自己寫個簡單的單播SNTP客戶端

一、功能實現

以NTP服務器爲基準，同步網內計算機或嵌入式設備的時間。

二、協議原理

協議：

客戶端向NTP服務器發送請求數據包，NTP服務器迴應一個數據包。

分別記錄客戶端發送請求包時的時刻t1和接收到迴應包的時刻t4，服務器迴應的數據包內包含了服務器接收到請求包的時刻t2和服務器發送迴應包的時刻t3。

t4-t1表示整個消息傳遞過程所需要的時間；

t3-t2表示消息傳遞過程在服務器停留的時間；

(t4-t1)-(t3-t2)是來回路上消耗的時間，如果來回傳輸所用的時間一樣，那麼，單程的時間爲：

t = ( (t4 - t1) - (t3 - t2) )/2;

假定客戶端相對於服務器的時間誤差是dis，則有下列等式：

t2 = t1 + dis + t;

t4 = t3 - dis + t;

則：dis = (( t2 - t1 ) + ( t3 - t4 )) / 2;

根據此差值重新設置時間即可。

說明：

服務器發返回的時間是以1900年爲基準計算的，而linux是以1970年爲基準的，所以需要用到一個常數做轉化：

#define JAN_1970   2208988800UL        /* 1970 - 1900 in seconds */

在計算與服務器的差值時會用到此值。

三、代碼實現

ntp.h

#ifndef __NTP_H
#define __NTP_H

#define JAN_1970    2208988800UL        /* 1970 - 1900 in seconds */
#define NTP_SERVER_NAME "10.13.0.21"    //my windows 7 ntp server
#define NTP_PORT    123

/* some ntp server
time-a.nist.gov
time-b.nist.gov
time-a.timefreq.bldrdoc.gov
time-b.timefreq.bldrdoc.gov
time-c.timefreq.bldrdoc.gov
utcnist.colorado.edu
ntp.tuxfamily.net
time-nw.nist.gov
nist1.datum.com
nist1-ny.glassey.com
nist1.aol-ca.truetime.com
ntp2.belbone.be
ntp.doubleukay.com
ntp0.cs.mu.OZ.AU
time.apple.com
0.pool.ntp.org
1.pool.ntp.org
2.pool.ntp.org
2.europe.pool.ntp.org
3.europe.pool.ntp.org
1.north-america.pool.ntp.org
2.north-america.pool.ntp.org
0.oceania.pool.ntp.org
1.oceania.pool.ntp.org
0.au.pool.ntp.org
3.au.pool.ntp.org
*/

typedef struct  
{
	unsigned char LiVnMode;
	unsigned char Stratum;
	unsigned char Poll;
	unsigned char Precision;
	long int RootDelay;
	long int RootDispersion;
	char RefID[4];
	long int RefTimeInt;
	long int RefTimeFraction;
	long int OriTimeInt;
	long int OriTimeFraction;
	long int RecvTimeInt;
	long int RecvTimeFraction;
	long int TranTimeInt;
	long int TranTimeFraction;
}STNP_Header;

int SYNC_Time(void);

#endif

ntp.c

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netdb.h>
#include "sntp.h"

int gethostIPbyname(char *name)
{
    struct hostent *host=NULL;
    struct in_addr *addr=NULL;

    host = gethostbyname (name);
    if(host->h_addr == NULL)
        return -1;
    addr = (struct in_addr*)host->h_addr;
    
    return addr->s_addr;
}

int GetNTPTime(STNP_Header *H_SNTP)
{
    int sockfd=0;
    struct sockaddr_in server;
    
    bzero((void*)H_SNTP, sizeof(STNP_Header));
    H_SNTP->LiVnMode = 0x1b;
    
    server.sin_family = AF_INET;
    server.sin_addr.s_addr = gethostIPbyname(NTP_SERVER_NAME);
    server.sin_port = htons(NTP_PORT);
    if(-1 == (int)server.sin_addr.s_addr)
        return -1;
    
    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if(sockfd<0)
    {
        perror("sockfd");
        return -1;
    }
    if(sendto(sockfd, (void*)H_SNTP, sizeof(STNP_Header), 0, (struct sockaddr*)&server, sizeof(server))<0)
    {
        perror("sendto");
        return -1;
    }
    
    fd_set r;
    FD_ZERO(&r);
    FD_SET(sockfd, &r);
    struct timeval timeout;
    timeout.tv_sec = 5;
    timeout.tv_usec = 0;
    
    if(1 != select(sockfd+1, &r, NULL, NULL, &timeout))
        return -1;
    if(recv(sockfd, (void*)H_SNTP, sizeof(STNP_Header), 0)<0)
        return -1;
    
    close(sockfd);
    return 0;
}

int SYNC_Time(void)
{
    STNP_Header HeaderSNTP;
    time_t t1,t2,t3,t4,dis;
    
    time(&t1);
    t1+=JAN_1970;
    
    printf("sync time from %s\n", NTP_SERVER_NAME);
    if(GetNTPTime(&HeaderSNTP)<0)
        return -1;
        
    time(&t4);
    t4+=JAN_1970;
    
    t2 = ntohl(HeaderSNTP.RecvTimeInt);
    t3 = ntohl(HeaderSNTP.TranTimeInt);
    
    dis = ( (t2-t1)+(t3-t4) )/2;
    if(dis<=0)
        printf("local time is faster then server %d seconds\n", (int)-dis);
    else
        printf("local time is slower then server %d seconds\n", (int)dis);
    
    struct timeval tv;
    gettimeofday (&tv, 0);
    tv.tv_sec+=dis;
    printf("%s", ctime(&tv.tv_sec));
    
    settimeofday (&tv, NULL);
    
    return 0;
}

main.c

#include <stdio.h>
#include "sntp.h"

int main(void)
{
    if(SYNC_Time()<0)
        printf("sync time Failed!\n");
    printf("sync time Succeed!\n");
    
    return 0;
}

配置windows 7爲NTP服務器