Firewall-Friendly Data Push

TL;DR

Pushing data to a client behind a firewall is tricky. WebSockets are often the best solution, but if those aren’t possible, consider long polling or Server-Sent Events (SSE). If even those are blocked, you might need to rely on the client initiating requests regularly (polling) – less efficient, but reliable.

Understanding the Problem

Firewalls block unsolicited incoming connections. Your client is likely behind a firewall that prevents direct pushes from your server. The goal is to find a method where the connection originates from the client, or uses ports/protocols the firewall allows.

Solution Steps

1. Check Existing Ports and Protocols: Before anything else, confirm what ports are open on the client’s firewall. Common ones include:
   • 80 (HTTP): Usually open for web browsing.
   • 443 (HTTPS): Usually open for secure web browsing.
2. WebSockets (Recommended): WebSockets provide a persistent, two-way communication channel over HTTP(S).
   • The initial connection is made via HTTP, so it usually gets through firewalls.
   • Once established, data can flow both ways in real-time.
   • Implementation: You’ll need a WebSocket server (e.g., using Node.js with the ws package, Python with websockets). The client connects to this server.

     // Example Node.js WebSocket Server
     const WebSocket = require('ws');
     
     const wss = new WebSocket.Server({ port: 8080 });
     
     wss.on('connection', ws => {
       console.log('Client connected');
       ws.send('Welcome to the server!');
     });
     

3. Long Polling: If WebSockets are blocked, long polling is a good alternative.
   • The client makes an HTTP request to your server.
   • Your server holds the connection open until it has new data for the client.
   • Once data is available, the server sends a response.
   • The client immediately re-establishes the connection.

     // Example (simplified) PHP Long Polling Server
     <?php
       while (true) {
         if ($newDataAvailable) {
           header('Content-Type: text/plain');
           echo $newData;
           break;
         } else {
           sleep(1);
         }
       }
     ?>

4. Server-Sent Events (SSE): SSE allows the server to push updates to the client over HTTP.
   • The client makes an HTTP request.
   • Your server sends a stream of data updates until the connection is closed.

     // Example Node.js Server-Sent Events
     const http = require('http');
     
     const server = http.createServer((req, res) => {
       res.setHeader('Content-Type', 'text/event-stream');
       res.setHeader('Cache-Control', 'no-cache');
     
       setInterval(() => {
         res.write('data: Hello from the server!nn');
       }, 1000);
     });
     
     server.listen(8080);
     

5. Regular Polling (Last Resort): If all else fails, the client must periodically request data from your server.
   • The client makes HTTP requests at fixed intervals.
   • This is less efficient than WebSockets, long polling or SSE because of overhead and potential delays.
   • Implementation: Use JavaScript’s setInterval to make regular AJAX calls.

     // Example JavaScript Polling
     setInterval(() => {
       fetch('/data')
         .then(response => response.json())
         .then(data => console.log(data));
     }, 5000); // Poll every 5 seconds
     

6. Consider a Message Queue: For more complex scenarios, use a message queue (e.g., RabbitMQ, Kafka).
   • The client can subscribe to messages from the queue.
   • This decouples your server from the client and provides greater scalability and reliability.

Important Considerations

• Security: Always use HTTPS for secure communication, especially when transmitting sensitive data.
• Firewall Configuration (Client-Side): If possible, work with the client to configure their firewall to allow connections on the necessary ports and protocols.
• Error Handling: Implement robust error handling in both your server and client code to deal with connection issues and data transmission failures.