[CentOS-virt] Issues with routing IPv6 to KVM Guests

David Hackl info at dh-online.net
Thu Sep 20 15:15:57 EDT 2012

Hi Joe,

strange thing but after setting multicast_router once to 2, ipv6 worked 
on all vnet devices. I also restarted the node and the vms and it is 
still working.

Seems like this fixes the problem....

Thanks for you help!


Am 20.09.2012 01:39, schrieb Joe Pruett:
> On 09/19/2012 04:29 PM, Joe Pruett wrote:
>> On 09/14/2012 03:16 PM, David Hackl wrote:
>>> IPv6 is enabled. As I said, it works good on the node itself. I can ping
>>> it and also ping out of it.
>> this is a really annoying bug in the linux bridge code. ipv6 neighbor
>> discovery relies on multicast, and the bridge code tries to optimize
>> multicast by not treating it as broadcast. but since v6 neighbor
>> discovery doesn't use v4 igmp, the bridge code never realizes what
>> bridge interfaces should get what multicast packets. the easiest fix is:
>> echo 2 > /sys/class/net/vif#.#/brport/multicast_router
>> with the appropriate #.# substituted (or just do them all), which tells
>> the bridge code that there is a multicast router on that port and
>> therefore it should receive all multicast traffic. i ran into this a few
>> months back and had forgotten it again until now. i never found a way to
>> make it happen at boot time, or via some script in xen, but i will now
>> dive into that hunt again...
> ah, the memory is going. i did come up with a hacked way to make this
> happen for xen vms, so you'd have to find another way to make it happen
> for kvm. for xen, there is a script that is called to create the vif
> interface, and i just hacked in the echo in there. i had to do it as a
> background job with a 5 second delay to get around some timing issue.
> and it only works for newly created vms, once a vm has been created, it
> remembers the original script name, so suspending and resuming gets you
> back to square one, even rebooting from within the guest.
> here is the code that was added to the vif-bridge script, just to see
> how easy it was:
> (sleep 5 ; echo 2 > /sys/class/net/$vif/brport/multicast_router) &
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