How to create a TCP listener with Netcat

A TCP client needs a receiving socket before it can send data, even when no application server is available. Netcat provides a temporary TCP listener that displays the bytes arriving from the client.

OpenBSD Netcat passes data between its terminal and a TCP connection without adding an application protocol. It provides neither peer authentication nor encryption, so test messages should contain no secrets.

On Ubuntu, the netcat-openbsd package supplies this Netcat variant, while iproute2 supplies the ss utility. A loopback listener accepts clients only from the same host, so both terminals must run there.

1. Check whether TCP port 9000 is already listening.

   $ ss -ltn 'sport = :9000'
   State Recv-Q Send-Q Local Address:Port Peer Address:Port

   A header with no socket rows means no TCP listener is using that port. If a row appears, choose another unused port for both terminals.

2. Create the TCP listener on loopback port 9000 in the first terminal.

   $ nc -l 127.0.0.1 9000

   The -l option waits for an incoming connection. The terminal stays in the foreground without output until a client sends data. Port 9000 does not require sudo.

3. Connect to the listener from a second terminal on the same host.

   $ nc 127.0.0.1 9000

   The client also waits without printing a prompt. If it reports Connection refused, the listener is not accepting connections at that address and port.

4. Send the text “listener test from client” from the client terminal with Enter.

   The text goes into the running nc client, rather than a shell prompt.

5. Verify that the first terminal displays the line sent by the client.

   $ nc -l 127.0.0.1 9000
   listener test from client

   The received line confirms that the listener accepted the TCP connection and received data. Both terminals remain connected for more text. When finished, Ctrl-C stops Netcat; a basic listener exits when its client disconnects.