[openstack-dev] [Neutron] auto configration of local_ip

Ian Wells ijw.ubuntu at cack.org.uk
Thu Jan 16 10:06:49 UTC 2014

On 16 January 2014 10:51, Robert Collins <robertc at robertcollins.net> wrote:

> > 1. assigned to the interface attached to default gateway

Which you may not have, or may be on the wrong interface (if I'm setting up
a control node I usually have the default gateway on the interface with the
API endpoints, which I emphatically don't use for internal traffic like

> > 2. being in the specified network (CIDR)
> 3. assigned to the specified interface
> >    (1 can be considered a special case of 3)

Except that (1) and (2) specify a subnet and a single address, and an
interface in (3) can have multiple addresses.

How about 4. Send a few packets with a nonce in them to any of the
> already meshed nodes, and those nodes can report what ip they
> originated from.

Which doesn't work unless you've discovered the IP address on another
machine - chicken, meet egg...

I appreciate the effort but I've seen people try this repeatedly and it's a
much harder problem than it appears to be.  There's no easy way, for a
given machine, to guess which interface you should be using.  Robert's
suggestion of a broadcast is actually the best idea I've seen so far - you
could, for instance, use MDNS to work out where the control node is and
which interface is which when you add a compute node, which would certainly
be elegant - but I'm concerned about taking a stab in the dark at an
important config item when there really isn't a good way of working it out.

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