bind_transport_exception: bind_transport_exception (Address already in use)

What does it mean?

While starting, an Elasticsearch node could not bind a socket to listen on the transport (inter-node communication) or HTTP port. It usually comes with the 'Address already in use' sub-error, and the node cannot start.

Why does it happen?

The requested port (default transport 9300, HTTP 9200) is already used by another process; most often a previous instance of the same node did not shut down cleanly, another ES instance/service on the same host conflicts, or network.host/port is misconfigured. Permission/access restrictions can also prevent binding.

How to fix it

Find who holds the port (Linux: ss -ltnp | grep 9300 or lsof -i:9300) and terminate the conflicting process, or assign the node a different port (transport.port / http.port). Verify that the previous ES process has actually exited. Make sure network.host and the port range settings are correct and bindable. Then restart the node.