[Bug 1664818] Re: Not possible to render pre-up, pre-down, post-up, or post-down snippets

[Dimitri John Ledkov]

I'm kind of concerned about any non-declarative functionality here. For
example, DHCP client and receiving/updating routes are all handled by
netwokrd already.

You can bind a systemd unit to e.g. sys-subsystem-net-devices-
eth0.device and then any number of systemd units should be started and
stopped in concert.

One can also block and wait until the relevant interface is up with
/lib/systemd/systemd-networkd-wait-online --help

However, I'd rather avoid running arbitrary code as all the examples
provided at the moment can be achieve as specifications to the networkd
units. (either as generated by netplan, or as drop-in snippets in /etc
or /run).

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https://bugs.launchpad.net/bugs/1664818

Title:
  Not possible to render pre-up, pre-down, post-up, or post-down
  snippets

Status in netplan:
  New
Status in systemd package in Ubuntu:
  Triaged

Bug description:
  Many configurations require scripts to run before or after an
  interface comes up or down.

  Example: I had a situation where I needed to render a post-up script
  to monitor an interface with ifupdown (and with the interface set to
  manual mode, because if the link was down I didn't want to require
  DHCP to run before the system continued booting). This is similar to
  what NetworkManager does in order to launch (or kill) a DHCP client.

  Other use cases involve setting up custom routes (such as when complex
  logic involving metrics or source routing is required) or anything
  else the designers of the netplan rendering engine didn't think of,
  such as invoking workarounds or special settings required for a
  particular NIC or environment, iptables rules that should be added or
  removed, etc.

  A follow-on issue is, it should be possible to ask that custom scripts
  run if the *link* is detected to come up or down. But I'll be happy if
  netplan first can simply replicate the custom-script-invoking behavior
  of ifupdown.

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