Ask the public address of a port from STUN server

port-puncher.go

package main

import (
	"fmt"
	"net"
	"os"
	"time"

	"github.com/pion/stun"
)

func main() {
	// Create a socket on a port
	localAddress := os.Args[1]
	fmt.Println("Listening on", localAddress)
	udpLocalAddress, err := net.ResolveUDPAddr("udp", localAddress)
	if err != nil {
		panic(err)
	}
	dialer := net.Dialer{
		LocalAddr: udpLocalAddress,
	}
	conn, err := dialer.Dial("udp", "stun.l.google.com:19302")
	if err != nil {
		panic(err)
	}

	// Creating a "connection" to STUN server.
	c, err := stun.NewClient(conn)
	if err != nil {
		panic(err)
	}

	// In a loop, just ask for IP/Port
	for {
		// Building binding request with random transaction id.
		message := stun.MustBuild(stun.TransactionID, stun.BindingRequest)
		// Sending request to STUN server, waiting for response message.
		if err := c.Do(message, func(res stun.Event) {
			if res.Error != nil {
				panic(res.Error)
			}
			// Decoding XOR-MAPPED-ADDRESS attribute from message.
			var xorAddr stun.XORMappedAddress
			if err := xorAddr.GetFrom(res.Message); err != nil {
				panic(err)
			}
			fmt.Println("Your IP is", xorAddr.IP)
			fmt.Println("Your port is", xorAddr.Port)
		}); err != nil {
			panic(err)
		}

		// Do not get us banned
		time.Sleep(5 * time.Second)
	}
}