 2.6.38-3
Severity: normal

Hi. I've got a system that hosts several kvm virtual hosts.  The VMs
access the network via tap devices bridged with a physical interface.
After upgrading to linux-image-2.6.38-2-amd64_2.6.38-4, I noticed that
the virtualhosts were not autoconfiguring their IPv6 interfaces.
Debugging revealed that no multicast was passing over the bridge.

The bridge configuration is:
bridge name     bridge id               STP enabled     interfaces
br0             8000.0002e3080eb5       no              eth1
                                                        tap0
                                                        tap1
                                                        tap2

If I attach tcpdump to br0, I can see multicast (e.g. IPv6 Neighbor
Solicitation) packets.  However, if I attach tcpdump to eth1, I do not
see multicast packets sourced from one of the VMs.

Downgrading to 2.6.38-3 solves the problem.

This is pretty weird.  Debian version 2.6.38-3 has a few bridging
changes from stable 2.6.38.3 and 2.6.38.4, but they don't look like they
would cause this.

Ben.

There are two possible explainations:
  1. In 2.6.37 and kernels the bridge uses IGMP snooping, there were several
     fixes to that in the stable kernel; especially related to IPv6.

  2. There was also a recent change to block link local multicast
     address. But that should impact what you are doing.