netplan and post-up/pre-down scripts

[Mike Pontillo]

Hi all,

   Recently, I was working on a project that led me to become frustrated
with the current state of `systemd` and `ifupdown` (e.g.
/etc/network/interfaces or /e/n/i) in Xenial. I remembered that
`netplan`[1] was under development, so I added the PPA for Xenial and gave
it a try. Unfortunately, it didn't solve the issue for me[2]... but I also
realized there was a blocking issue in the way of my adoption of it.

   Let me explain my use case: when an interface goes up or down, I want to
be able to do event-driven things with the network configuration, such as
add or remove routes, run a DHCP client, etc. My first attempt to make this
happen was to add `post-up` and `pre-down` scripts to do this. However,
this had a fatal flaw for my application: `ifupdown` doesn't separate the
concept of operational status from the concept of administrative status.
(That is, in `ifupdown`, an interface is "up" if the admin says it is up.
Link up or link down does not seem to matter; it's strictly an
/administrative/ status[3].)

   Long story short: in order to get the behavior I wanted, I wrote a
custom script that monitors *operational status* (aka physical link up/down
status), and I launch it using /e/n/i's `post-up`, and bring it down using
/e/n/i's `pre-down` scripts.

   Looking at the `netplan` spec[4], I don't see a way to achieve that
functionality. I know that many people are using the script-callout
functionality in /e/n/i to achieve what they need to achieve, so it seems
to me that having this in `netplan` is critical to achieving parity with
what we have in Xenial with `ifupdown`.

   In an ideal world, I think `netplan` would be able to model my use case
out-of-the-box.[5] But since we can't expect to model everyone's use case,
it seems like custom scripting functionality is a hard requirement, though
perhaps one that could have tricky cross-platform implications.

   Thoughts?

Regards,
Mike

[1]: https://lists.ubuntu.com/archives/ubuntu-devel/2016-July/039464.html

[2]: The behavior was exactly the same, which is actually good for
consistency.

[3]: Well, it is after an annoying 5 minute timeout, in some cases.

[4]: https://git.launchpad.net/netplan/tree/doc/netplan.md

[5]: As an aside, this brings up an interesting subtle difference between
rendering a networking configuration with (networkd, ifupdown) vs
NetworkManager. In NetworkManager, there *IS* a concept of operational
status, and I would have been able to get closer to the behavior I wanted,
if I was willing to pull in all those dependencies. We should be mindful
that rendering the same configuration on NetworkManager, even for simple
things like a static IP address and a DHCP interface, could produce
drastically different behavior than with networkd or ifupdown.
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