Broadcast Wake on LAN packet

wol.go

// Broadcast Wake On LAN packet, see https://en.wikipedia.org/wiki/Wake-on-LAN#Magic_packet
package main

import (
	"fmt"
	"log"
	"net"
	"os"
	"strings"
	"time"
)

var (
	// Wake-on-LAN packet body preamble
	wolPreamble = []byte{0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}
)

// Supports either - or : separators
func parseMacArg(mac string) ([]byte, error) {
	m := strings.Replace(mac, "-", ":", -1)

	a := make([]byte, 6)
	n, err := fmt.Sscanf(m, "%2x:%2x:%2x:%2x:%2x:%2x", &a[0], &a[1], &a[2], &a[3], &a[4], &a[5])
	if n < 6 || err != nil {
		return nil, err
	}

	return a, nil
}

func main() {
	log.SetFlags(0)

	args := os.Args[1:]
	if len(args) < 1 {
		log.Fatal("No MAC address provided")
	}

	targetMAC, err := parseMacArg(args[0])
	if err != nil {
		log.Fatal(err)
	}

	// Assemble the magic packet
	packet := make([]byte, 0, 102)
	packet = append(packet, wolPreamble...)
	for i := 0; i < 16; i++ {
		packet = append(packet, targetMAC...)
	}

	// Broadcast packet to all 3 ports just to be sure
	// OS X will not allow a connection to port 0
	for _, port := range []int{0, 7, 9} {
		conn, err := net.DialUDP("udp4", nil, &net.UDPAddr{
			IP:   net.IPv4bcast,
			Port: port,
		})
		if err != nil {
			log.Printf("Error opening UDP: %v\n", err)
			continue
		}
		defer conn.Close()

		// Write the packet
		n, err := conn.Write(packet)
		if err != nil {
			log.Fatalf("Failed to send packet: %v\n", err)
		}
		if n != 102 {
			fmt.Printf("Expected to send 102 bytes, sent %d", n)
		}

		<-time.After(250 * time.Millisecond)
	}
}