How to keep a Netcat listener open for multiple connections

Temporary TCP tests often involve several clients connecting one after another. An OpenBSD Netcat listener can stay open between connections, so each test reaches the same receiving terminal.

Keep-open mode waits for a new TCP connection after the previous client disconnects. It handles clients in sequence, so a client that leaves its connection open can delay the next test.

Both terminals must use OpenBSD Netcat, available as netcat-openbsd on Ubuntu. The local-only listener accepts connections from the same machine without encryption or authentication, so it suits temporary tests with nonsensitive text.

1. Start a keep-open listener on the loopback address in the first terminal.

   $ nc -lk 127.0.0.1 9000

   The -l option listens, and -k accepts another connection after the previous one finishes. No output appears until a client sends data. If port 9000 is already in use, choose an unused port for both the listener and clients. If nc rejects -k, the installed variant may not be OpenBSD Netcat.
   Related: How to install Netcat on Ubuntu

2. Send a first message from a second terminal on the same machine.

   $ printf 'first connection\n' | nc -N 127.0.0.1 9000

   The -N option shuts down the client's sending side after printf finishes, allowing the connection to end. The client returns to the shell prompt without printing a reply.

3. Check that the listener terminal displays the first message.

   first connection

   The listener stays running without returning to a shell prompt.

4. Send a second message from the second terminal without restarting the listener.

   $ printf 'second connection\n' | nc -N 127.0.0.1 9000

5. Confirm that the original listener displays both messages while remaining open.

   first connection
   second connection

   The second line comes from a separate client connection, proving that the listener accepted another client after the first disconnected. A returned shell prompt or a failed second connection means the listener did not remain open. When testing is finished, Ctrl-C in the listener terminal stops it.