Port 8009: Google Cast / AJP
Port 8009 is the Google Cast control channel on Chromecasts, and also Apache Tomcat's AJP connector.
What runs on port 8009
Cast senders, such as the YouTube or Netflix app on your phone, connect to Cast devices on TCP 8009 over TLS to start and control playback. Separately, Apache Tomcat has long used 8009 for AJP, a protocol for connecting a front-end web server to Tomcat; the 2020 Ghostcat flaw showed how dangerous an exposed AJP port can be.
On a home network
Chromecasts, Nest speakers and displays, and TVs with Chromecast built in listen on 8009. Tomcat on a home server might too.
If port 8009 is reachable from the internet
Cast devices exposed to the internet can be controlled by strangers, as happened in the 2019 Chromecast hijacking campaign. An exposed Tomcat AJP port can leak files from the web application. Neither should be reachable from outside.
How to check whether it is open on your network
Look for 8009 in your router's UPnP table and port forwarding list. A LAN port scan shows which devices are Cast receivers.
To find out whether port 8009 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.