UDP hole punching can't connect externally

Question

I have previously made a simple peer to peer chat program using UDP hole punching that worked and now I am trying to do something similar but within a game made using libGDX. The game itself runs fine and the connections work on a LAN but I have difficult trying with external connections. I understand how UDP hole punching works as follows:

If both person A and B know each other's IP addresses and ports then:

1. A sends a UDP packet to B which punches a hole in A's NAT but is dropped by B's firewall
2. A waits for a reply
3. B sends a UDP packet to A which punches a hole in its own NAT and goes through A's firewall
4. B waits for a reply
5. A receives B's initial message and sends a second message to B
6. B receives A's message

My networking code is in one class:

private boolean connected;
private DatagramSocket socket;
private DatagramPacket packet;

private InetAddress peerIP;

public NetworkManager(InetAddress peerIP){
    this.peerIP = peerIP;
    log("Created with peer ip: " + peerIP.getHostAddress());
    connected = false;
}

@Override
public void run(){
    try{
        log("Setting up socket");
        socket = new DatagramSocket(Constants.CLIENT_PORT);
        log("Socket successfully setup");

        //Punch hole
        log("Punching UDP Hole");
        sendBytes("one");

        //Receive
        log("Waiting for peer reply");
        receiveBytes(3);

        //Send second message 
        if(Arrays.equals(packet.getData(), "one".getBytes()) ){
            sendBytes("two");
        }

    }catch(Exception e){
        log("Error connecting to peer");
        return;
    } 

    log("Successfully connected");
    connected = true;
}


private synchronized void receiveBytes(int length) throws Exception {
    packet = new DatagramPacket(new byte[length], length);
    log("Receiving " + length + " bytes...");
    socket.receive(packet);
    log("Received bytes " + packet.getData()+ " from " + packet.getAddress());
}

private synchronized void sendBytes(String s) throws Exception {
    byte[] sendBytes = s.getBytes();
    packet = new DatagramPacket(sendBytes, sendBytes.length, peerIP, Constants.CLIENT_PORT);

    log("Sending " + sendBytes.length + " bytes...");

    socket.send(packet);

    log("Bytes sent");
}

Answer 1

If both A and B are in the same network, i.e if they are in the same LAN, A can connect to B and vice versa. You don't need UDP Hole punching here.

But if they are in different networks, or behind different NATs, you can try to achieve a direct connection using UDP hole punching. Here you need a mediator server. And A and B need to ping mediator server, which will pass the IP and port number to the other corresponding clients.

Most important point to note here is that not all NATs supports hole punching. It is upto the NAT implementation, which is mostly not available in public domain. So even if you use UDP Hole Punching, you need a relay server as a fallback. In case it doesn't get hole punched, the relay server can pass the messages to each other.

http://www.brynosaurus.com/pub/net/p2pnat/

This link explains UDP Hole Punching architecture.