We managed to finally (hopefully) fix an annoying problem with one of our FreeBSD boxes and IPv6.

For some background; we used the ng_fec (4) device on FreeBSD 6.2-RELEASE to make a logical network interface out of two physical interfaces. This is referred to as bonding on Linux, IP Multipathing on Solaris and I think Cisco call it Ether Channel. Whatever way you go about it, the end result is that you get a network interface that is reachable via a multiple different paths. If each physical interface is plugged into a different switch, you get a nice amount of resiliency.

This works great in FreeBSD with ng_fec, however if you try to use an IPv6 address on the logical interface thats where things start to get interesting. Without getting into too much detail – the problems relate to how IPv6 maps IP addresses to MAC addresses, this is done by way of “neighbour advertisement” messages sent via multicast. However the ng_fec driver was not telling the physical network interfaces to join the correct multicast groups so these neighbour advertisement messages could not be processed correctly, which led to very flaky and mostly unavailable IPv6 connectivity. From what I can gather, you can use other drivers in -CURRENT that don’t exhibit this problem.

But with some diagnoses and a patch to ng_fec.c from Dave Malone we managed to correct the issue. This may make it up into the FreeBSD source tree at some point.

Hopefully things will stay stable from now on :)

Edit: Ah, there is a bug report raised for this already.

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