What runs on Port 9999?
Port 9999 is often used when standard ports (like 8888 or 8080) are busy, or for specific administrative tools.
1. Jupyter Notebook: "Password or Token"
If you see a Jupyter login screen but don't know the password, you need the access token generated when you started the server.
Check your terminal for a URL containing ?token=... or run this command:
It will show currently running servers and their tokens.
2. Java Remote Debugging (JDWP)
Backend developers often use port 9999 to attach a debugger (IntelliJ IDEA / Eclipse) to a running application.
To enable this, start your Java app with these JVM arguments:
3. Port 9999 is Busy?
If you cannot start your server because the port is taken, identify the process.
kill -9 [PID]
4. HTTP vs HTTPS
Some admin panels (like Solr or custom dashboards) might require HTTPS. If the connection resets, try:
https://localhost:9999You may need to accept a "Self-Signed Certificate" warning.