Getting original destination address on a UDP socket

udp_socket_addr.cc

#include <sys/socket.h>
#include <netinet/in.h>
#include <iostream>
#include <string.h>

int main()
{
    // --------------------------------------------

    struct sockaddr_in addr;
    addr.sin_family      = AF_INET;
    addr.sin_port        = htons(9876);
    addr.sin_addr.s_addr = INADDR_ANY;
    int udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
    int ret = bind(udp_socket, (sockaddr*)&addr, sizeof(addr));

    int flags = 1;
    ret = setsockopt(udp_socket, IPPROTO_IP, IP_RECVORIGDSTADDR, &flags, sizeof(flags));

    // --------------------------------------------

    struct sockaddr_in cl_addr;
    cl_addr.sin_family      = AF_INET;
    cl_addr.sin_port        = htons(9879);
    cl_addr.sin_addr.s_addr = INADDR_ANY;
    int client_socket = socket(AF_INET, SOCK_DGRAM, 0);
    std::cout << "client socket: " << client_socket << std::endl;

    ret = bind(client_socket, (sockaddr*)&cl_addr, sizeof(cl_addr));
    std::cout << "client bind ret= " << ret << std::endl;

    // --------------------------------------------

    char buf[1] = {'f'};
    ret = sendto(client_socket, buf, 1, 0, (sockaddr*)&addr, sizeof(addr));
    std::cout << "client send ret=" << ret << std::endl;

    // --------------------------------------------

    char buf2[1] = { 0 };
    struct iovec indata = {0};
    indata.iov_base = buf2;
    indata.iov_len  = sizeof(buf2);

    char inmsg[1000];

    struct msghdr incoming_msg = { 0 };
    incoming_msg.msg_iov = &indata;
    incoming_msg.msg_iovlen = 1;
    incoming_msg.msg_control = inmsg;
    incoming_msg.msg_controllen = sizeof(inmsg);

    ret = recvmsg(udp_socket, &incoming_msg, 0);

    sockaddr_in dstaddr = { 0 };
    for (struct cmsghdr* cmsg = CMSG_FIRSTHDR(&incoming_msg);
         cmsg != NULL;
         cmsg = CMSG_NXTHDR(&incoming_msg,cmsg))
    {
        if ((cmsg->cmsg_level == SOL_IP) && (cmsg->cmsg_type == IP_RECVORIGDSTADDR))
        {
            memcpy (&dstaddr, CMSG_DATA(cmsg), sizeof (dstaddr));
            dstaddr.sin_family = AF_INET;
        }
    }

    std::cout << " original dst address " << std::hex <<  ntohl(dstaddr.sin_addr.s_addr) << std::dec <<  ":" << ntohs(dstaddr.sin_port) << std::endl;
    std::cout << "buf2[0] = " << buf2[0] << std::endl;
}