Why is my ICMP sequence number not incrementing? (C Socket Programming)

Question

is it possible someone can help me workout why my ICMP sequence number isn't incrementing on every request, when this was used as a ping program it would increment the sequence number on each ping. Also does anyone have any idea why my Round Trip Times are displaying as negative numbers? This also worked fine when this was a ping program.

Please assume all the code 'works', i have removed some for easier reading.

void
respond (int signum) {
   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 = 0;
   long int length;
   fd_set rdfds;
   int ready;
   int rtt;
   char buff [BUF_SIZE];

   /* Create and check Socket Number */
   skt = socket (AF_INET, SOCK_RAW, IPPROTO_ICMP);


    int ttl = 0;
    setsockopt(skt, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl)) < 0;


       /* Check Socket */
       if (skt < 0) {
          perror ("socket()");
          exit (1);
       }

       /* Set IP Addresses */
       addr.sin_family      = AF_INET;
       addr.sin_port        = 0;
       addr.sin_addr.s_addr = INADDR_ANY;


       /* Check Socket Bind */
       if (bind (skt, (struct sockaddr *)&addr, sizeof(struct sockaddr_in))) {
          perror ("Can't bind socket");
          exit (1);
       }

/* START SEND LOOP*/
int i;
for (i = 0; i < 7; i++){
        ttl+=1;
        setsockopt(skt, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));

        /* IP Buffer */
       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;

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


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

       /*Calculating packet size*/
       length = sizeof(struct icmphdr) + sizeof(struct timeval);
       icmp->checksum = ~(sum (0, buff, length));



       /* Packet too small, ERROR
       SEND Request             */
       if (sendto (skt, buff, length, 0,
             (struct sockaddr *) &dstaddr, sizeof(struct sockaddr_in)) <= 0) {
          perror ("sendto()");
          exit (1);
        }

       /* Define File Descriptor */
       timeout.tv_sec  = 1;
       timeout.tv_usec = 1;
       FD_ZERO(&rdfds);
       FD_SET (skt, &rdfds);

       /* Select Data from File Descriptor */
       ready = select (skt + 1, &rdfds, NULL, NULL, &timeout);
       if (ready < 0) {
          perror ("Select()");
          exit (1);
       }

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



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


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


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


       /* Check ICMP response type*/
       if (icmp->type == 11){
           printf("");
           }

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

       /*   Find the difference between sent and end times in 10s of ms */
       sent = (struct timeval *)icmp->data;
       if ((rtt = (end.tv_usec - sent->tv_usec) / 10) < 0)
          rtt += 10000;  // We've cycled to a new second
       rtt += (end.tv_sec - sent->tv_sec) * 10000;  // Add any seconds

       /* PRINT ICMP REPLY*/
       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);


} /*END SEND LOOP


   /* 3 Second Probe */
   alarm (5);
}

score 2 · Accepted Answer · answered Feb 08 '13 at 00:18

2

You're never setting sequence to 0 in this code, only declaring that it's an int.

answered Feb 08 '13 at 00:18

Eric

2,056
13
11

Same problem still, it worked in the ping version of this program – A CSc Student---- Feb 08 '13 at 00:22
Is it staying at 0 for all 7 iterations? also can you update the code with whatever fixes you've added. – Eric Feb 08 '13 at 00:26
1

Oh I see what's going on. you're reassigning the value of "icmp" during each iteration of the loop. At first it's for your outgoing packet, then reassigning the response to that variable. What's happening is that you're setting the sequence to say 3, but a TTL expired is being sent back which resets the sequence to 0. You can't expect the sequence to be expected when some random router between you and the target is intercepting and replying to the request with an error message. – Eric Feb 08 '13 at 00:34
Ouch damm, never thought of the networking side of this as i was so involved in trying to get the code to work. From some debugging i can see what you're saying, the actual variable value is changing... In wireshark i can see a continuous sequence number so I was confused... – A CSc Student---- Feb 08 '13 at 00:40

score 1 · Answer 2 · answered Feb 08 '13 at 00:33

In this piece of code:

   /* IP Buffer */
   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;

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

you are treating the buffer buff as starting with both an IP header and an ICMP header. I believe you need to offset the icmphdr pointer by the size of the IP header, unless your icmphdr structure includes an iphdr at its start (which the Linux version does not):

icmp = (struct icmphdr*)(buff + sizeof(iphdr));

Also, you are overswriting the IP header by doing memset (buff, 0, sizeof(buff)); after writing the IP header to the buffer.

Why is my ICMP sequence number not incrementing? (C Socket Programming)

2 Answers2