Error listen EADDRINUSE address already in use: finding what holds the port

The application will not start and the message says Error: listen EADDRINUSE: address already in use :::3000. In Python, the version is OSError: [Errno 98] Address already in use. The short answer: another process is already listening on that port, and only one program can listen on a port at a time. Nearly always it is the same application, still running from an earlier attempt. This article is for a VPS; on shared hosting the case is different (see the end).

Finding who holds the port

1 See who is listening on the port (here 3000; swap in yours): sudo ss -tlnp | grep :3000The last column shows the program name and the process number (pid=1234). If nothing shows, the port is already free and the error came from an earlier attempt.
2 Work out what it is. If it is a node or python from your application, it is an old instance. If it is something else (nginx, a Docker container), the port really is reserved for that service and your application has to use another.
3 Stop the old instance with care. If PM2 or systemd manages it, stop it there: pm2 stop name or sudo systemctl stop name. Only for a stray process, end it with kill 1234 (the pid number). Use kill -9 only if the first does not work.
4 Start again and confirm with the same command as step 1.

The causes behind it

Cause How to recognise it What to do
Two instances of the same application ss shows your node; pm2 list has the application twice, or one runs by hand and another as a service. Keep one only: PM2 or systemd, not both, and not an open session at the same time.
A service restarting in a loop The service status flips and the log repeats the error. Stop the service, sort out the port, start again. See reading systemctl status.
A container publishes the same port docker ps shows the published port. Change the left-hand port in -p: your first container.
Another legitimate program ss shows a service you know. Pick another port for your application. Below 1024 only root can listen.
Development with auto-reload An old process got stuck after an error. End the old process, as in step 3.
Do not blindly kill the process ss shows. Check what it is first: it may be nginx or the database, and stopping those takes other sites down. If in doubt, ask whoever set the server up. On an unmanaged VPS, managing processes is yours: how far our support goes.
On shared hosting (cPanel) you do not choose ports: Passenger connects the domain to the application. If you see this error there, the code is trying to open a second server, or a fixed port with an old server still alive. Use process.env.PORT || 3000 in listen and restart with the button on the application screen. For the rest, see ports and firewall on a VPS.

Done all this and the port is still taken? Send us the ss output and the service name.

Open a support ticket

SEE ALSO

Ports and firewall on a VPS: why your app is not reachable

Keeping a Node.js app running on a VPS with PM2

A service will not start on my VPS: reading systemctl status

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