//p21.2.c ping程序實現#include <stdio.h>#include <stdlib.h>#include <string.h> #include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>#include <unistd.h>#include <sys/types.h>#include <netinet/ip_icmp.h>#define WAIT_TIME 3#define BUFFER_SIZE 1024u_short cal_cksum(const u_short *addr, register int len, u_short csum){	register int nleft = len;	const u_short *w = addr;	register u_short answer;	register int sum = csum;		/**  使用32位累加器實現校驗碼的計算	*  聯系對16位數進行累加，最后將高16位與低16位進行累加	*/		while (nleft > 1)  {			sum += *w++;			nleft -= 2;		}				/* 如果最后剩余8位，補齊到16位（補零，同時注意網絡字節與	主機字節間的轉換） */		if (nleft == 1)			sum += htons(*(u_char *)w << 8);				/** 將高低16位相加，以消除累加時進位問題*/		sum = (sum >> 16) + (sum & 0xffff);	/* 高低16位相加 */		sum += (sum >> 16);			/* 消除高低位相加的進位問題 */		answer = ~sum;				/* 取反 */		return (answer);}//ICMP報頭解析代碼int unpack_packet_header(char *buf,int len,struct sockaddr_in *from){       int i,iphdrlen;        struct ip *ip;        struct icmp *icmp;        struct timeval *tvsend;	struct timeval tvrecv;        double rtt;        ip=(struct ip *)buf;                //ip報頭長度= ip_hl*4        iphdrlen=ip->ip_hl*4;            //得到ICMP報文信息        icmp=(struct icmp *)(buf+iphdrlen);         //icmp報文總長度        len-=iphdrlen;         //如果長度小于要求，退出程序                   if( len<8)                        {       printf("ICMP packets\'s length is less than 8\n");                return -1;        }        if( icmp->icmp_type==ICMP_ECHOREPLY)        {       tvsend=(struct timeval *)icmp->icmp_data;		gettimeofday(&tvrecv,NULL);                //得到接收時間，并計算兩者時間差                if((tvrecv.tv_usec-=tvsend->tv_usec)<0){			tvrecv.tv_sec--;			tvrecv.tv_usec+=1000000;  		}				tvrecv.tv_sec-=tvsend->tv_sec;                rtt=tvrecv.tv_sec*1000+tvrecv.tv_usec/1000;                 //顯示時間信息                printf("%d byte from %s: icmp_seq=%u ttl=%d rtt=%.3f ms\n",                        len,                        inet_ntoa(from->sin_addr),                        icmp->icmp_seq,                        ip->ip_ttl,                        rtt);        } else                    return -1;}		intmain(int argc, char * argv[]){    int sock;    struct sockaddr_in sin;    unsigned short local_port;    unsigned char protocol;    char * buffer, * buffer2, * dnsdata;    struct iphdr * ip_header;    struct icmphdr * icmp_header;    char * remote_ip_str;    unsigned short buffer_size, buffer_size2;    int tmp, len;    short randomseq;     //判斷參數是否符合程序運行要求    if (argc != 3) {        fprintf(stderr, "USAGE: %s port destination\n", argv[0]);        return 1;    }     protocol = IPPROTO_ICMP;     local_port = atoi(argv[1]);    remote_ip_str = argv[2];    //創建通信所需套接字，注意套接字類型為原始套接字    if ((sock = socket(PF_INET, SOCK_RAW, protocol)) < 0) {         perror("socket");        exit(1);    }    memset(& sin, 0, sizeof(sin));    sin.sin_port = htons(local_port);     //完成套接字與端口、地址信息的綁定    if ((bind(sock, (struct sockaddr *)& sin, sizeof(sin))) < 0) {        perror("bind");        exit(1);    }     tmp = 1;    setsockopt(sock, 0, IP_HDRINCL, & tmp, sizeof(tmp));     //申請內存空間，并填寫包頭信息    buffer_size = sizeof(struct iphdr) + sizeof(struct icmphdr)		+ sizeof(struct timeval);    buffer_size2 = BUFFER_SIZE;    buffer = (char *) malloc(buffer_size);    buffer2 = (char *) malloc(buffer_size2);     memset(buffer, 0, sizeof(buffer));     memset(& sin, 0, sizeof(sin));    sin.sin_family = AF_INET;    sin.sin_addr.s_addr = inet_addr(remote_ip_str);     ip_header = (struct iphdr *) buffer;    ip_header->ihl = 5;    ip_header->version = 4;    ip_header->tos = 0;    ip_header->tot_len = htons(buffer_size);    ip_header->id = rand();    ip_header->ttl = 64;    ip_header->frag_off = 0x40;    ip_header->protocol = protocol;    ip_header->check = 0;     ip_header->daddr = inet_addr(remote_ip_str);    ip_header->saddr = 0;      icmp_header = (struct icmphdr *) (ip_header + 1);    icmp_header->type = ICMP_ECHO;    icmp_header->code = 0;    icmp_header->un.echo.id = htons(local_port);    icmp_header->un.echo.sequence = 0;	struct timeval *tp = (struct timeval *)&buffer[28];	gettimeofday(tp, NULL);		icmp_header->checksum = cal_cksum((const u_short *) icmp_header, sizeof(struct icmphdr) + sizeof(struct timeval), 0);				      //發送該IP數據包    if (sendto(sock, buffer, buffer_size, 0, (struct sockaddr *) &sin,sizeof(struct sockaddr_in)) < 0)     {        perror("sendto");	return 1;    }     printf("send ICMP inquery\n"); len = sizeof(sin);    alarm(WAIT_TIME);//接收對方回應的數據，并給出提示    if (recvfrom(sock, buffer2, buffer_size2, 0, (struct sockaddr *) &sin, &len) < 0) {        perror("recvfrom");        return 1;    }    printf("received Destionation response\n");   //顯示報文信息    unpack_packet_header(buffer2,buffer_size2,(struct sockaddr_in *)&sin);     close(sock);    return 0;}