When working with WebRTC applications, testing STUN server availability is more nuanced than simple TCP connectivity checks. Google's public STUN servers (stun.l.google.com and its variants) implement the STUN protocol over UDP, not TCP.

Attempting to telnet to port 19302 won't work because:

• STUN operates over UDP (telnet uses TCP)
• The protocol requires specific binary message formats
• Servers may drop unauthenticated TCP connections

Method 1: Using netcat with STUN binding request

echo -ne "\x00\x01\x00\x00\x21\x12\xA4\x42\x00\x00\x00\x00\x00\x00\x00\x00" | nc -u -w 2 stun.l.google.com 19302 | hexdump -C

This sends a basic STUN binding request and displays the raw response.

Method 2: JavaScript WebRTC Test

const testStun = async (server) => {
  const pc = new RTCPeerConnection({
    iceServers: [{ urls: stun:${server} }]
  });
  
  return new Promise((resolve) => {
    pc.onicecandidate = (e) => {
      if (!e.candidate) return;
      console.log(STUN ${server} working. Got candidate:, e.candidate);
      resolve(true);
    };
    
    setTimeout(() => resolve(false), 3000);
    pc.createDataChannel('');
    pc.createOffer().then(offer => pc.setLocalDescription(offer));
  });
};

// Usage:
testStun('stun.l.google.com:19302').then(console.log);

Method 3: Python STUN Client

import socket
import struct

def check_stun(host, port=19302):
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.settimeout(2)
    
    # STUN binding request header
    msg = struct.pack('!HHI12s',
        0x0001,  # message type
        0x0000,  # message length
        0x2112A442,  # magic cookie
        b'\x00'*12  # transaction ID
    )
    
    try:
        s.sendto(msg, (host, port))
        data = s.recv(1024)
        return len(data) > 0
    except:
        return False
    finally:
        s.close()

print(check_stun('stun.l.google.com'))

If Google's servers appear unresponsive, consider these alternatives:

• stun.services.mozilla.com:3478
• stun.voipbuster.com:3478
• stun.ekiga.net:3478

For production applications:

• Always implement fallback STUN servers
• Consider running your own STUN server (coturn, restund)
• Monitor STUN server availability periodically
• Handle NAT traversal failures gracefully

When working with WebRTC applications, testing STUN server availability goes beyond simple telnet checks. Google's public STUN servers (stun.l.google.com and its variants) implement the STUN protocol (RFC 5389) which requires proper protocol-level communication rather than raw TCP connections.

A telnet attempt to port 19302 or 3478 might timeout because:

1. STUN servers expect proper STUN protocol messages
2. Many public STUN servers implement rate limiting
3. The server may drop non-STUN traffic immediately

Here are three reliable ways to test STUN server functionality:

1. Using Node.js STUN Client

const stun = require('stun');

async function testStunServer(server, port) {
  try {
    const response = await stun.request(stun:${server}:${port});
    console.log(Server ${server}:${port} response:, response);
    return response && response.getType() === stun.constants.STUN_BINDING_RESPONSE;
  } catch (err) {
    console.error(STUN test failed for ${server}:${port}:, err.message);
    return false;
  }
}

// Test all Google STUN servers
const servers = [
  'stun.l.google.com',
  'stun1.l.google.com',
  'stun2.l.google.com',
  'stun3.l.google.com',
  'stun4.l.google.com'
];

servers.forEach(server => {
  testStunServer(server, 19302);
});

2. Command Line with stunclient

# On Linux (install via package manager)
stunclient stun.l.google.com 19302

# Expected successful output:
# Binding test: success
# Local address: A.B.C.D:PORT
# Mapped address: W.X.Y.Z:PORT

3. Browser-based WebRTC Test

// Simple browser test
const testConnection = () => {
  const pc = new RTCPeerConnection({
    iceServers: [{ urls: 'stun:stun.l.google.com:19302' }]
  });
  
  pc.createDataChannel('test');
  pc.onicecandidate = (e) => {
    if (e.candidate) {
      console.log('STUN server working - got candidate:', e.candidate);
    }
  };
  pc.createOffer()
    .then(offer => pc.setLocalDescription(offer));
};

testConnection();

For production applications, consider implementing:

• Scheduled pings using the methods above
• Fallback to alternative STUN servers
• Local caching of successful server responses

If Google's servers appear unavailable, consider these alternatives:

const backupStunServers = [
  'stun.voipbuster.com:3478',
  'stun.voipstunt.com:3478',
  'stun.xten.com:3478',
  'stun.sipgate.net:3478'
];