ping 命令工作原理详细解析

ping的原理就是首先建立通道，然后发送包，对方接受后返回信息，这个包至少包括以下内容，发送的时候，包的内容包括对方的ip地址和自己的地址，还有序列数，回送的时候包括双方地址，还有时间等，主要是接受方在都是在操作系统内核里做好的，时刻在监听，提供一段c程序的代码，希望对大家有用。

＃i nclude <stdio.h> 
＃i nclude <signal.h> 
＃i nclude <arpa/inet.h> 
＃i nclude <sys/types.h> 
＃i nclude <sys/socket.h> 
＃i nclude <unistd.h> 
＃i nclude <netinet/in.h> 
＃i nclude <netinet/ip.h> 
＃i nclude <netinet/ip_icmp.h> 
＃i nclude <netdb.h> 
＃i nclude <setjmp.h> 
＃i nclude <errno.h> 

#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 -----------*/

本文转自loveme2351CTO博客，原文链接：http://blog.51cto.com/loveme23/7953 ，如需转载请自行联系原作者