How to set up a socket for UDP multicast with 2 network cards present?

Question

I'm trying to get udp multicast data using sockets and c++ (c). I have a server with 2 network cards so I need to bind socket to specific interface. Currently I'm testing on another server that has only one network card.

When I use INADDR_ANY I can see the udp data, when I bind to specific interface I don't see any data. Function inet_addr is not failing (I removed checking for return value for now).

Code is below. On a server with one network card, my IP address is 10.81.128.44. I receive data when I run as: ./client 225.0.0.37 12346

This gives me no data: ./client 225.0.0.37 12346 10.81.128.44

Any suggestions? (Hope the code compiles, I removed comments ...)

   #include <stdlib.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>
    #include <time.h>
    #include <string.h>
    #include <stdio.h>

    #include <iostream>
    #include <string>

    using namespace std;

    #define HELLO_PORT 12345
    #define HELLO_GROUP "225.0.0.37"
    #define MSGBUFSIZE 256

    int main(int argc, char *argv[])
    {
        string source_iface;
        string group(HELLO_GROUP);
        int port(HELLO_PORT);

        if (!(argc < 2)) group = argv[1];
        if (!(argc < 3)) port = atoi(argv[2]);
        if (!(argc < 4)) source_iface = argv[3];

        cout << "group: " << group << " port: " << port << " source_iface: " << source_iface << endl;

        int fd;
        if ((fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
        {
            perror("socket");
            exit(1);
        }

        u_int yes = 1;
        if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes)) < 0)
        {
            perror("Reusing ADDR failed");
            exit(1);
        }

        struct sockaddr_in addr;
        memset(&addr, 0, sizeof(addr));
        addr.sin_family = AF_INET;
        addr.sin_port = htons(port);
        addr.sin_addr.s_addr = (source_iface.empty() ? htonl(INADDR_ANY) : inet_addr(source_iface.c_str()));

   if (bind(fd,(struct sockaddr *)&addr, sizeof(addr)) < 0)
   {
        perror("bind");
        exit(1);
   }

   struct ip_mreq mreq;
   mreq.imr_multiaddr.s_addr = inet_addr(group.c_str());
   mreq.imr_interface.s_addr = (source_iface.empty() ? htonl(INADDR_ANY) : inet_addr(source_iface.c_str()));

    if (setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)) < 0)
    {
        perror("setsockopt");
        exit(1);
    }

    socklen_t addrlen;
    int nbytes;
    char msgbuf[MSGBUFSIZE];

   while (1)
    {
        memset(&msgbuf, 0, MSGBUFSIZE);

        addrlen = sizeof(addr);
        if ((nbytes = recvfrom(fd, msgbuf, MSGBUFSIZE, 0, (struct sockaddr *)&addr, &addrlen)) < 0)
        {
            perror("recvfrom");
            exit(1);
        }
       cout.write(msgbuf, nbytes);
        cout.flush();
    }

    return 0;
}

Thanks in advance ...

Answer 1

After some searching and testing I found out here that when binding udp multicast socket we specify port and leave address empty e.g. specify INADDR_ANY.

So the following

addr.sin_family = AF_INET;
addr.sin_port = htons(port);
addr.sin_addr.s_addr = (source_iface.empty() ?
                        htonl(INADDR_ANY) : 
                        inet_addr(source_iface.c_str()));

should be look like:

COMMENT: If I understand your code you should be binding to your multicast address not the wildcard address. If you bind to the wildcard address you will be able to receive unicast packets on your multicast port. Binding to your multicast address will prevent this and ensure you only get multicast packets on that port.

EDIT: Fixed the code based on above comment, binding to multicast address, stored in 'group', as opposed to INADDR_ANY to receive only multicast packets sent to multicast address.

addr.sin_family = AF_INET;
addr.sin_port = htons(port);
addr.sin_addr.s_addr = (group.empty() ?
                        htonl(INADDR_ANY) :
                        inet_addr(group.c_str()));

This solved the problem. Adding IP_MULTICAST_IF will not help because that is for selecting specific interface for sending udp data, the problem above was on receiving side.