nat-detector.go

package main

import (
	"fmt"
	"net"
	"time"

	"github.com/pion/stun/v3"
)

func mappedAddr(server string) (string, error) {
	conn, err := net.DialTimeout("udp", server, 3*time.Second)
	if err != nil {
		return "", err
	}
	defer conn.Close()

	c, err := stun.NewClient(conn)
	if err != nil {
		return "", err
	}
	defer c.Close()

	var xorAddr stun.XORMappedAddress
	if err = c.Do(stun.MustBuild(stun.TransactionID, stun.BindingRequest), func(res stun.Event) {
		if res.Error != nil {
			err = res.Error
			return
		}
		err = xorAddr.GetFrom(res.Message)
	}); err != nil {
		return "", err
	}
	return xorAddr.String(), nil
}

func main() {
	a, err := mappedAddr("stun.l.google.com:19302")
	if err != nil {
		fmt.Printf("Error getting mapped address: %v\n", err)
		return
	}
	b, err := mappedAddr("stun1.l.google.com:19302")
	if err != nil {
		fmt.Printf("Error getting mapped address: %v\n", err)
		return
	}

	switch {
	case a == "" || b == "":
		fmt.Println("NAT Type F (UDP blocked)")
	case a != b:
		fmt.Println("NAT Type D (Symmetric)")
	default:
		fmt.Println("NAT Type B (Cone-type)")
	}
}