How can I trigger a systemd unit on suspend before networking is shut down?

Inspired by https://unix.stackexchange.com/a/139664/160746 I upgraded my start/stop scripts to a full blown daemon and made it listen for PrepareToShutdown signal. This is a race against NetworkManager every start/stop, but it seems to work reliably on my system.

I have uploaded my code and systemd unit at https://github.com/davidn/av.

Acccording to the systemd-suspend documentation, as well as the systemctl man page systemctl suspend activates the suspend.target.

systemctl list-dependencies suspend.target --after --all shows that suspend.target calls systemctl-suspend.service then sleep.target. This means when you call systemctl suspend the default order of operations are:

suspend.target
|-systemd-suspend.service
  |-sleep.target

---

If you placed Before=sleep.target, then your order of operations is likely:

suspend.target
|-systemd-suspend.service
  |-[custom service]
    |-sleep.target

So you're service runs after systemd-suspend.service does its thing, which is likely your issue.

---

You can add to your service file to get the correct results:

Before=systemd-suspend.service

After calling systemctl daemon-reload you should be able to use systemctl list-dependencies suspend.target --after --all to see your service appear between suspend.target and systemd-suspend.service. Your final order of operations should be:

suspend.target
|-[custom service]
  |-systemd-suspend.service
    |-sleep.target