NTP Go Client

main.go

package main

import (
	"bufio"
	"bytes"
	"encoding/binary"
	"fmt"
	"net"
	"os"
	"time"
)

var (
	// 1970-01-01 00:00:00
	ref uint32 = 2208988800
	// The NTP message consists of a 384 bit or 48 byte data structure containing 17 fields.
	buff []byte = make([]byte, 48)
	ntp         = &NTPPacket{
		// 0x23 for 00100011
		// leap indicator: 00
		// version: 100 (version 4)
		// mode: 011 (mode client)
		LIVNMode: 0x21,
	}
)

// NTPPacket is the struct defined in
// https://tools.ietf.org/html/rfc5905#section-7.3
type NTPPacket struct {
	// LIVNMode is the Leap Indicator + Version + Mode (LLVVVMMM)
	LIVNMode       byte
	Stratum        byte
	Poll           byte
	Precision      byte
	RootDelay      uint32
	RootDispersion uint32
	RefID          uint32
	RefTms         uint32
	RefTmf         uint32
	OrigTms        uint32
	OrigTmf        uint32
	RxTms          uint32
	RxTmf          uint32
	TxTms          uint32
	TxTmff         uint32
}

func main() {
	addr, err := net.ResolveUDPAddr("udp", "pool.ntp.org:123")
	if err != nil {
		fmt.Print(err)
		os.Exit(1)
	}

	client, err := net.DialUDP(addr.Network(), nil, addr)
	if err != nil {
		fmt.Print(err)
		os.Exit(1)
	}

	if err := binary.Write(client, binary.BigEndian, *ntp); err != nil {
		fmt.Print(err)
		os.Exit(1)
	}

	if _, err := bufio.NewReader(client).Read(buff); err != nil {
		fmt.Print(err)
		os.Exit(1)
	}

	buf := bytes.NewBuffer(buff)
	// resultat needs to be decoded in network byte order (BigEndian)
	if err := binary.Read(buf, binary.BigEndian, ntp); err != nil {
		fmt.Print(err)
		os.Exit(1)
	}

	sec := ntp.TxTms
	sec -= ref
	t := time.Unix(int64(sec), 0)
	fmt.Println(t)
}