package main

import (
	"bytes"
	"flag"
	"log"
	"net"
	"os"
	"time"
)

var targetAddr = flag.String("tgt", "127.0.0.1", "target address")

func main() {
	flag.Parse()
	id := os.Getpid() & 0xffff
	exchangeICMPEcho(*targetAddr, newICMPEchoRequest(id, 1, 128, []byte("Go Go Gadget Ping!!!")))
}

func exchangeICMPEcho(target string, echo []byte) {
	c, err := net.ListenPacket("ip4:icmp", "") // talk with anybody who interest in specified protocol
	if err != nil {
		log.Fatal(err)
	}
	c.SetDeadline(time.Now().Add(300 * time.Millisecond))
	defer c.Close()

	taddr, err := net.ResolveIPAddr("ip4", target)
	if err != nil {
		log.Fatal(err)
	}
	_, err = c.WriteTo(echo, taddr)
	if err != nil {
		log.Fatal(err)
	}

	reply := make([]byte, 128)
	for {
		_, raddr, err := c.ReadFrom(reply)
		if err != nil {
			log.Fatal(err)
		}
		if !raddr.(*net.IPAddr).IP.Equal(taddr.IP) {
			continue
		}
		if reply[0] != ICMP4_ECHO_REPLY {
			continue
		}
		xid, xseqnum := parseICMPEchoReply(echo)
		rid, rseqnum := parseICMPEchoReply(reply)
		if rid != xid || rseqnum != xseqnum {
			log.Fatalf("MISMATCH: id = %v, seqnum = %v; expected id = %v, seqnum = %v", rid, rseqnum, xid, xseqnum)
		}
		log.Println("done")
		break
	}
}

const (
	ICMP4_ECHO_REQUEST = 8
	ICMP4_ECHO_REPLY   = 0
)

func newICMPEchoRequest(id, seqnum, msglen int, filler []byte) []byte {
	b := newICMPInfoMessage(id, seqnum, msglen, filler)
	b[0] = ICMP4_ECHO_REQUEST

	// calculate ICMP checksum
	cklen := len(b)
	s := uint32(0)
	for i := 0; i < cklen-1; i += 2 {
		s += uint32(b[i+1])<<8 | uint32(b[i])
	}
	if cklen&1 == 1 {
		s += uint32(b[cklen-1])
	}
	s = (s >> 16) + (s & 0xffff)
	s = s + (s >> 16)
	// place checksum back in header; using ^= avoids the
	// assumption the checksum bytes are zero
	b[2] ^= uint8(^s & 0xff)
	b[3] ^= uint8(^s >> 8)

	return b
}

func newICMPInfoMessage(id, seqnum, msglen int, filler []byte) []byte {
	b := make([]byte, msglen)
	copy(b[8:], bytes.Repeat(filler, (msglen-8)/len(filler)+1))
	b[0] = 0                    // type
	b[1] = 0                    // code
	b[2] = 0                    // checksum
	b[3] = 0                    // checksum
	b[4] = uint8(id >> 8)       // identifier
	b[5] = uint8(id & 0xff)     // identifier
	b[6] = uint8(seqnum >> 8)   // sequence number
	b[7] = uint8(seqnum & 0xff) // sequence number
	return b
}

func parseICMPEchoReply(b []byte) (id, seqnum int) {
	id = int(b[4])<<8 | int(b[5])
	seqnum = int(b[6])<<8 | int(b[7])
	return
}