自己动手学TCP/IP--ICMP（ping报文）
ping是ICMP的一种，是用来检查网络是否通畅或者网络连接速度的命令。
报文长度（98bytes）= 以太网头（14bytes）+ IP头（20bytes）+ICMP头（8）+ ICMP数据内容（56字节）
ICMP回显请求报文类型是8（ICMP_ECHO），标识符字段是进程的ID，用于接受应答包的处理，序列号是不断加1，ICMP数据内容选项则包含了发送包时系统的时间。
ICMP回显应答报文类型是1（ICMP_ECHOREPLY），标识符字段是发送进程的ID号，ICMP数据内容保存的是发送请求的时间，ping程序收到ICMP的回显应答包时，获取当前系统时间，然后与回显包中数据内容（发送的时间）相减，算出时间差。
下面是一张wireshark抓到的ping请求报文
下面是模拟简单的ping功能的代码，详细的代码可以查看ping.c
#include <stdio.h>#include <signal.h>#include <arpa/inet.h>#include <sys/types.h>#include <sys/socket.h>#include <unistd.h>#include <netinet/in.h>#include <netinet/ip.h>#include <netinet/ip_icmp.h>#include <netdb.h>#include <setjmp.h>#include <errno.h>#define PACKET_SIZE 4096#define MAX_WAIT_TIME 5#define MAX_NO_PACKETS 3char sendpacket[PACKET_SIZE];char recvpacket[PACKET_SIZE];int sockfd,datalen=56;int nsend=0,nreceived=0;struct sockaddr_in dest_addr;pid_t pid;struct sockaddr_in from;struct timeval tvrecv;void statistics(int signo);unsigned short cal_chksum(unsigned short *addr,int len);int pack(int pack_no);void send_packet(void);void recv_packet(void);int unpack(char *buf,int len);void tv_sub(struct timeval *out,struct timeval *in);void statistics(int signo){ printf("\n--------------------PING statistics-------------------\n"); printf("%d packets transmitted, %d received , %%%d lost\n",nsend,nreceived, (nsend-nreceived)/nsend*100); close(sockfd); exit(1);}/*校验和算法*/unsigned short cal_chksum(unsigned short *addr,int len){ int nleft=len; int sum=0; unsigned short *w=addr; unsigned short answer=0;/*把ICMP报头二进制数据以2字节为单位累加起来*/ while(nleft>1) { sum+=*w++; nleft-=2; }/*若ICMP报头为奇数个字节，会剩下最后一字节。把最后一个字节视为一个2字节数据的高字节，这个2字节数据的低字节为0，继续累加*/ if( nleft==1) { *(unsigned char *)(&answer)=*(unsigned char *)w; sum+=answer; } sum=(sum>>16)+(sum&0xffff); sum+=(sum>>16); answer=~sum; return answer;}/*设置ICMP报头*/int pack(int pack_no){ int i,packsize; struct icmp *icmp; struct timeval *tval; icmp=(struct icmp*)sendpacket; icmp->icmp_type=ICMP_ECHO; icmp->icmp_code=0; icmp->icmp_cksum=0; icmp->icmp_seq=pack_no; icmp->icmp_id=pid; packsize=8+datalen; tval= (struct timeval *)icmp->icmp_data; gettimeofday(tval,NULL); /*记录发送时间*/ icmp->icmp_cksum=cal_chksum( (unsigned short *)icmp,packsize); /*校验算法*/ return packsize;}/*发送三个ICMP报文*/void send_packet(){ int packetsize; while( nsend<MAX_NO_PACKETS) { nsend++; packetsize=pack(nsend); /*设置ICMP报头*/ if( sendto(sockfd,sendpacket,packetsize,0, (struct sockaddr *)&dest_addr,sizeof(dest_addr) )<0 ) { perror("sendto error"); continue; } sleep(1); /*每隔一秒发送一个ICMP报文*/ }}/*接收所有ICMP报文*/void recv_packet(){ int n,fromlen; extern int errno; signal(SIGALRM,statistics); fromlen=sizeof(from); while( nreceived<nsend) { alarm(MAX_WAIT_TIME); if( (n=recvfrom(sockfd,recvpacket,sizeof(recvpacket),0, (struct sockaddr *)&from,&fromlen)) <0) { if(errno==EINTR)continue; perror("recvfrom error"); continue; } gettimeofday(&tvrecv,NULL); /*记录接收时间*/ if(unpack(recvpacket,n)==-1)continue; nreceived++; }}/*剥去ICMP报头*/int unpack(char *buf,int len){ int i,iphdrlen; struct ip *ip; struct icmp *icmp; struct timeval *tvsend; double rtt; ip=(struct ip *)buf; iphdrlen=ip->ip_hl<<2; /*求ip报头长度,即ip报头的长度标志乘4*/ icmp=(struct icmp *)(buf+iphdrlen); /*越过ip报头,指向ICMP报头*/ len-=iphdrlen; /*ICMP报头及ICMP数据报的总长度*/ if( len<8) /*小于ICMP报头长度则不合理*/ { printf("ICMP packets\'s length is less than 8\n"); return -1; } /*确保所接收的是我所发的的ICMP的回应*/ if( (icmp->icmp_type==ICMP_ECHOREPLY) && (icmp->icmp_id==pid) ) { tvsend=(struct timeval *)icmp->icmp_data; tv_sub(&tvrecv,tvsend); /*接收和发送的时间差*/ rtt=tvrecv.tv_sec*1000+tvrecv.tv_usec/1000; /*以毫秒为单位计算rtt*/ /*显示相关信息*/ 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;}main(int argc,char *argv[]){ struct hostent *host; struct protoent *protocol; unsigned long inaddr=0l; int waittime=MAX_WAIT_TIME; int size=50*1024; if(argc<2) { printf("usage:%s hostname/IP address\n",argv[0]); exit(1); } if( (protocol=getprotobyname("icmp") )==NULL) { perror("getprotobyname"); exit(1); } /*生成使用ICMP的原始套接字,这种套接字只有root才能生成*/ if( (sockfd=socket(AF_INET,SOCK_RAW,protocol->p_proto) )<0) { perror("socket error"); exit(1); } /* 回收root权限,设置当前用户权限*/ setuid(getuid()); /*扩大套接字接收缓冲区到50K这样做主要为了减小接收缓冲区溢出的 的可能性,若无意中ping一个广播地址或多播地址,将会引来大量应答*/ setsockopt(sockfd,SOL_SOCKET,SO_RCVBUF,&size,sizeof(size) ); bzero(&dest_addr,sizeof(dest_addr)); dest_addr.sin_family=AF_INET; /*判断是主机名还是ip地址*/ if( inaddr=inet_addr(argv[1])==INADDR_NONE) { if((host=gethostbyname(argv[1]) )==NULL) /*是主机名*/ { perror("gethostbyname error"); exit(1); } memcpy( (char *)&dest_addr.sin_addr,host->h_addr,host->h_length); } else /*是ip地址*/ memcpy( (char *)&dest_addr,(char *)&inaddr,host->h_length); /*获取main的进程id,用于设置ICMP的标志符*/ pid=getpid(); printf("PING %s(%s): %d bytes data in ICMP packets.\n",argv[1], inet_ntoa(dest_addr.sin_addr),datalen); send_packet(); /*发送所有ICMP报文*/ recv_packet(); /*接收所有ICMP报文*/ statistics(SIGALRM); /*进行统计*/ return 0;}/*两个timeval结构相减*/void tv_sub(struct timeval *out,struct timeval *in){ if( (out->tv_usec-=in->tv_usec)<0) { --out->tv_sec; out->tv_usec+=1000000; } out->tv_sec-=in->tv_sec;}/*------------- The End -----------*/