Receiving multiple multicast feeds on the same port - C, Linux

Question

I have an application that is receiving data from multiple multicast sources on the same port. I am able to receive the data. However, I am trying to account for statistics of each group (i.e. msgs received, bytes received) and all the data is getting mixed up. Does anyone know how to solved this problem? If I try to look at the sender's address, it is not the multicast address, but rather the IP of the sending machine.

I am using the following socket options:

struct ip_mreq mreq;         
mreq.imr_multiaddr.s_addr = inet_addr("224.1.2.3");         
mreq.imr_interface.s_addr = INADDR_ANY;         
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));

and also:

setsockopt(s, SOL_SOCKET, SO_REUSEPORT, &reuse, sizeof(reuse));

Answer 1

After some years facing this linux strange behaviour, and using the bind workaround describe in previous answers, I realize that the ip(7) manpage describe a possible solution :

IP_MULTICAST_ALL (since Linux 2.6.31)
This option can be used to modify the delivery policy of multicast messages to sockets bound to the wildcard INADDR_ANY address. The argument is a boolean integer (defaults to 1). If set to 1, the socket will receive messages from all the groups that have been joined globally on the whole system. Otherwise, it will deliver messages only from the groups that have been explicitly joined (for example via the IP_ADD_MEMBERSHIP option) on this particular socket.

Then you can activate the filter to receive messages of joined groups using :

int mc_all = 0;
if ((setsockopt(sock, IPPROTO_IP, IP_MULTICAST_ALL, (void*) &mc_all, sizeof(mc_all))) < 0) {
    perror("setsockopt() failed");
}

This problem and the way to solve it enabling IP_MULTICAST_ALL is discussed in Redhat Bug 231899, this discussion contains test programs to reproduce the problem and to solve it.

Answer 2

[Edited to clarify that bind() may in fact include a multicast address.]

So the application is joining several multicast groups, and receiving messages sent to any of them, to the same port. SO_REUSEPORT allows you to bind several sockets to the same port. Besides the port, bind() needs an IP address. INADDR_ANY is a catch-all address, but an IP address may also be used, including a multicast one. In that case, only packets sent to that IP will be delivered to the socket. I.e. you can create several sockets, one for each multicast group. bind() each socket to the (group_addr, port), AND join group_addr. Then data addressed to different groups will show up on different sockets, and you'll be able to distinguish it that way.

I tested that the following works on FreeBSD:

#include <sys/socket.h>
#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/param.h>
#include <unistd.h>
#include <errno.h>

int main(int argc, const char *argv[])
{
    const char *group = argv[1];

    int s = socket(AF_INET, SOCK_DGRAM, 0);
    int reuse = 1;
    if (setsockopt(s, SOL_SOCKET, SO_REUSEPORT, &reuse, sizeof(reuse)) == -1) {
        fprintf(stderr, "setsockopt: %d\n", errno);
        return 1;
    }

    /* construct a multicast address structure */
    struct sockaddr_in mc_addr;
    memset(&mc_addr, 0, sizeof(mc_addr));
    mc_addr.sin_family = AF_INET;
    mc_addr.sin_addr.s_addr = inet_addr(group);
    mc_addr.sin_port = htons(19283);

    if (bind(s, (struct sockaddr*) &mc_addr, sizeof(mc_addr)) == -1) {
        fprintf(stderr, "bind: %d\n", errno);
        return 1;
    }

    struct ip_mreq mreq;
    mreq.imr_multiaddr.s_addr = inet_addr(group);
    mreq.imr_interface.s_addr = INADDR_ANY;
    setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));

    char buf[1024];
    int n = 0;
    while ((n = read(s, buf, 1024)) > 0) {
        printf("group %s fd %d len %d: %.*s\n", group, s, n, n, buf);
    }
}

If you run several such processes, for different multicast addresses, and send a message to one of the addresses, only the relevant process will receive it. Of course, in your case, you probably will want to have all the sockets in one process, and you'll have to use select or poll or equivalent to read them all.

Answer 3

Use setsockopt() and IP_PKTINFO or IP_RECVDSTADDR depending on your platform, assuming IPv4. This combined with recvmsg() or WSARecvMsg() allows you to find the source and destination address of every packet.

Unix/Linux, note FreeBSD uses IP_RECVDSTADDR whilst both support IP6_PKTINFO for IPv6.

• http://www.kernel.org/doc/man-pages/online/pages/man7/ip.7.html

Windows, also has IP_ORIGINAL_ARRIVAL_IF

• http://msdn.microsoft.com/en-us/library/ms741645(v=VS.85).aspx