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 <firstname.lastname@example.org> wrote:
> On Fri, 2011-05-06 at 13:12 -0700, Noah Meyerhans wrote:
> > 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 126.96.36.199 and 188.8.131.52, but they don't look like they
> would cause this.
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.