WatchYourLAN: Lightweight Network IP Scanner

Speaking about LAN, today, I encountered an unexpected event I had never imagined or experienced before.

I was working on a simple HTML/CSS game in VS Code, with Live Server running on port 5500 to serve the site. Feeling a bit tired, I decided to take a break. I put my Windows PC to sleep and moved to another room in my house. There, I spotted my Android tablet and thought it would be interesting to see how the game would perform on a tablet.

I unlocked the tablet, opened Chrome, entered my PC's local IP address and port, and hit 'Go'. To my surprise, the loading spinner appeared and spun for about 3-4 seconds. I was puzzled as to why the request was taking so long to get a response, and then it hit me—my PC was supposed to be sleeping.

Yet, just as I was processing this realization, the game’s web page loaded on the tablet. I was stunned, thinking, "Wait a minute—didn't I put my PC to sleep?" I went back to check my PC, and sure enough, it was awake but showing the lock screen. Out of curiosity, I repeated the experiment: I put the PC to sleep again, then accessed the webpage from my phone, and, once again, my PC awoke in response to the request. It was an eye-opening moment to see how the network request could wake my PC from sleep!

I googled this behavior and turned out to be called Wake on LAN or, WOL for short [1].

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1. https://learn.microsoft.com/en-us/troubleshoot/windows-clien...

WoL relies on a special magic Ethernet frame being sent to the MAC of the sleeping computer. A normal ARP or TCP SYN from an incoming HTTP request won't do it. The wikipedia article has the exact frame format: https://en.wikipedia.org/wiki/Wake-on-LAN

I've seen setups where the router is configured to send the magic WoL packet when it sees an ARP for the IP of a computer it knows is sleeping, but you'd almost certainly know if you had an exotic configuration like that on your network.