Trouble with IPv6 and a bridge?

While trying to debug some really weird issue, I’ve noticed that you really really really should use bridge_hw or a pre-up/up/post-up whatever rule to assign a permanent MAC to your bridge.

Assume two hosts namely node1 and node2. From node1 I do ping an IPv6 address which is set on a bridge in node2. The following configuration in node2:

auto vmbr0
iface vmbr0 inet static
    address x.x.x.4
    broadcast x.x.x.31
    gateway x.x.x.1
    bridge_ports eth1
    bridge_maxwait 0
    bridge_fd 2
    bridge_ageing 0
    bridge_stp off
iface vmbr0 inet6 static
    address yyyy:yyyy:yy::3
    netmask 64
    gateway yyyy:yyyy:yy::1

There are ~30 connected containers to this bridge. When starting and restarting containers it somehow happened that the bridge took over the MAC of such a container.

If I do ping the IPv6 address ::3 from node1 I can see those icmp echo requests quite fine in tcpdump. They’re however never answered. I found a stackoverflow article in which one wrote „check the mac“ without any helpful „why“ „for“ .. So I couldn’t do anything with the information at a first glance. Later I noticed that the MAC the request was sent to matches the MAC of a container. Why?

It seems black magic happens here. A bridge in Linux will use the MAC of the first connected port. Usually that will be eth0/eth1 whatever physical device you attach to it. But there’s more: A bridge in Linux seems to use the lowest MAC of the enslaved interfaces.

That does explain another problem I am having, too. If I stop my container with ID 14 the result is, that my whole system is not reachable for a few seconds up to minutes. Surely, if the bridge had the same MAC the container had and the container is stopped (so the MAC is gone) you’ll have a problem.

The „fix“ for this is to assign the MAC. You can do so in Debian when using bridge_hw. First find out the MAC of the primary device you’re enslaving (most likely eth0, here it is eth1)

ip link show eth1 | awk '/ether/{print $2}'

then in your /etc/network/interfaces add

    bridge_hw xx:xx:xx:xx:xx:xx

among the other settings you’re using for your bridge like bridge_stp, bridge_fd etc. Reboot (or restart networking) and you’re fine. IPv6 pings from node1 do work now as they are expected to.

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