UDP: Listening to the same port for two different multicast streams

Question

I need to listen to 2 different multicast groups using the same port. Program A will listen from 230.0.0.1 and Program B from 230.0.0.2. Both multicast groups use the same port 2000 and I have no control over it.

When I run my programs I receive both multicast streams in each program, that is both the data packets broadcasted on 230.0.0.1 and 230.0.0.2. I suspect the problem is due to the common port. This is the code I am using to subscribe to the multicast:

if( (sd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0 ) {
  perror("socket");
  return -1;
}

if( setsockopt(sd, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse)) < 0 ) {
  perror("setsockopt SO_REUSEADDR");
  return -1;
}

memset(&in_addr, 0, sizeof(in_addr));
in_addr.sin_family = AF_INET;
in_addr.sin_addr.s_addr = htonl(INADDR_ANY);
in_addr.sin_port = htons(2000);
if( bind(sd, (struct sockaddr*)&in_addr, sizeof(in_addr)) < 0 ) {
  perror("bind");
  return -1;
}

memset(&req, 0, sizeof(req));
inet_aton(intfc_ip, &req.imr_interface);
inet_aton("230.0.0.1", &req.imr_multiaddr);
if( setsockopt(sd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &req, sizeof(req)) < 0 ) {
  perror("setsockopt IP_ADD_MEMBERSHIP");
  return -1;
}

recv()...

How can I filter a specific multicast group in each program?

Answer 1

If you change

in_addr.sin_addr.s_addr = htonl(INADDR_ANY);

to

inet_aton(<your wanted IP address>, &in_addr.sin_addr.s_addr);

you could have more success.

(And if you change your program to work with getaddrinfo(), you make it future-proof.)

Answer 2

"connect" might be what you need after all. Typically, for connecting TCP sockets, the man page also suggests that it can be used for filtering out UDP packets from other addresses:

From the man page posted here:

If the socket sockfd is of type SOCK_DGRAM then addr is the address to which datagrams are sent by default, and the only address from which datagrams are received.