ntp.c

NTP协议的C语言实现程序。程序实现功能：更新本地主机时间

/* ntp.c */

#include <sys/socket.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/un.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <netinet/in.h>
#include <string.h>
#include <netdb.h>

#define NTP_PORT			123
#define TIME_PORT	        	37
#define NTP_SERVER_IP			"210.72.145.44"
#define NTP_PORT_STR			"123"
#define NTPV1   			"NTP/V1"
#define NTPV2   			"NTP/V2"
#define NTPV3   			"NTP/V3"
#define NTPV4   			"NTP/V4"
#define TIME    			"TIME/UDP"

#define NTP_PCK_LEN 48
#define LI 0
#define VN 3
#define MODE 3
#define STRATUM 0
#define POLL 4 
#define PREC -6

#define JAN_1970	0x83aa7e80  /* 从1900年到1970年之间的时间秒数 */
#define NTPFRAC(x)	(4294 * (x) + ((1981 * (x)) >> 11))
#define USEC(x)		(((x) >> 12) - 759 * ((((x) >> 10) + 32768) >> 16))

typedef struct _ntp_time 
{
    unsigned int coarse;
    unsigned int fine;
} ntp_time;

struct ntp_packet
{
	unsigned char leap_ver_mode;
	unsigned char startum; 
	char poll;
	char precision;
	int  root_delay;
	int  root_dispersion;
	int reference_identifier;
	ntp_time reference_timestamp;
	ntp_time originage_timestamp;
	ntp_time receive_timestamp;
	ntp_time transmit_timestamp;
};

char protocol[32];

/*构建NTP协议包*/
int construct_packet(char *packet)
{
	char version = 1;
	long tmp_wrd;
	int port;
	time_t timer;

	strcpy(protocol, NTPV3);
		
	/*判断协议版本*/
	if(!strcmp(protocol, NTPV1)||!strcmp(protocol, NTPV2)||!strcmp(protocol, NTPV3)||!strcmp(protocol, NTPV4))
	{
		memset(packet, 0, NTP_PCK_LEN);
		port = NTP_PORT;
		/*设置16字节的包头*/
		version = protocol[6] - 0x30;
		tmp_wrd = htonl((LI << 30)|(version << 27)|(MODE << 24)|(STRATUM << 16)|(POLL << 8)|(PREC & 0xff));
		memcpy(packet, &tmp_wrd, sizeof(tmp_wrd));
		
		/*设置Root Delay、Root Dispersion和Reference Indentifier */
		tmp_wrd = htonl(1<<16);
		memcpy(&packet[4], &tmp_wrd, sizeof(tmp_wrd));
		memcpy(&packet[8], &tmp_wrd, sizeof(tmp_wrd));
		
		/*设置Timestamp部分*/
		time(&timer);
		tmp_wrd = htonl(JAN_1970 + (long)timer);
		memcpy(&packet[40], &tmp_wrd, sizeof(tmp_wrd)); /*Transmit Timestamp coarse*/
		tmp_wrd = htonl((long)NTPFRAC(timer));
		memcpy(&packet[44], &tmp_wrd, sizeof(tmp_wrd)); /*Transmit Timestamp fine*/
		return NTP_PCK_LEN;
	}
	else if (!strcmp(protocol, TIME))/* "TIME/UDP" */
	{
		port = TIME_PORT;
		memset(packet, 0, 4);
		return 4;
	}
	
	return 0;
}

/*获取NTP时间*/
int get_ntp_time(int sk, struct addrinfo *addr, struct ntp_packet *ret_time)
{
	fd_set pending_data;
	struct timeval block_time;
	char data[NTP_PCK_LEN * 8];
	int  packet_len, data_len = addr->ai_addrlen, count=0, result, i, re;	

	if (!(packet_len = construct_packet(data)))
	{
		return 0;
	}
	
	/*客户端给服务器端发送NTP协议数据包*/
	if ((result = sendto(sk, data, packet_len, 0, addr->ai_addr, data_len)) < 0)
	{
		perror("sendto");
		return 0;
	}
	
	/*调用select函数，并设定超时时间为1s*/
	FD_ZERO(&pending_data);
	FD_SET(sk, &pending_data);
	block_time.tv_sec=10;
	block_time.tv_usec=0;

	if (select(sk + 1, &pending_data, NULL, NULL, &block_time) > 0)
	{
		/*接收服务器端的信息*/
		if ((count = recvfrom(sk, data, NTP_PCK_LEN * 8, 0, addr->ai_addr, &data_len)) < 0)
		{
			perror("recvfrom");
			return 0;
		}
		
		if (protocol == TIME)
		{
			memcpy(&ret_time->transmit_timestamp, data, 4);
			return 1;
		}
		else if (count < NTP_PCK_LEN)
		{
			return 0;
		}
		
		ret_time->leap_ver_mode = ntohl(data[0]);
		ret_time->startum = ntohl(data[1]);
		ret_time->poll = ntohl(data[2]);
		ret_time->precision = ntohl(data[3]);
		ret_time->root_delay = ntohl(*(int*)&(data[4]));
		ret_time->root_dispersion = ntohl(*(int*)&(data[8]));
		ret_time->reference_identifier = ntohl(*(int*)&(data[12]));
		ret_time->reference_timestamp.coarse = ntohl(*(int*)&(data[16]));
		ret_time->reference_timestamp.fine = ntohl(*(int*)&(data[20]));
		ret_time->originage_timestamp.coarse = ntohl(*(int*)&(data[24]));
		ret_time->originage_timestamp.fine = ntohl(*(int*)&(data[28]));
		ret_time->receive_timestamp.coarse = ntohl(*(int*)&(data[32]));
		ret_time->receive_timestamp.fine = ntohl(*(int*)&(data[36]));
		ret_time->transmit_timestamp.coarse = ntohl(*(int*)&(data[40]));
		ret_time->transmit_timestamp.fine = ntohl(*(int*)&(data[44]));

		return 1;		
	} /* end of if select */	

	return 0;
}

int set_local_time(struct ntp_packet * pnew_time_packet)
{
	struct timeval tv;
	tv.tv_sec = pnew_time_packet->transmit_timestamp.coarse - JAN_1970;
	tv.tv_usec = USEC(pnew_time_packet->transmit_timestamp.fine);
	return settimeofday(&tv, NULL);
}

int main()
{
	int sockfd, rc;
	struct addrinfo hints, *res = NULL;
	struct ntp_packet new_time_packet;
		
	memset(&hints, 0, sizeof(hints));
	hints.ai_family = AF_UNSPEC;
	hints.ai_socktype = SOCK_DGRAM;
	hints.ai_protocol = IPPROTO_UDP;
	
	/*调用getaddrinfo函数，获取地址信息*/
	rc = getaddrinfo(NTP_SERVER_IP, NTP_PORT_STR, &hints, &res);
	if (rc != 0) 
	{
		perror("getaddrinfo");
		return 1;
	}
	
	sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
	if (sockfd <0 ) 
	{
		perror("socket");
		return 1;
	}
		
	/*调用取得NTP时间函数*/
	if (get_ntp_time(sockfd, res, &new_time_packet))
	{
		/*调整本地时间*/
		if (!set_local_time(&new_time_packet))
		{
			printf("NTP client success!\n");
		}
	}
	
	close(sockfd);
	return 0;
}