WebSocket connections proxying not working (uses http, not ws)

[LeoVasanko]

Describe the bug

I have my vite config setup as follows:

 server: {
    proxy: {
      "/api": {
        target: "ws://localhost:8078",
        changeOrigin: false,
        ws: true
      }

Despite the ws setting and ws:// target, WebSocket requests to /api get processed as HTTP. This is apparent from Connection: keep-alive header sent to backend (should be Connection: Upgrade), and from the fact that actual WebSocket communications don't appear to be passing through even if I force-set the header on backend side so that it still gets processed as a WebSocket connection there. Upgrade: websocket header is present from the request the browser makes to Vite, to the backend.

For the sake of simplicity, testing only what headers get sent to the backend port (8078):

$ nc -l 8078    # new WebSocket("ws://localhost:8077/api") // Vite
GET /api HTTP/1.1
accept: */*
accept-encoding: gzip, deflate, br, zstd
accept-language: en-US,en;q=0.9
host: localhost:8077
referer: http://localhost:8077/register
sec-ch-ua: "Brave";v="137", "Chromium";v="137", "Not/A)Brand";v="24"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"
sec-fetch-dest: empty
sec-fetch-mode: cors
sec-fetch-site: same-origin
sec-gpc: 1
user-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/137.0.0.0 Safari/537.36
Connection: keep-alive

^C⏎
$ nc -l 8078    # new WebSocket("ws://localhost:8078/api") // Directly to nc (backend)
GET /api HTTP/1.1
Host: localhost:8078
Connection: Upgrade
Pragma: no-cache
Cache-Control: no-cache
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/137.0.0.0 Safari/537.36
Accept-Language: en-US,en;q=0.9
Upgrade: websocket
Origin: http://localhost:8077
Sec-WebSocket-Version: 13
Accept-Encoding: gzip, deflate, br, zstd
Sec-WebSocket-Key: ulPCryvhCoCiS9yI3Timiw==
Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits

^C⏎

I am also pretty sure that the same configuration worked with the same project before, using some earlier version of Vite and possibly NPM or Yarn rather than Bun.

Reproduction

https://git.zi.fi/leo/wsproxytest

Steps to reproduce

The repository contains minimal Vite project and server. See the README for further instructions.

System Info

System:
    OS: Linux 6.6 Ubuntu 24.04.2 LTS 24.04.2 LTS (Noble Numbat)
    CPU: (28) x64 Intel(R) Core(TM) i7-14700
    Memory: 115.91 GB / 125.57 GB
    Container: Yes
    Shell: 3.7.0 - /usr/bin/fish
  Binaries:
    npm: 10.9.2 - /mnt/c/Program Files/nodejs/npm
    bun: 1.2.6 - /tmp/bun-node-8ebd5d53d/bun
  npmPackages:
    vite: ^6.3.5 => 6.3.5

Used Package Manager

bun

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[sapphi-red]

It works fine for me on both Windows and WSL. I used Bun 1.2.13 which is newer than yours though.

[LeoVasanko]

I did bun upgrade and bun update now.

I've tried wsproxytest on MacOS and Windows, and it appears to be working. Only when Vite is running in WSL2 I get this problem (WS hangs on (pending) and keep-alive in header).

Vite on Windows, and server.js on Windows or WSL2 works fine. For completeness, I tried also Python WS client on WSL2 (rather than Brave on Windows), but that didn't change anything (fails if Vite is on WSL2, works if Vite is on Windows, does not matter where the server runs).

Any ideas on what more I can debug?

[LeoVasanko]

Tried installing a fresh new Ubuntu on WSL2, no difference, still doesn't work. Changing port numbers makes no difference either.

I also tried on a native Linux desktop (Garuda/Arch), same problem as WSL2 Ubuntu (keep-alive).

I've updated the repo with server.py that also prints the headers and that can be easily run on any platform with uv

[sapphi-red]

Hmm, I tried it again but cannot reproduce it.

Probably you can see what's happening here:

vite/packages/vite/src/node/server/middlewares/proxy.ts

Lines 167 to 211 in 3bf3a8a

	httpServer.on('upgrade', async (req, socket, head) => {
	const url = req.url!
	for (const context in proxies) {
	if (doesProxyContextMatchUrl(context, url)) {
	const [proxy, opts] = proxies[context]
	if (
	opts.ws ||
	opts.target?.toString().startsWith('ws:') ||
	opts.target?.toString().startsWith('wss:')
	) {
	if (opts.bypass) {
	try {
	const bypassResult = await opts.bypass(req, undefined, opts)
	if (typeof bypassResult === 'string') {
	debug?.(`bypass: ${req.url} -> ${bypassResult}`)
	req.url = bypassResult
	return
	}
	if (bypassResult === false) {
	debug?.(`bypass: ${req.url} -> 404`)
	socket.end('HTTP/1.1 404 Not Found\r\n\r\n', '')
	return
	}
	} catch (err) {
	config.logger.error(
	`${colors.red(`ws proxy bypass error:`)}\n${err.stack}`,
	{
	timestamp: true,
	error: err,
	},
	)
	return
	}
	}
	if (opts.rewrite) {
	req.url = opts.rewrite(url)
	}
	debug?.(`${req.url} -> ws ${opts.target}`)
	proxy.ws(req, socket, head)
	return
	}
	}
	}
	})

You can link the local Vite build with
https://github.com/vitejs/vite/blob/main/CONTRIBUTING.md#repo-setup
https://github.com/vitejs/vite/blob/main/CONTRIBUTING.md#testing-vite-against-external-packages