Setting the source IP for a UDP socket

Question

I have a UDP socket that is bound to INADDR_ANY to listen to packets on all the IPs my server has. I\'m sending out replies through the same socket.

Right now the se

Answer 1

I encountered the same problem recently.

What I do to solve this problem is

1. get the interface name from received packet
2. bind socket to specific interface
3. unbind socket

Example:

  struct ifreq ifr;
  ...
  recvmsg(fd, &msg...)
  ...      
  if (msg.msg_controllen >= sizeof(struct cmsghdr))
    for (cmptr = CMSG_FIRSTHDR(&msg); cmptr; cmptr = CMSG_NXTHDR(&msg, cmptr))
      if (cmptr->cmsg_level == SOL_IP && cmptr->cmsg_type == IP_PKTINFO)
      {
        iface_index = ((struct in_pktinfo *)CMSG_DATA(cmptr))->ipi_ifindex;
      }
  if_indextoname(iface_index , ifr.ifr_name);
  mret=setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, &ifr, sizeof(ifr));

  sendmsg(...);

  memset(&ifr, 0, sizeof(ifr));
  snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "");
  mret=setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, &ifr, sizeof(ifr));