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    ping的原理就是首先建立通道，然后發送包，對方接受后返回信息，這個包至少包括以下內容，發送的時候，包的內容包括對方的ip地址和自己的地址，還有序列數，回送的時候包括雙方地址，還有時間等，主要是接受方在都是在操作系統內核里做好的，時刻在監聽，提供一段c程序的代碼，希望對大家有用。

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include

#define PACKET_SIZE 4096
#define MAX_WAIT_TIME 5
#define MAX_NO_PACKETS 3

char 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);
}

/*校驗和算法*/
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報頭二進制數據以2字節為單位累加起來*/
while(nleft>1)
{ sum+=*w++;
nleft-=2;
}

/*若ICMP報頭為奇數個字節，會剩下最后一字節。把最后一個字節視為一個2字節數據的高字節，這個2字節數據的低字節為0，繼續累加*/
if( nleft==1)
{ *(unsigned char *)(&answer)=*(unsigned char *)w;
sum+=answer;
}
sum=(sum>>16)+(sum&0xffff);
sum+=(sum>>16);
answer=~sum;
return answer;
}

/*設置ICMP報頭*/
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); /*記錄發送時間*/
icmp->icmp_cksum=cal_chksum( (unsigned short *)icmp,packsize); /*校驗算法*/
return packsize;
}

/*發送三個ICMP報文*/
void send_packet()
{ int packetsize;
while( nsend{ nsend++;
packetsize=pack(nsend); /*設置ICMP報頭*/
if( sendto(sockfd,sendpacket,packetsize,0,
(struct sockaddr *)&dest_addr,sizeof(dest_addr) )<0 )
{ perror("sendto error");
continue;
}
sleep(1); /*每隔一秒發送一個ICMP報文*/
}
}

/*接收所有ICMP報文*/
void recv_packet()
{ int n,fromlen;
extern int errno;

signal(SIGALRM,statistics);
fromlen=sizeof(from);
while( nreceived{ 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); /*記錄接收時間*/
if(unpack(recvpacket,n)==-1)continue;
nreceived++;
}
}

/*剝去ICMP報頭*/
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報頭長度,即ip報頭的長度標志乘4*/
icmp=(struct icmp *)(buf+iphdrlen); /*越過ip報頭,指向ICMP報頭*/
len-=iphdrlen; /*ICMP報頭及ICMP數據報的總長度*/
if( len<8) /*小于ICMP報頭長度則不合理*/
{ printf("ICMP packets\'s length is less than 8\n");
return -1;
}

/*確保所接收的是我所發的的ICMP的回應*/
if( (icmp->icmp_type==ICMP_ECHOREPLY) && (icmp->icmp_id==pid) )
{ tvsend=(struct timeval *)icmp->icmp_data;
tv_sub(&tvrecv,tvsend); /*接收和發送的時間差*/
rtt=tvrecv.tv_sec*1000+tvrecv.tv_usec/1000; /*以毫秒為單位計算rtt*/

/*顯示相關信息*/
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,這種套接字只有root才能生成*/
if( (sockfd=socket(AF_INET,SOCK_RAW,protocol->p_proto) )<0)
{ perror("socket error");
exit(1);
}

/* 回收root權限,設置當前用戶權限*/
setuid(getuid());

/*擴大套接字接收緩沖區到50K這樣做主要為了減小接收緩沖區溢出的
的可能性,若無意中ping一個廣播地址或多播地址,將會引來大量應答*/
setsockopt(sockfd,SOL_SOCKET,SO_RCVBUF,&size,sizeof(size) );
bzero(&dest_addr,sizeof(dest_addr));
dest_addr.sin_family=AF_INET;

/*判斷是主機名還是ip地址*/
if( inaddr=inet_addr(argv[1])==INADDR_NONE)
{ if((host=gethostbyname(argv[1]) )==NULL) /*是主機名*/
{ 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);
/*獲取main的進程id,用于設置ICMP的標志符*/
pid=getpid();
printf("PING %s(%s): %d bytes data in ICMP packets.\n",argv[1],
inet_ntoa(dest_addr.sin_addr),datalen);
send_packet(); /*發送所有ICMP報文*/
recv_packet(); /*接收所有ICMP報文*/
statistics(SIGALRM); /*進行統計*/
return 0;
}

/*兩個timeval結構相減*/
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;
}