[Lxc-users] Bug discussion: implementing high virtual device MAC addresses

Mon Oct 24 18:41:44 UTC 2011

Hi,

Here we are using lxc intensively with bridges. Since we don't use STP, the
downtime for each a mac@ change is unnoticeable. In fact, we discovered it
when reading this mailinglist. After some test I can confirm that most of
the time we are spawning/destroying a container, the bridge's mac@ change,
but there is no loss of connectivity, since arp tables
are instantly refreshed.

So an easy workaround for the moment is to disable STP on the brige (brctl
br0 stp off). If you are using a bridge in a controlled environment, you
really dont need STP anyway.

My 2cents,

On Mon, Oct 24, 2011 at 11:09 AM, Derek Simkowiak <derek at simkowiak.net>wrote:

>     Hello,
>     Just following up re: this bug.  I think it's a pretty serious issue.
>
>     I am looking to work on this, but I am seeking some feedback and
> direction from one of the core LXC devs.
>
> - Do you agree with my analysis?
> - Has anyone else worked on this already?
> etc.
>
>
> Thanks,
> Derek
>
> On 10/18/2011 04:31 PM, Derek Simkowiak wrote:
> >       There is a behavior in the Linux kernel which can cause a bridge
> > device to change MAC address, thus causing a network blackout of several
> > seconds (while everybody ARPs the new MAC address flushes the old one).
> > This happens when bridging an enslaved interface, like we do with LXC.
> >
> >       The symptom is that the LXC host will black out for several seconds
> > when starting or stopping an LXC container.  Your SSH terminal on the
> > host will freeze and become unresponsive.  (It is a random symptom,
> > because the blackout only happens if the randomly-assigned MAC address
> > of the virtual device is lower than that of the physical eth0 device).
> >
> >       This behavior was first observed by the libvirt folks when creating
> > virtual machines.  You can read more details about it (and how they
> > fixed it) here:
> >
> > https://www.redhat.com/archives/libvir-list/2010-July/msg00450.html
> > https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/584048
> >
> >       I have observed the symptom under LXC, and the workaround for it
> > has been independently confirmed for LXC in this bug report (ID:
> 3411497):
> >
> >
> http://sourceforge.net/tracker/index.php?func=detail&aid=3411497&group_id=163076&atid=826303
> >
> >
> >       The workaround for the bug is to give the virtual device a high MAC
> > address, thus discouraging the bridge device from adapting its MAC
> > address as its own.
> >
> >       I have mentioned this bug on the list before, however, I was
> > confused about which MAC address was causing the problem.  This is NOT
> > the mac address specified in lxc.conf, like this:
> >
> > lxc.network.hwaddr = fe:16:3e:fd:5a:5b
> >
> >       That MAC address has nothing to do with the bug; the host's bridge
> > device (br0) will never assume a configured LXC MAC address as its own.
> > Instead, the MAC address in question is the one of the virtual vethXXXX
> > device, as shown with "ifconfig" on the host:
> >
> > veth0IEDlk Link encap:Ethernet  HWaddr 4e:34:7c:dc:92:e8
> > [...snip...]
> >
> >       That HWaddr should be given a high prefix to avoid the network
> > blackouts, just like they've done for libvirt.  That does not exist in
> > any config file anywhere; it must be fixed in the LXC source code.
> >
> >       I looked in network.c for the LXC source code and I think the fix
> > should go in lxc_bridge_attach() near line 991.  The fix would put a
> > manually-generated MAC address -- one with a high prefix -- into
> > ifr.ifr_hwaddr.sa_data and thus replace the random one assigned by the
> > kernel.
> >
> >       However, I'm new to the LXC source and would like some input and
> > analysis from a more seasoned contributor.  I would be happy to test and
> > maybe even contribute a patch, but I'd like some feedback first.
> >
> >
> > Thank You,
> > Derek Simkowiak
> >
> >
> >
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François-Xavier Bourlet
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