Setting the source IP for a UDP socket

by

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.

Note also that Windows does not support an equivalent to ipi_spec_dst in the in_pktinfo struct, so you cannot use this method to set the source address on an outgoing winsock2 packet.

(man pages referenced – getting around 1 hyperlink limit)

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

Leave a Comment