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)


报文格式


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 
#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( nsendip_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 -----------*/

标签:C/C++, Linux, TCP/IP

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