C++ DGRAM socket get the RECEIVER address - c++

Question

In C++,
how can I get the receiver address of the UDP packet which I have received using recvfrom. I know that it should be the same host on which I am receiving the packet, but I need to extract it from the received packet, in order to verify something. How can I do this?
I found that one way of doing this is:
int r = getsockopt(receiver_sock, SOL_IP, SO_ORIGINAL_DST, (struct sockaddr *) &sender_addr, (socklen_t *)&addr_len);`
But I get the error:
error: ‘SO_ORIGINAL_DST’ was not declared in this scope
I am using the appropriate headers
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include </usr/src/linux-headers-2.6.32-21/include/linux/netfilter_ipv4.h>
#include <arpa/inet.h>
#include <linux/netfilter.h>
Using netfilter_ipv4 gives other errors like INT_MIN not declared. However, I think the mistake is something more fundamental rather than inclusion of proper header.
Please help.

Answer 1

On Linux you want to use IP_PKTINFO option, see ip(7), and the recvmsg(2) call.
Stevens has examples of doing this but with IP_RECVDSTADDR and IP_RECVIF options that are not available on Linux.

Answer 2

I've constructed an example that extracts the source, destination and interface addresses. For brevity, no error checking is provided. See this duplicate: Get destination address of a received UDP packet.
// sock is bound AF_INET socket, usually SOCK_DGRAM
// include struct in_pktinfo in the message "ancilliary" control data
int opt = 1;
setsockopt(sock, IPPROTO_IP, IP_PKTINFO, &opt, sizeof(opt));
// the control data is dumped here
char cmbuf[0x100];
// the remote/source sockaddr is put here
struct sockaddr_in peeraddr;
// if you want access to the data you need to init the msg_iovec fields
struct msghdr mh = {
.msg_name = &peeraddr,
.msg_namelen = sizeof(peeraddr),
.msg_control = cmbuf,
.msg_controllen = sizeof(cmbuf),
};
recvmsg(sock, &mh, 0);
for ( // iterate through all the control headers
struct cmsghdr *cmsg = CMSG_FIRSTHDR(&mh);
cmsg != NULL;
cmsg = CMSG_NXTHDR(&mh, cmsg))
{
// ignore the control headers that don't match what we want
if (cmsg->cmsg_level != IPPROTO_IP ||
cmsg->cmsg_type != IP_PKTINFO)
{
continue;
}
struct in_pktinfo *pi = CMSG_DATA(cmsg);
// at this point, peeraddr is the source sockaddr
// pi->ipi_spec_dst is the destination in_addr
// pi->ipi_addr is the receiving interface in_addr
}