Why is my TTL value increasing by 2 each time? (C Socket programming)

Question

i'm trying to implement a Traceroute program but i've ran into two problems, one is that the TTL and RTT print out wrong; though they printed out right when implemented as a ping program. Finally my main issue is that when i'm incrementing the TTL it's going up by 2 instead of 1.

I've only included the code i think is necessary, thanks.

Thanks in advance :)

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;
   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;


    /* START SEND LOOP*/
    int i;
    for (i = 0; i < 4; i++){
        ttl+=1;
        setsockopt(skt, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));


   /* 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);
   }

    /* 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;

   ip->ttl=(ttl++);

   /* 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  = 2;
   timeout.tv_usec = 0;
   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("Type 11: ICMP....");
       }

  /* 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) / 100) < 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),
      /*Set initial TTL */
      ip->ttl,
      ((float)rtt) / 10);


    } /*END SEND LOOP

   /* Invalid Signal returned */
   if (signum == SIGINT) {
      printf ("\nGoodbye!\n");
      exit(0);
   }

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

Answer 1

/* START SEND LOOP*/
int i;
for (i = 0; i < 4; i++){
    ttl+=1;
    setsockopt(skt, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));

Here you increase ttl by one each time the loop iterates.

 ip->ttl=(ttl++);

And here you increase it by one again. So if it was 1, it will be 2 after this line. However, once the loop iterates, you increase it again, so it will now be 3. That's why your ttl goes up by two on each iteration.

The line ip->ttl=(ttl++) means: Take the value of ttl and copy it to ip->ttl, then after doing so, increase ttl by one.

Answer 2

You're incrementing the ttl twice in that loop. Once when the for loop begins:

    ttl+=1;

Then again while assigning it to the struct

   ip->ttl=(ttl++);

As for the rtt, well that code just looks wrong to me.