I have a Debian Linux running on a RasPi. I'm looking for a way to run a script when another system becomes available on the network (aka logs in on my wifi AP). The AP is an AVM Fritzbox and as such can't do what I would like to achieve. So I'd like to use one of the RasPis that I have available in my network to do this. It's always on and can monitor the network 24/7. Once a specific host logs into the network I'd like to run a script that for example notifies me of the event. 
Linux; run script when a specific host becomes reachable (logs in on AP)
linux;networking;scripting;wifi