Joining multiple Multicast Streams

[Pieter de Goeje]

On Thursday 17 December 2009 19:45:24 Dex Nada wrote:
> Hi:
>
> I am writing an application that joins a multicast stream on a specific UDP
> port. But when I run more than one instance of the same application, the
> second instance complains that the port is already in use. For example if I
> join stream 229.10.10.133:2000 on one instance and 229.10.10..134:2000 on
> another instance, the second one fails to join - I can however join it if I
> kill the first instance. I have compiled that application with SOCKET_REUSE
> option, but I wonder if I need to enable/recompile-with any special
> multicast kernel option for this to work. I do not have this problem when I
> run this code on Linux (Ubuntu 9.10) but I really want to get this working
> on FreeBSD.
>
> Thanks in advance.
>
> -DxN

Try SO_REUSEPORT. The manpage (getsockopt(2)) specifically mentions multiple 
listeners for the same multicast stream. The following pseudo C seems to work 
fine:

int reuseport = 1;

memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_ANY);
addr.sin_port = htons(port);

mreq.imr_multiaddr.s_addr = maddr;
mreq.imr_interface.s_addr = INADDR_ANY;

int fd = socket(AF_INET, SOCK_DGRAM, 0);
setsockopt(fd, SOL_SOCKET, SO_REUSEPORT, &reuseport, sizeof(reuseport));
bind(fd, (struct sockaddr *)&addr, sizeof(addr));
setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
recvfrom(fd, ....);

Good luck,

Pieter de Goeje