Re: Multicast & Tunnel devices

> On 29 Apr 2024, at 03:09, Rodney W. Grimes <freebsd-rwg@gndrsh.dnsmgr.net> wrote:
> 
>> 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
> 
> Is 10.0.0.1 the IP address of tun0, or is it the address of some other interface?
> I suspect that the IP address of the tun0 interface is 10.31.0.6 from your tcpdump above.

That is correct 10.0.0.1/8. 10.31.0.6 is another machine at the other end of the tunnel broadcasting.

> IIRC you have to join multicast group on all interfaces you expect to receive mustcast packets on.
> 
>> 	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);
>>     }
>>