udpsockopt.go

package main

import (
	"encoding/binary"
	"fmt"
	"log"
	"net"
	"syscall"
	"time"
)

func main() {
	addrChan := make(chan *net.UDPAddr)
	go func() {
		log.Fatal(runServer(addrChan))
	}()
	if err := runClient(<-addrChan); err != nil {
		log.Fatal(err)
	}
}

func setSockOpt(conn *net.UDPConn) error {
	rawConn, err := conn.SyscallConn()
	if err != nil {
		return err
	}
	return rawConn.Control(func(fd uintptr) {
		if err := syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_PKTINFO, 1); err != nil {
			panic(err)
		}
	})
}

func runServer(addrChan chan<- *net.UDPAddr) error {
	laddr, err := net.ResolveUDPAddr("udp4", "localhost:0")
	if err != nil {
		return err
	}
	conn, err := net.ListenUDP("udp4", laddr)
	if err != nil {
		return err
	}
	if err := setSockOpt(conn); err != nil {
		return err
	}
	addrChan <- conn.LocalAddr().(*net.UDPAddr)
	for {
		b := make([]byte, 1000)
		oob := make([]byte, 0, 1000)
		n, oobn, flags, addr, err := conn.ReadMsgUDP(b, oob)
		if err != nil {
			return err
		}
		fmt.Println("Received packet:")
		fmt.Printf("\tn: %d, oobn: %d, flags: %d, addr: %s\n", n, oobn, flags, addr)
		fmt.Printf("\toob: %#x\n", oob[:oobn])
	}
}

func runClient(raddr *net.UDPAddr) error {
	laddr, err := net.ResolveUDPAddr("udp4", "localhost:0")
	if err != nil {
		return err
	}
	conn, err := net.ListenUDP("udp4", laddr)
	if err != nil {
		return err
	}
	if err := setSockOpt(conn); err != nil {
		return err
	}
	for i := 0; i < 5; i++ {
		b := make([]byte, 8)
		binary.BigEndian.PutUint64(b, uint64(i))
		if _, _, err := conn.WriteMsgUDP(b, nil, raddr); err != nil {
			return err
		}
		time.Sleep(200 * time.Millisecond)
	}
	return nil
}