MacOS Monterey Port 5000 Already In Use
Many web development frameworks are using some abstract port on local machine for development purposes, such as serving the results (pages, API endpoints) locally for the testing/validation purposes. Those ports are usually chosen to be easy to remember, for instance 8080, 8000, 3000, 5000 etc. And since (somewhat) standardised port range is only those below 1024, the port usage was always a bit of a chaos.
The problem I faced recently was with a default development port (5000) suggested by Python Flask framework.
I recently updated my laptop to OSX Monterey, and when trying to launch the Flask development server I faced the "Port 5000 already in use" error. At first, I thought it might be a previous dev server instance hanging around, but looking at it with
lsof -i tcp:5000
showed that it's something else:
[halien@airhydra:~/devel]# lsof -i tcp:5000 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME ControlCe 18536 halien 19u IPv4 0x4008cac8d519c5c9 0t0 TCP *:commplex-main (LISTEN) ControlCe 18536 halien 20u IPv6 0x4008cad72f646471 0t0 TCP *:commplex-main (LISTEN)
After some googling, turned out that OSX Control Center is now listening on port 5000 as an AirPlay server. According to the comments in this thread, port 5000 usage for AirPlay is nothing new, it's just the OSX now supports it:
The Remote Audio Output Protocol, AKA AirTunes, AKA AirPlay, has been using port 5000 since the year 2004. This is nothing new. All AirPlay receivers including AirPort Express and Apple TV use port 5000. If you've ever used Airfoil on your Mac (Airfoil Speakers was released in 2008), that also uses 5000 for the same reason. AirPlay receiving is new to macOS Monterey, but AirPlay itself is very old, predating Flask and these other web development environments.
A more robust solution would be to use a different development port - even a small change like using 5001 would do. However, that could mean changing it in a lot of places, and if you're working in a team that could potentially (depending on the collaboration setup) affect other developers.
Another way is to disable AirPlay server via System Preferences › Sharing, and unchecking AirPlay Receiver