How to receive multicast UDP packets correctly? - c++

Question

new to multicast networking, I need to receive UDP packets from a multicast channel through one of the NICs on my windows box, followed Microsoft docs and some blog entry, but still having issues.
I create a socket via
ls = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
call.
Then setsockopt to SO_REUSEADDR
unsigned int reuse = 1;
if( setsockopt(ls, SOL_SOCKET, SO_REUSEADDR, (const char*)&reuse, sizeof(reuse)) < 0 )
{
LOG4CXX_ERROR(logger, "Reusing ADDR failed. Err: " << WSAGetLastError());
}
If socket is good
int result = bind(ls, reinterpret_cast<SOCKADDR*>(&server), sizeof(server));
where
server.sin_family = AF_INET;
server.sin_addr.s_addr = htonl(INADDR_ANY);
server.sin_port = htons(myport);
If bind succedes
struct ip_mreq mreq;
mreq.imr_multiaddr.s_addr = inet_addr("e.f.g.h");
mreq.imr_interface.s_addr = htonl(INADDR_ANY);
if( setsockopt(ls, IPPROTO_IP, IP_ADD_MEMBERSHIP, (const char*)&mreq, sizeof(mreq)) < 0 )
{
LOG4CXX_ERROR(logger, "setsockopt multicast group add membership failed. Err: " << WSAGetLastError());
}
The box on which I need to receive packets has four NICs, network administrators told me that I have to use the third one, let's say that it has a.b.c.d IPv4 address
They told me also that mcast IP is e.f.g.h
If I run windump.exe -i 3 on my windows box I see something like this
... 12:53:58.454987 IP i.k.l.m.xxxxx > e.f.g.h.myport: UDP, length 58
...
After initializing my UDP socket I call recvfrom
sz = recvfrom(ls, buffer, DATA_BLOCK_SIZE, 0, reinterpret_cast<SOCKADDR*>(&client), &size);
where sz is an int, ls is my socket, buffer is a "suitable buffer", DATA_BLOCK_SIZE is buffer size, client is a SOCKADDR pointer to receive info from the sender, and size is the received message size.
My code stucks in the recvfrom call never receiving anything.
I'm clearly making a mistake somewhere but not understanding where and worse why.
If someone can explain me what's happening it will be very appreciated.

Answer 1

SOLVED ...
I changed these lines only
struct ip_mreq mreq;
mreq.imr_multiaddr.s_addr = inet_addr("e.f.g.h");
mreq.imr_interface.s_addr = htonl(INADDR_ANY);
to
struct ip_mreq mreq;
inet_pton(AF_INET, "e.f.g.h", &(mreq.imr_multiaddr));
inet_pton(AF_INET, "a.b.c.d", &(mreq.imr_interface));
As I have guessed I was not correctly indicating in my struct ip_mreq which was the network interface to use for multicast messages.
It was my fault. Sorry for the noise.