Can I trigger a systemd service when radios are turned on?

noblman · March 1, 2023, 11:41pm

I use the following systemd service to restart openvpn after a suspend:

  systemd.services.openvpn-reconnect = {
    enable = true;
    description = "Restart OpenVPN after suspend";
    script = "${pkgs.procps}/bin/pkill --signal SIGHUP --exact openvpn";
    wantedBy = [ "sleep.target" ];
  };

It works great.
However, if I put my laptop into airplane mode by hitting the airplane mode button (which I presume turns off all radios), when I hit the airplane mode button again to turn the radios back on, I need to manually restart openvpn using the above command.
Is there a way to have the above systemd service run after re-enabling the radios? Is this the correct approach?

ericgundrum · March 2, 2023, 2:18am

I think that what you really want is for the vpn to reconnect when access to its endpoint becomes available. Why endpoint access went away should be irrelevant. But I do not know systemd well enough to find a suitable target.

Does systemd send a notification when a network link goes up or down? How about when an interface gets an IP address? These would be useful triggers.

networking.networkmanager.dispatcherScripts runs a script when link status changes. If you use NetworkManager, maybe you can use such a script to signal openvpn-reconnect.

Hopefully someone else has a better solution. I’ve wanted to solve a similar problem with Nebula VPN but I’ve never taking the time to really dig into it.

nixinator · March 2, 2023, 2:52pm

Just some out of the box thinking, does pressing this button cause acpi event?

You maybe able to get nixos to do a openvpn systemd.restart when the button is disables or re enables wifi.

Populating /etc/acpi/events/ with configuration to launch a script. There is a example here of someone doing for headphones.

https://sourcegraph.com/github.com/chris-martin/home/-/blob/nix/os/renzo.nix?L116:3&subtree=true

github.com

NixOS/nixpkgs/blob/d70f5cd5c3bef45f7f52698f39e7cc7a89daa7f0/nixos/modules/services/hardware/acpid.nix

{ config, lib, pkgs, ... }:

with lib;

let
  cfg = config.services.acpid;

  canonicalHandlers = {
    powerEvent = {
      event = "button/power.*";
      action = cfg.powerEventCommands;
    };

    lidEvent = {
      event = "button/lid.*";
      action = cfg.lidEventCommands;
    };

    acEvent = {
      event = "ac_adapter.*";

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just an idea, there is might be a way to for systemd to pickup acpi event also, but i’m not sure how to do that.

The non-nixos way for doing it is here.

Good luck and let us know how you get on.