Migrate deprecated "CriticalConnection" attribute to "KeepConfiguration" (systemd-networkd)

[Chris Aumann]

With systemd-v243 (which is the default since focal), the "CriticalConnection" attribute was
revamped into "KeepConfiguration" [1], addressing several issues, especially in high-availability
environents [2][3].

The new "KeepConfiguration" setting can take multiple arguments, according to the documentation
[4]:

> Takes a boolean or one of "static", "dhcp-on-stop", "dhcp". When "static", systemd-networkd will
> not drop static addresses and routes on starting up process. When set to "dhcp-on-stop",
> systemd-networkd will not drop addresses and routes on stopping the daemon. When "dhcp", the
> addresses and routes provided by a DHCP server will never be dropped even if the DHCP lease
> expires. This is contrary to the DHCP specification, but may be the best choice if, e.g., the
> root filesystem relies on this connection. The setting "dhcp" implies "dhcp-on-stop", and "yes"
> implies "dhcp" and "static". Defaults to "no".

If I understand the code correctly, it still uses "CriticalConnection" when setting "critical: true" in the netplan configuration.

- [1] https://github.com/systemd/systemd/pull/12511/files
- [2] https://github.com/systemd/systemd/issues/12050
- [3] https://chr4.org/blog/2019/01/21/make-keepalived-play-nicely-with-netplan-slash-systemd-network/
- [4] https://systemd.network/systemd.network.html#KeepConfiguration=

I could dedicate some time working working on this if help is wanted.

[Lukas Märdian (slyon)]

Thanks for your report, pull requests are always welcome!

This might also be related to: https://bugs.launchpad.net/netplan/+bug/1815101

[Hans van den Bogert (hbogert)]

If some guidance can be given, then I'm up to implementing this as well.

[Lukas Märdian (slyon)]

Thank you very much, have a look how the "CriticalConnection" stanza is written here and feel free to open a pull request on GitHub: https://github.com/canonical/netplan/blob/main/src/networkd.c#L924

[Chris Aumann (chr4)]

I've added a pull request here: https://github.com/canonical/netplan/pull/409, feel free to review/ contribute/ give feedback!

[Danilo Egea Gondolfo (danilogondolfo)]

Hi Chris, thanks for bringing this to our attention and for your PR.

I just took a look at it today. Please check my comments in https://github.com/canonical/netplan/pull/409#issuecomment-1729410106