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 server chooses automatically which IP is used as the source IP when packets are sent out, but I would like to be able to set the outgoing source IP myself.

Is there any way to do that without having to create a separate socket for each IP ?

Answer 1

You either bind(2) to each interface address and manage multiple sockets, or let the kernel do the implicit source IP assignment with INADDR_ANY. There is no other way.

My question would be - why do you need this? Is normal IP routing not working for you?

Answer 2

Nikolai, using a separate socket and bind(2) for each address or messing with routing tables is often not a feasible option e.g. with dynamic addresses. A single IP_ADDRANY-bound UDP server should be able to appear to respond on the same dynamically-assigned IP address a packet is received on.

Luckily, there is another way. Depending on your system's support you can make use of the IP_PKTINFO socket options to set or receive ancillary data about a message. Ancillary data (via cmsg(3)) is covered in many places online though comp.os.linux.development.system had a full code sample specific to IP_PKTINFO.

The code in the link uses IP_PKTINFO (or IP_RECVDSTADDR depending on the platform) to get the destination address of a UDP message from the ancillary cmsg(3) data. Paraphrased here:

struct msghdr msg;
struct cmsghdr *cmsg;
struct in_addr addr;
// after recvmsg(sd, &msg, flags);
for(cmsg = CMSG_FIRSTHDR(&msg);
    cmsg != NULL;
    cmsg = CMSG_NXTHDR(&msg, cmsg)) {
  if (cmsg->cmsg_level == IPPROTO_IP && cmsg->cmsg_type == IP_PKTINFO) {
    addr = ((struct in_pktinfo*)CMSG_DATA(cmsg))->ipi_addr;
    printf("message received on address %s\n", inet_ntoa(addr));
  }
}

Gene, your question asked how to set the source address on outgoing packets. With IP_PKTINFO it is possible to set the ipi_spec_dst field of the struct in_pktinfo in the ancillary data passed to sendmsg(2). See the post referenced above, cmsg(3), and sendmsg(2) for guidelines on how to create and manipulate the ancillary data in a struct msghdr. An example (no guarantee here) might be:

struct msghdr msg;
struct cmsghdr *cmsg;
struct in_pktinfo *pktinfo;
// after initializing msghdr & control data to CMSG_SPACE(sizeof(struct in_pktinfo))
cmsg = CMSG_FIRSTHDR(&msg);
cmsg->cmsg_level = IPPROTO_IP;
cmsg->cmsg_type = IP_PKTINFO;
cmsg->cmsg_len = CMSG_LEN(sizeof(struct in_pktinfo));
pktinfo = (struct in_pktinfo*) CMSG_DATA(cmsg);
pktinfo->ipi_ifindex = src_interface_index;
pktinfo->ipi_spec_dst = src_addr;
// bytes_sent = sendmsg(sd, &msg, flags);

Note this is different in IPv6: use struct in6_pktinfo::ipi6_addr in both the recvmsg and sendmsg cases.

(man pages referenced - getting around 1 hyperlink limit)

http:// linux.die.net/man/2/sendmsg
http:// linux.die.net/man/3/cmsg