Add a relay server support

[fttx]

Here’s a secure Node.js-based relay server that does the following:

• Allows desktop servers (behind NAT) to register.
• Allows mobile clients to connect via WebSocket using a QR code-based pairing system.
• Forwards messages securely only between matched client-server pairs.
• Supports LAN fallback (users can skip relay and connect directly in trusted networks).

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🔐 Pairing Strategy (QR-based)

• When the desktop app (server) starts, it generates a unique pairing code (UUID or short code).
• It opens a WebSocket to the relay and registers itself with that code.
• The mobile app (client) scans a QR code from the desktop and uses the same code to connect to the relay.
• The relay pairs the two sockets and relays messages securely between them.

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🧠 Tech Stack

• Node.js
• ws for WebSocket
• uuid for secure unique IDs

---

📦 Dependencies

npm install ws uuid

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🌐 relay-server.js

const WebSocket = require('ws');
const { v4: uuidv4 } = require('uuid');

const wss = new WebSocket.Server({ port: 8080 });
console.log("Relay server listening on port 8080");

const servers = new Map();  // pairingCode -> ws
const clients = new Map();  // clientWs -> serverWs

wss.on('connection', (ws) => {
  ws.isAlive = true;

  ws.on('pong', () => ws.isAlive = true);

  ws.on('message', (msg) => {
    try {
      const data = JSON.parse(msg);

      if (data.type === 'register-server') {
        // Register a server with a unique pairing code
        const code = data.code || uuidv4();  // can be random short string
        ws.role = 'server';
        ws.pairingCode = code;
        servers.set(code, ws);
        ws.send(JSON.stringify({ type: 'registered', code }));
        console.log(`Server registered with code: ${code}`);

      } else if (data.type === 'connect-client') {
        const serverWs = servers.get(data.code);
        if (!serverWs) {
          ws.send(JSON.stringify({ type: 'error', message: 'Invalid code' }));
          return;
        }
        ws.role = 'client';
        ws.serverWs = serverWs;
        serverWs.clientWs = ws;
        clients.set(ws, serverWs);
        clients.set(serverWs, ws);
        ws.send(JSON.stringify({ type: 'connected' }));
        serverWs.send(JSON.stringify({ type: 'client-connected' }));
        console.log(`Client connected to server with code: ${data.code}`);

      } else if (data.type === 'message') {
        // Relay message to paired client/server
        const target = clients.get(ws);
        if (target) {
          target.send(JSON.stringify({ type: 'message', payload: data.payload }));
        }
      }

    } catch (err) {
      console.error("Error parsing message", err);
    }
  });

  ws.on('close', () => {
    if (ws.role === 'server') {
      servers.delete(ws.pairingCode);
      if (ws.clientWs) {
        ws.clientWs.close();
        clients.delete(ws.clientWs);
      }
    } else if (ws.role === 'client' && ws.serverWs) {
      ws.serverWs.send(JSON.stringify({ type: 'client-disconnected' }));
      ws.serverWs.clientWs = null;
      clients.delete(ws.serverWs);
    }
    clients.delete(ws);
  });
});

// Optional: ping-pong to clean up dead sockets
setInterval(() => {
  wss.clients.forEach(ws => {
    if (!ws.isAlive) return ws.terminate();
    ws.isAlive = false;
    ws.ping();
  });
}, 10000);

---

🖥️ Desktop App (server) Example

const ws = new WebSocket('wss://yourrelayserver.com');

ws.onopen = () => {
  const code = 'ABCD1234'; // generate or reuse
  ws.send(JSON.stringify({ type: 'register-server', code }));
};

ws.onmessage = (msg) => {
  const data = JSON.parse(msg.data);
  if (data.type === 'client-connected') {
    console.log('Client connected!');
  } else if (data.type === 'message') {
    console.log('Received from client:', data.payload);
  }
};

// To send message:
function sendToClient(payload) {
  ws.send(JSON.stringify({ type: 'message', payload }));
}

---

📱 Mobile App (client) Example

const ws = new WebSocket('wss://yourrelayserver.com');

ws.onopen = () => {
  const code = scannedFromQRCode; // user scans QR
  ws.send(JSON.stringify({ type: 'connect-client', code }));
};

ws.onmessage = (msg) => {
  const data = JSON.parse(msg.data);
  if (data.type === 'message') {
    console.log('Received from server:', data.payload);
  }
};

// To send message:
function sendToServer(payload) {
  ws.send(JSON.stringify({ type: 'message', payload }));
}

---

🧾 Security Recommendations

1. Use TLS (wss://) — Cloud relay must use HTTPS/WSS only.
2. Expire pairing codes — Limit to 10 minutes or until first connection.
3. Add JWT or shared secret — For more secure client-server handshakes.
4. Rate limit and firewall — Protect against abuse and DDoS.

---

Would you like:

• A QR code generator snippet for the desktop app?
• A React Native or Ionic QR scanner example for the mobile app?
• A production-ready Docker setup for the relay?

Let me know how far you want to take it and I’ll build it out for you.

https://chatgpt.com/c/67e13278-29f0-8012-a5ae-515186fcc004