debugging-websocket-issues

Testing & Quality

Use when seeing WebSocket errors like "Invalid frame header", "RSV1 must be clear", or "WS_ERR_UNEXPECTED_RSV_1" - covers multiple WebSocketServer conflicts, compression issues, and raw frame debugging techniques

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Debugging WebSocket Issues

Overview

WebSocket "invalid frame header" errors often stem from raw HTTP being written to an upgraded socket, not actual frame corruption. The most common cause is multiple WebSocketServer instances conflicting on the same HTTP server.

When to Use

Error: Invalid WebSocket frame: RSV1 must be clear
Error: WS_ERR_UNEXPECTED_RSV_1
Error: Invalid frame header
WebSocket connects then immediately disconnects with code 1006
Server logs success but client receives garbage data

Quick Reference

Symptom	Likely Cause	Fix
RSV1 must be clear	Multiple WSS on same server OR compression mismatch	Use `noServer: true` mode
Hex starts with `48545450`	Raw HTTP on WebSocket (0x48='H')	Check for conflicting upgrade handlers
Code 1006, no reason	Abnormal closure, often server-side abort	Check `abortHandshake` calls
Works isolated, fails in app	Something else writing to socket	Audit all upgrade listeners

The Multiple WebSocketServer Bug

Problem

When attaching multiple WebSocketServer instances to the same HTTP server using the server option:

// ❌ BAD - Both servers add upgrade listeners, causing conflicts
const wss1 = new WebSocketServer({ server, path: '/ws' });
const wss2 = new WebSocketServer({ server, path: '/ws/other' });

What happens:

Client connects to /ws
BOTH upgrade handlers fire (Node.js EventEmitter calls all listeners)
wss1 matches path, handles upgrade successfully
wss2 doesn't match, calls abortHandshake(socket, 400)
Raw HTTP/1.1 400 Bad Request written to the now-WebSocket socket
Client receives HTTP text as WebSocket frame data
First byte 0x48 ('H') interpreted as: RSV1=1, opcode=8 → invalid frame

Solution

Use noServer: true and manually route upgrades:

// ✅ GOOD - Single upgrade handler routes to correct server
const wss1 = new WebSocketServer({ noServer: true, perMessageDeflate: false });
const wss2 = new WebSocketServer({ noServer: true, perMessageDeflate: false });

server.on('upgrade', (request, socket, head) => {
  const pathname = new URL(request.url || '', `http://${request.headers.host}`).pathname;

  if (pathname === '/ws') {
    wss1.handleUpgrade(request, socket, head, (ws) => {
      wss1.emit('connection', ws, request);
    });
  } else if (pathname === '/ws/other') {
    wss2.handleUpgrade(request, socket, head, (ws) => {
      wss2.emit('connection', ws, request);
    });
  } else {
    socket.destroy();
  }
});

Debugging Techniques

Raw Frame Inspection

Hook into the socket to see actual bytes received:

ws.on('open', () => {
  const socket = ws._socket;
  const originalPush = socket.push.bind(socket);

  socket.push = function (chunk, encoding) {
    if (chunk) {
      console.log('First 20 bytes (hex):', chunk.slice(0, 20).toString('hex'));
      const byte0 = chunk[0];
      console.log(`FIN: ${!!(byte0 & 0x80)}, RSV1: ${!!(byte0 & 0x40)}, Opcode: ${byte0 & 0x0f}`);

      // Check if it's actually HTTP text
      if (chunk.slice(0, 4).toString() === 'HTTP') {
        console.log('*** RECEIVED RAW HTTP ON WEBSOCKET ***');
      }
    }
    return originalPush(chunk, encoding);
  };
});

Key Hex Patterns

81 = FIN + text frame (normal)
82 = FIN + binary frame (normal)
88 = FIN + close frame (normal)
48545450 = "HTTP" - raw HTTP on WebSocket (bug!)
c1 or similar with bit 6 set = compressed frame (RSV1=1)

Common Mistakes

Mistake	Result	Fix
Multiple WSS with `server` option	HTTP 400 written to socket	Use `noServer: true`
`perMessageDeflate: true` (default in older ws)	RSV1 set on frames	Explicitly set `perMessageDeflate: false`
Not checking upgrade headers	Miss compression negotiation	Log `sec-websocket-extensions` header
Assuming RSV1 error = compression	Could be raw HTTP	Check if bytes decode as ASCII "HTTP"

Verification Checklist

After fixing, verify:

RSV1: false in frame inspection
Extensions header: NONE in upgrade response
No HTTP/1.1 in raw frame data
Messages received match sent payload size
Multiple broadcasts work (test interval sends)