webcat - netcat for websockets

Connect to a websocket server:

webcat ws://example.com:3000/

Start a websocket server listening on port 3000:

webcat -l 3000

The server only accepts one client at a time. The client auto-reconnects.

Setting up a MITM debugging proxy

To debug stateful protocols on top of websockets, it is helpful to be able to inject messages to the server or the client in a real session, acting as a man-in-the-middle (MITM). On Linux you can achieve this using webcat and FIFOs as follows:

mkfifo client-in server-in
webcat ws://example.com:3000/ < client-in > server-in
webcat -l 4000 < server-in > client-in
echo > server-in # unblock the FIFO deadlock

You can now connect your client to ws://localhost:4000/. And inject messages by writing to the named pipes:

echo "Hello from webcat" > client-in

Note that when redirecting stdout the messages are automatically printed to stderr, so you can still observe what's happening.

Limitations

• no support for messages containing newlines (cannot send them, cannot distinguish them from separate messages)

• no support for WebSocket Secure (wss://) (webcat is meant for local testing & debugging, wss is out of scope)