Newbie question: why do we need this tool, since we already have socat?

[oucfei]

Sorry for the newbie question, I have almost zero experience on networks, hope anyone can teach me: if I want to forward traffic from/to WebSocket, why can't I just use socat? My understanding is socat can be used to forward TCP traffic, and WebSocket is just a protocol on top of TCP, so isn't the underlying traffic still TCP? I'm asking because I did try to use socat to forward a ws traffic and there was some issues. Even with websocat I'm having trouble making it work, but I want to know which is the right direction... Thanks in advance!

[oucfei]

By the way, I know there's a docker container for this and I've started one like this: docker run -p 9229:9229 --name websocat solsson/websocat --binary -v -E ws-listen:127.0.0.1:9229 ws://172.17.0.2:4848 The container starts successfully and prints some message. I've verified the connectivity to ws://172.17.0.2:4848 inside the container is fine. However when I try to connect from host echo "some data" | websocat ws://127.0.0.1:9229 I got: websocat: WebSocketError: WebSocket protocol error websocat: error running

and the container seems didn't receive anything because it's not printing anything. Shouldn't it listening on port 9229 for ws traffic now? Why it doesn't work?

[vi]

if I want to forward traffic from/to WebSocket, why can't I just use socat?

For plain ws-l:127.0.0.1:9229 -> ws://172.17.0.2:4848 case socat (or websocat tcp-l:127.0.0.1:9229 tcp:172.17.0.2:4848) may indeed be more appropriate.

Main usage of websocat is when you want to extract WebSocket messages and do something with them. Another case is when you want websocat's autoreconnect: or reuse: features.

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I may answer later about debugging WebSocket protocol error.

[vi]

WebSocket protocol error

This looks like as if client Websocat is connected to a TCP socket, but something other than a proper WebSocket reply (including immediate connection drop) is received.

You can get traffic dump of how Websocat tries to connect to the socket by combining it with socat:

websocat -t - ws-c:sh-c:'socat -v -x - tcp:127.0.0.1:9229'

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The port is not properly forwaded to websocat running inside Docker. Either you should listen 0.0.0.0 instead of 127.0.0.1 (docker run -p 9229:9229 --name websocat solsson/websocat --binary -v -E ws-listen:0.0.0.0:9229 ws://172.17.0.2:4848) or use --network=host (docker run --network=host -p 9229:9229 --name websocat solsson/websocat --binary -v -E ws-listen:127.0.0.1:9229 ws://172.17.0.2:4848)

[birdinforest]

I encounted one case of WebSocket protocol error. It turned out that the server request a Sec-WebSocket-Protocol header, but I didn't provide in connecting. Fixed by websocat ws://[ip]:[port] --protocol [sec-websocket-protocol]

Maybe irrelevant to this issue, just for reference to others.