Writing a basic traceroute script in C

Question

I have to write a trceroute script but I'm not sure if my attempts are correct.

Right now I'm doing it like that (please correct me if I'm doing wrong or clumsy):

1. Got an struct for ip- and udpheader
2. A checksum function
3. Opening 2 sockets: One for sending UDP-packets in SOCK_RAW mode (to manipulate ttl) and one to receive ICMP-answers from the routers.
4. Using sendto() to send UDP packet
5. Having no clue how to receive and process an ICMP answer

Are there any more comfortable ways to change the TTL than using sock_raw where I have to define all header stuff by myself? What parameters should I use for socket() when opening ICMP sock? How to receive the ICMP answer?

Answer 1

What platform are you targeting? Here's a BSD flavor from OpenBSD source:

if ((s = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP)) < 0)
    err(5, "icmp socket");
if ((sndsock = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0)
    err(5, "raw socket");

On Linux, I believe, you need to use IP_RECVERR and recvmsg(2) with the MSG_ERRQUEUE, see ip(7).

Answer 2

As far as setting the TTL is concerned, you can use setsockopt(). Here's an extract from the iputils' source for ping on Linux:

if (setsockopt(icmp_sock, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, 1) == -1) {
    perror ("ping: can't set multicast time-to-live");
    exit(2);
}

if (setsockopt(icmp_sock, IPPROTO_IP, IP_TTL, &ittl, sizeof(ittl)) == -1) {
    perror ("ping: can't set unicast time-to-live");
    exit(2);
}