Read ICMP payload from a recvmsg with MSG_ERRQUEUE flag - c

Question

I'm using BSD sockets to build an advanced traceroute program that doesn't need root privileges (like tracepath).
Using UDP and a bound socket, I call:
recvmsg(socket, header, MSG_ERRQUEUE)
I receive info about the supposed ICMP notification that a previously sent packet has triggered.
Do you know if it's possible to access the ICMP payload (which should be the previously-sent packet)?
I read from recvmsg man page:
[..] The payload of the original packet that caused the error
is passed as normal data via msg_iovec. [..]
But I can't find anything useful there, it just seems random data (I used wireshark to cross-check data).

Answer 1

You can use this sample code to check which ICMP Error you got and handle it (The sample contain some comments and links):
#define BUFFER_MAX_SIZE 1024
int on = 1;
/* Set the option, so we can receive errors */
setsockopt(socket, SOL_IP, IP_RECVERR,(char*)&on, sizeof(on));
/* Other code here */
/* .... */
/* Handle receving ICMP Errors */
int return_status;
char buffer[BUFFER_MAX_SIZE];
struct iovec iov; /* Data array */
struct msghdr msg; /* Message header */
struct cmsghdr *cmsg; /* Control related data */
struct sock_extended_err *sock_err; /* Struct describing the error */
struct icmphdr icmph; /* ICMP header */
struct sockaddr_in remote; /* Our socket */
for (;;)
{
iov.iov_base = &icmph;
iov.iov_len = sizeof(icmph);
msg.msg_name = (void*)&remote;
msg.msg_namelen = sizeof(remote);
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
msg.msg_flags = 0;
msg.msg_control = buffer;
msg.msg_controllen = sizeof(buffer);
/* Receiving errors flog is set */
return_status = recvmsg(socket, &msg, MSG_ERRQUEUE);
if (return_status < 0)
continue;
/* Control messages are always accessed via some macros
* http://www.kernel.org/doc/man-pages/online/pages/man3/cmsg.3.html
*/
for (cmsg = CMSG_FIRSTHDR(&msg);cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg))
{
/* Ip level */
if (cmsg->cmsg_level == SOL_IP)
{
/* We received an error */
if (cmsg->cmsg_type == IP_RECVERR)
{
fprintf(stderror, "We got IP_RECVERR message\n");
sock_err = (struct sock_extended_err*)CMSG_DATA(cmsg);
if (sock_err)
{
/* We are interested in ICMP errors */
if (sock_err->ee_origin == SO_EE_ORIGIN_ICMP && sock_err->ee_type == ICMP_DEST_UNREACH)
{
/* Handle ICMP destination unreachable error codes */
switch (sock_err->ee_code)
{
case ICMP_NET_UNREACH:
/* Handle this error */
fprintf(stderror, "Network Unreachable Error\n");
break;
case ICMP_HOST_UNREACH:
/* Handle this error */
fprintf(stderror, "Host Unreachable Error\n");
break;
/* Handle all other cases. Find more errors :
* http://lxr.linux.no/linux+v3.5/include/linux/icmp.h#L39
*/
}
}
}
}
}
}
}