c - 需要帮助修复此 C Socket ICMP 程序

Question

我有一个 C 程序，它能够接收 ICMP 请求并显示它们，但根据我使用 Wireshark 所看到的情况，它本身不会发送任何 ICMP 请求。

我已经包含了所需的库，并且正在使用省略的 IP 头文件和 ICMP 头文件。

如有任何帮助，我们将不胜感激。请忽略随机的 Printf 语句，它们只是为了帮助我。

谢谢

**

***IP.h*****

#ifndef __IP_H__
#define __IP_H__

struct iphdr {

   uint8_t   hdrlen:4;    
   uint8_t   version:4;
   uint8_t   ecn:2;       
   uint8_t   dscp:6;     
   uint16_t  length;
   uint16_t  ident;
   uint16_t  fragoff:13;
   uint16_t  flags:3;
   uint8_t   ttl;
   uint8_t   protocol;
   uint16_t  checksum;
   uint32_t  srcip;
   uint32_t  dstip;
   uint32_t  options[ ];  
} __attribute__((__packed__));

#endif // __IP_H__

****ICMP.H****

#ifndef __ICMP_H__
#define __ICMP_H__

struct icmphdr {
   uint8_t   type;         
   uint8_t   code;          
   uint16_t  checksum;      
   uint16_t  id;          
   uint16_t  seqNum;       
   uint32_t  data[ ];       
} __attribute__((__packed__));


char * messages[ ] = {
   "Echo reply",
   "Type 1",
   "Type 2",
   "Destination unreachable",
   "Source quench",
   "Redirect",
   "Type 6",
   "Type 7",
   "Echo request",
   "Router advertisement",
   "Router discovery",
   "Time exceeded",
   "Parameter problem",
   "Timestamp request",
   "Timestamp reply",
   "Information request (obsol.)",
   "Information reply (obsol.)",
   "Address mask request",
   "Address mask reply"
};

#endif // __ICMP_H__


****Main C File ****

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/time.h>

    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>
    #include <netdb.h>

    #include "ip.h"
    #include "icmp.h"

    struct timeval timeout;
    struct timeval end;


    #define BUF_SIZE   1024
    #define ICMP       1
    #define ECHO_REQ   8
    #define ECHO_REPL  0
    #define HOSTADDR   "8.8.8.8"

    char *
    iptos (uint32_t ipaddr) {
       static char ips[16];

       sprintf(ips, "%d.%d.%d.%d",
          (ipaddr >> 24),
          (ipaddr >> 16) & 0xff,
          (ipaddr >>  8) & 0xff,
          (ipaddr      ) & 0xff );
       return ips;
    }

    uint16_t
    sum (uint16_t initial, void * buffer, int bytes) {
       uint32_t   total;
       uint16_t * ptr;
       int        words;

       total = initial;
       ptr   = (uint16_t *) buffer;
       words = (bytes + 1) / 2; // +1 & truncation on / handles any odd byte at end

       while (words--) total += *ptr++;
       while (total & 0xffff0000) total = (total >> 16) + (total & 0xffff);

       return (uint16_t) total;
    }




    void
    some_function ( ) {

       struct sockaddr_storage peer_addr;
       socklen_t               peer_addrlen;

       struct sockaddr_in      addr;
       struct sockaddr_in      dstaddr;
       struct iphdr   *        ip;
       struct icmphdr *        icmp;
       struct timeval *        sent;
       int skt;
       int sequence;
       long int length;
       fd_set rdfds;
       int ready;
       int rtt;

       char buff [BUF_SIZE];



       skt = socket (AF_INET, SOCK_RAW, IPPROTO_ICMP);
       if (skt < 0) {
          perror ("socket()");
          exit (1);
       }

       addr.sin_family      = AF_INET;
       addr.sin_port        = 0;
       addr.sin_addr.s_addr = INADDR_ANY;

       if (bind (skt, (struct sockaddr *)&addr, sizeof(struct sockaddr_in))) {
          perror ("Can't bind socket");
          exit (1);
       }
    printf("aaa");
       ip = (struct iphdr *)buff;
       peer_addrlen = (socklen_t) sizeof (struct sockaddr_storage);

       memset (&dstaddr, 0, sizeof(struct sockaddr_in));
       dstaddr.sin_addr.s_addr = inet_addr(HOSTADDR);
       dstaddr.sin_family = AF_INET;

       memset (buff, 0, sizeof(buff));
       icmp = (struct icmphdr *) buff;
       icmp->type   = ECHO_REQ;
       icmp->id     = htons(getpid( ) & 0xffff);
       icmp->seqNum = htons(sequence++);

    printf("aabba");


       if (gettimeofday ((struct timeval *)icmp->data, NULL)) {
          perror ("Can't establish send time");
          exit (1);
       }

       length = sizeof(struct icmphdr) + sizeof(struct timeval);
       icmp->checksum = ~htons(sum (0, buff, length));


    printf("aaa");

       if (sendto (skt, buff, length, 0,
             (struct sockaddr *) &dstaddr, sizeof(struct sockaddr_in)) <= 0) {
          perror ("sendto()");
          exit (1);


    }
    printf("aaa");
     timeout.tv_sec  = 1;
       timeout.tv_usec = 0;
       FD_ZERO(&rdfds);
       FD_SET (skt, &rdfds);

       ready = select (skt + 1, &rdfds, NULL, NULL, &timeout);
       if (ready < 0) {
          perror ("Select()");
          exit (1);
       }

     memset (buff, 0, sizeof(buff));
       if (recvfrom (skt, buff, sizeof(buff), 0,
            (struct sockaddr *) &peer_addr, &peer_addrlen) <= 0) exit (1);

       if (gettimeofday (&end, NULL)) {   // Timestamp reception
          perror ("Can't establish time of receipt");
          exit (1);
       }

       if (ip->version != 4 ||
           sum (0, buff, sizeof(struct iphdr)) != 0xffff ||
           ip->protocol != ICMP)
          exit(1);

       length = ntohs(ip->length) - ip->hdrlen * 4;       // Length of IP payload
       icmp = (struct icmphdr *)((uint32_t *)ip + ip->hdrlen);   // Find ICMP hdr

       if (icmp->type != ECHO_REPL || sum (0, icmp, length) != 0xffff) {
          fprintf (stderr, "Received %s\n", messages[icmp->type]);
          exit (1);
       }


       sent = (struct timeval *)icmp->data;
       if ((rtt = (end.tv_usec - sent->tv_usec) / 100) < 0)
          rtt += 10000;  // We've cycled to a new second
       rtt += (end.tv_sec - sent->tv_sec) * 10000;  // Add any seconds

       printf ("%ld bytes from %s: icmp_req=%d ttl=%d time=%0.1f ms\n",
          length,
          iptos(ntohl(ip->srcip)),
          ntohs(icmp->seqNum),
          ip->ttl,
          ((float)rtt) / 10);

    }
    int main(){
    printf("aaa");
    some_function();
    return(0);

    }

Answer 1

问题是校验和错误。

更改此行

icmp->checksum = htons(sum (0, buff, length));

与

icmp->checksum = ~(sum (0, buff, length));

sum 函数已经关心字节顺序，无需使用 htons 再次翻译