Node.js TCP Port Scanner - Using net.Socket with Timeout

Problem

Need to check if a port is open on a host. ping only works at ICMP level (no port info), and this is too simple for an external library.

Solution

const net = require('net');

async function checkPort(host, port, timeout = 500) {
  return new Promise((resolve) => {
    const socket = new net.Socket();

    const timer = setTimeout(() => {
      socket.destroy();
      resolve({ host, port, open: false });
    }, timeout);

    socket.on('connect', () => {
      clearTimeout(timer);
      socket.destroy();
      resolve({ host, port, open: true });
    });

    socket.on('error', () => {
      clearTimeout(timer);
      resolve({ host, port, open: false });
    });

    socket.connect(port, host);
  });
}

// Usage: scan multiple ports concurrently
async function scanPorts(host, ports) {
  const results = await Promise.all(
    ports.map(port => checkPort(host, port))
  );
  return results.filter(r => r.open);
}

Key Points

• Always resolve instead of reject — a closed port is a valid result, not an error. This keeps Promise.all from short-circuiting.
• 500ms timeout is sufficient for LAN. For internet hosts, increase to 2000-3000ms.
• Always call socket.destroy(). Without it, sockets stay open and you’ll exhaust file descriptors when scanning hundreds of ports.