/*
 *
 * (C) 2014 David Lettier.
 *
 * http://www.lettier.com/
 *
 * NTP client.
 *
 * Compiled with gcc version 4.7.2 20121109 (Red Hat 4.7.2-8) (GCC).
 *
 * Tested on Linux 3.8.11-200.fc18.x86_64 #1 SMP Wed May 1 19:44:27 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux.
 *
 * To compile: $ gcc main.c -o ntpClient.out
 *
 * Usage: $ ./ntpClient.out
 *
 */

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

#define NTP_TIMESTAMP_DELTA 2208988800ull

#define LI(packet)   (uint8_t) ((packet.li_vn_mode & 0xC0) >> 6) // (li   & 11 000 000) >> 6
#define VN(packet)   (uint8_t) ((packet.li_vn_mode & 0x38) >> 3) // (vn   & 00 111 000) >> 3
#define MODE(packet) (uint8_t) ((packet.li_vn_mode & 0x07) >> 0) // (mode & 00 000 111) >> 0

void error( char* msg )
{
		perror( msg ); // Print the error message to stderr.

		exit( 0 ); // Quit the process.
}

int main( int argc, char* argv[ ] )
{
	int sockfd, n; // Socket file descriptor and the n return result from writing/reading from the socket.

	int portno = 123; // NTP UDP port number.

	char* host_name = "us.pool.ntp.org"; // NTP server host-name.

	// Structure that defines the 48 byte NTP packet protocol.

	typedef struct
	{

		uint8_t li_vn_mode;      // Eight bits. li, vn, and mode.
														 // li.   Two bits.   Leap indicator.
														 // vn.   Three bits. Version number of the protocol.
														 // mode. Three bits. Client will pick mode 3 for client.

		uint8_t stratum;         // Eight bits. Stratum level of the local clock.
		uint8_t poll;            // Eight bits. Maximum interval between successive messages.
		uint8_t precision;       // Eight bits. Precision of the local clock.

		uint32_t rootDelay;      // 32 bits. Total round trip delay time.
		uint32_t rootDispersion; // 32 bits. Max error aloud from primary clock source.
		uint32_t refId;          // 32 bits. Reference clock identifier.

		uint32_t refTm_s;        // 32 bits. Reference time-stamp seconds.
		uint32_t refTm_f;        // 32 bits. Reference time-stamp fraction of a second.

		uint32_t origTm_s;       // 32 bits. Originate time-stamp seconds.
		uint32_t origTm_f;       // 32 bits. Originate time-stamp fraction of a second.

		uint32_t rxTm_s;         // 32 bits. Received time-stamp seconds.
		uint32_t rxTm_f;         // 32 bits. Received time-stamp fraction of a second.

		uint32_t txTm_s;         // 32 bits and the most important field the client cares about. Transmit time-stamp seconds.
		uint32_t txTm_f;         // 32 bits. Transmit time-stamp fraction of a second.

	} ntp_packet;              // Total: 384 bits or 48 bytes.

	// Create and zero out the packet. All 48 bytes worth.

	ntp_packet packet = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };

	memset( &packet, 0, sizeof( ntp_packet ) );

	// Set the first byte's bits to 00,011,011 for li = 0, vn = 3, and mode = 3. The rest will be left set to zero.

	*( ( char * ) &packet + 0 ) = 0x1b; // Represents 27 in base 10 or 00011011 in base 2.

	// Create a UDP socket, convert the host-name to an IP address, set the port number,
	// connect to the server, send the packet, and then read in the return packet.

	struct sockaddr_in serv_addr; // Server address data structure.
	struct hostent *server;      // Server data structure.

	sockfd = socket( AF_INET, SOCK_DGRAM, IPPROTO_UDP ); // Create a UDP socket.

	if ( sockfd < 0 )
		error( "ERROR opening socket" );

	server = gethostbyname( host_name ); // Convert URL to IP.

	if ( server == NULL )
		error( "ERROR, no such host" );

	// Zero out the server address structure.

	bzero( ( char* ) &serv_addr, sizeof( serv_addr ) );

	serv_addr.sin_family = AF_INET;

	// Copy the server's IP address to the server address structure.

	bcopy( ( char* )server->h_addr, ( char* ) &serv_addr.sin_addr.s_addr, server->h_length );

	// Convert the port number integer to network big-endian style and save it to the server address structure.

	serv_addr.sin_port = htons( portno );

	// Call up the server using its IP address and port number.

	if ( connect( sockfd, ( struct sockaddr * ) &serv_addr, sizeof( serv_addr) ) < 0 )
		error( "ERROR connecting" );

	// Send it the NTP packet it wants. If n == -1, it failed.
	
	sleep(2);

	n = write( sockfd, ( char* ) &packet, sizeof( ntp_packet ) );
	
	sleep(2);

	if ( n < 0 )
		error( "ERROR writing to socket" );

	// Wait and receive the packet back from the server. If n == -1, it failed.

	n = read( sockfd, ( char* ) &packet, sizeof( ntp_packet ) );
	
	sleep(2);

	if ( n < 0 )
		error( "ERROR reading from socket" );

	// These two fields contain the time-stamp seconds as the packet left the NTP server.
	// The number of seconds correspond to the seconds passed since 1900.
	// ntohl() converts the bit/byte order from the network's to host's "endianness".

	packet.txTm_s = ntohl( packet.txTm_s ); // Time-stamp seconds.
	packet.txTm_f = ntohl( packet.txTm_f ); // Time-stamp fraction of a second.

	// Extract the 32 bits that represent the time-stamp seconds (since NTP epoch) from when the packet left the server.
	// Subtract 70 years worth of seconds from the seconds since 1900.
	// This leaves the seconds since the UNIX epoch of 1970.
	// (1900)------------------(1970)**************************************(Time Packet Left the Server)

	time_t txTm = ( time_t ) ( packet.txTm_s - NTP_TIMESTAMP_DELTA );

	// Print the time we got from the server, accounting for local timezone and conversion from UTC time.

	printf( "Time: %s", ctime( ( const time_t* ) &txTm ) );

	return 0;
}