Bridged nodes sharing local IP to be used as gateway for LXC

[Mike Machuidel]

Hello Nick,

That is indeed what happened. But those nodes are not yet in
production and I was curious of what would happen. Hoped that may be
Tinc would block the ARP communication, but Tinc is already doing
enough. Some wishful thinking of me ;)
LXC containers do not support live migration meaning they will always
be stopped (if possible) and started on another node. The only thing I
would need to do is sending an ARP announcement to cause (hopefully)
all ARP caches to update.
I wrote a sort of HA daemon to do all this for me.

Mike

On Thu, Mar 14, 2013 at 2:18 PM, Nick Hibma <nick at anywi.com> wrote:
>> Well if you have two hosts with the same IP addresses on a network, the one
>> which responds first to an ARP request will win. The MAC address discovered
>> via ARP will be cached, so if a LXC container migrates to another node it will
>> not automatically change to the local gateway. And even if no containers
>> migrate, it could be that the local host has a higher load than the remote
>> node, and the remote node's ARP replies will arrive before the local node's.
>
> Which will cause flapping... Using the same IP address is not recommended for normal working setups, unless you know exactly what you do.
>
> Moving a container however should be trivial. If I am not mistaken an interface that is taken down and up again will send an ARP request for its own IP address, which will prime all connected host's ARP caches. Think gateway failover/VRRP, they do the same. So a simple 'ifconfig eth0 down; ifconfig eth0 up' in the container should do the trick.
>
> Nick
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