When is a link actually, y’know, UP?

TL;DR Use device names such as eno1, enp7s0 rather than nic1, nic2

I’ve been chasing an issue with a TripleO-based installation whereby the nodes were provisioning but failing to configure networking correctly.

Debugging TripleO deployments is fiendishly hard and this was made more complex by being unable to connect to the failed nodes. Deployed TripleO nodes only allow key-based ssh authentication. It’s great to see security being so good even the sysadmin can’t access the node I guess.

If you want to login to a node at the console, you basically have to roll your own deployment image. I was on the verge of heading down this route when I considered the following:

TripleO deployments have two methods of specifying usable, active network interfaces. The first (and unfortunately, the default) is to number them nic1 , nic2, nic3 etc in the config. Unfortunately this introduces some logic from os-net-config to determine which links are actually connected to switches.

This would be fine on most machines but sadly the hardware I’m working on has a built-in ethernet-over-usb device for out of band access. This reports itself as having a link (for reasons unknown, maybe a link to the usb interface?) and therefore fulfills the following criteria:

1. Not the local loopback
2. Has an address
3. Reports carrier signal as active in /sys/class/net/<device>/carrier
4. Has a subdirectory of device information in /sys/class/net/<device>/device/

Despite reporting the link state as UNKNOWN in the ip command, this meant that the logic of os-net-config concluded that this was the management nic and attempted to configure it as such, obviously to no avail.

Happily this has resulted in my first OpenStack patch:

https://review.openstack.org/#/c/291243

which may even get accepted.