NAT超え over UDP

main.go

package main

import (
	"flag"
	"fmt"
	"net"
	"strings"
	"time"
)

var addr = flag.String("addr", "", "")

func write(dst, src string) (raddr, laddr []byte, err error) {
	var udpaddr *net.UDPAddr
	udpaddr, err = net.ResolveUDPAddr("udp4", dst)
	if err != nil {
		return
	}

	var udpladdr *net.UDPAddr
	if src != "" {
		udpladdr, err = net.ResolveUDPAddr("udp4", src)
		if err != nil {
			return
		}
	}

	var conn *net.UDPConn
	conn, err = net.DialUDP("udp4", udpladdr, udpaddr)
	if err != nil {
		return
	}
	defer conn.Close()

	fmt.Println("Remote Addr:", conn.RemoteAddr().String())
	fmt.Println("Local  Addr:", conn.LocalAddr().String())

	for {
		_, err = conn.Write([]byte(fmt.Sprintf("%v", time.Now())))
		if err != nil {
			return
		}

		rxbuf := make([]byte, 65535)
		var n int
		var ip net.Addr
		n, ip, err = conn.ReadFrom(rxbuf)
		if err != nil {
			return
		}

		fmt.Println(ip.String(), "received:", string(rxbuf[:n]))

		if strings.HasPrefix(string(rxbuf[:n]), "addr:") {
			raddr = rxbuf[5:n]
			laddr = []byte(conn.LocalAddr().String())

			return
		}
	}
}

func main() {
	flag.Parse()

	dstAddr, srcAddr, err := write(*addr, "")
	if err != nil {
		panic(err)
	}

	msg, _, err := write(string(dstAddr), string(srcAddr))
	if err != nil {
		panic(err)
	}

	fmt.Println(string(msg))
}