Multicast & Tunnel devices

Would anyone know if there is something special with tunnel devices and multicast ? 

I’ve got some code that happily processes multicast packets on a normal interface; but appears not to do this on a tunnel interface. Tun0 shows multicast enabled:
	
	tun0: flags=8043<UP,BROADCAST,RUNNING,MULTICAST> metric 0 mtu 1500

Tcpdump on that interface gives the expected thing (here with mDNS):

	tcpdump -n -i tun0 port 5353
	listening on tun0, link-type NULL (BSD loopback), capture size 262144 bytes
	19:26:03.976259 IP 10.31.0.6.5353 > 224.0.0.251.5353: 0 PTR (QM)? _raop._tcp.local. (34)

And code, with a simple IP_ADD_MEMBERSHIP  of the MC group on the IP of the local interface below works on a normal interface (e.g. igb0/10.0.0.1/24). 

	./listener 10.0.0.1 224.0.0.251 5353
	Received packet, len=128
	etc

But yields no output if ran against above tun0 interface (while tcpdump on same is fine). Does that ring a bell with anyone ?

Dw


int main(int argc, char *argv[])
{
    struct sockaddr_in addr;
    struct ip_mreq mreq;

	// skip error trapping command line arguments

    char* ip = argv[1]; 
    char* group = argv[2]; 
    int port = atoi(argv[3]); // 0 if error, which is an invalid port

    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_interface.s_addr = inet_addr(ip); 
    mreq.imr_multiaddr.s_addr = inet_addr(group);

	// skip error trapping on inet_addr

    int fd = socket(AF_INET, SOCK_DGRAM, 0);
	// skip error trapping socket

    if (bind(fd, (struct sockaddr*) &addr, sizeof(addr)) < 0) {
	// skip error trapping

    if (setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*) &mreq, sizeof(mreq)) < 0 ){
	// skip error trapping argumetns

    while (1) {
	..
        int nbytes = recvfrom(fd,msgbuf,MSGBUFSIZE,0,(struct sockaddr *) &addr,&addrlen);
        if (nbytes < 0) {
            perror("recvfrom");
            return 1;
        }
	printf(“Received packet, len=%d\n", nbytes);
     }