自己动手学TCP/IP--ICMP（ping报文）

梦里梦外; 2022-06-09 07:27 224阅读 0赞

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）

![1342966148_1420.png][]

## 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

![1342966426_9015.png][]

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  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 -----------*/

[1342966148_1420.png]: https://img-my.csdn.net/uploads/201207/22/1342966148_1420.png
[1342966426_9015.png]: https://img-my.csdn.net/uploads/201207/22/1342966426_9015.png
[ping.c]: http://research.microsoft.com/en-us/um/redmond/projects/invisible/src/games/ping/ping.c.htm