• I've searched for any related issues and avoided creating a duplicate
  issue.

Description

Reproducible in:

• version: current
• Node.js version(s): 14
• OS version(s): alpine
• Container base image: node:14-alpine

Steps to reproduce:

I have 2 container which are works like a websocket proxy. So each container is a server and a client to and run the same code.
In my case the first one can handle the incoming traffic and create a new connection to the second container and the second container is trying to forward the traffic to the end.

Code looks like this:
var ws = new WebSocket(url, s.metadata.HTTP["headers"]["sec-websocket-protocol"], options);
Where the options contain the headers.

Expected result:

The traffic flow through the two 2 container with the sub protocol.

Actual result:

The second container cannot forward the traffic due Error: Server sent no subprotocol
In the first container I have these options:

{
  headers: {
    upgrade: 'websocket',
    origin: '192.168.99.1',
    'sec-websocket-version': '13',
    connection: 'upgrade'
  }
}

And the second one works with this:

{
  headers: {
    'sec-websocket-version': '13',
    connection: 'upgrade',
    upgrade: 'websocket',
    origin: '192.168.99.1',
    'sec-websocket-extensions': 'permessage-deflate; client_max_window_bits'
  }
}

I don't modify any of these headers.

I saw that the error is generated because the serverProt is undefined. I tried to insert sec-websocket-protocol field by hand, into the second headers, but I got the same result.

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