Was looking at making something that would interact with SystemConfiguration.framework and registers itself on the DynamicStore's callback (something akin to dns-heaven[1]). From the introductory tutorials included with xcode I have no idea what is the equivalent to applicationDidFinishLaunching() where this kind of things would be put.
It looked like maybe BackgroundTask is what I'm looking for, but it seems arbitrarily limited on macOS to an urlsession, whatever that is ?
[1]: https://github.com/greenboxal/dns-heaven/