How to multicast with ipv6 udp socket in C/C++ on linux? - c++

Question

(English is not my native tongue, don't worry if some sentences are strange ;) ).
I was developing a PONG game and by the way creating some classes to help me managing window, event ... and network because I added a LAN feature to the game but currently you have to enter the address of the one with who you want to play with. And a solution to that was a broadcast (scanning LAN for player). This was easy with ipv4, just use the address 255.255.255.255 but we are in 2017 and provide a feature that works only with ipv4 sucks...
Then I look for a way to broadcast with ipv6 and I learn about multi-cast but this part just get me lost. =(
I use standard libraries on Linux in C++, I found several example of multi-cast that didn't work with me. The best I have done at this time is sending a udp packet from one instance of the program to an other on the same computer.
How can I multi-cast with ipv6 udp socket on Linux in C/C++ ?
The best code found on Internet (I rearranged it) that almost work
(there is client and serv all in one, choice is made by adding 1 or 0 to argv) :
int main(int argc, char const *argv[]) {
struct sockaddr_in6 groupSock;
int sd = -1;
char databuf[10];
int datalen = sizeof databuf;
/* Create a datagram socket on which to send/receive. */
if((sd = socket(AF_INET6, SOCK_DGRAM, 0)) < 0) {
perror("Opening datagram socket error");
return 1;
} else {
cout << "Opening the datagram socket...OK." << endl;;
}
/* Enable SO_REUSEADDR to allow multiple instances of this */
/* application to receive copies of the multicast datagrams. */
int reuse = 1;
if(setsockopt(sd, SOL_SOCKET, SO_REUSEADDR, (char *)&reuse, sizeof reuse) < 0) {
perror("Setting SO_REUSEADDR error");
close(sd);
return 1;
} else {
cout << "Setting SO_REUSEADDR...OK." << endl;
}
/* Initialize the group sockaddr structure with a */
memset((char *) &groupSock, 0, sizeof groupSock);
groupSock.sin6_family = AF_INET6;
// address of the group
inet_pton(AF_INET6, "ff0e::/16", &groupSock.sin6_addr);
groupSock.sin6_port = htons(4321);
/* Set local interface for outbound multicast datagrams. */
/* The IP address specified must be associated with a local, */
/* multicast capable interface. */
int ifindex = if_nametoindex ("enp3s0");
cout << "ifindex is " << ifindex << endl;
if(setsockopt(sd, IPPROTO_IPV6, IPV6_MULTICAST_IF, &ifindex, sizeof ifindex)) {
perror("Setting local interface error");
return 1;
} else {
cout << "Setting the local interface...OK" << endl;
}
// choice is 0 for sending and 1 for receiving
int choice;
if (argc < 2) {
cout << "missing argv[1]" << endl;
return 1;
}
sscanf (argv[1], "%d", &choice);
// if sending
if (choice == 0) {
memset(databuf, 'a', datalen);
databuf[sizeof databuf - 1] = '\0';
if (sendto(sd, databuf, datalen, 0, (sockaddr*)&groupSock, sizeof groupSock) < 0) {
cout << "Error in send" << endl;
} else {
cout << "Send okay!" << endl;
}
}
// if receiving
else if (choice == 1) {
groupSock.sin6_addr = in6addr_any;
if(bind(sd, (sockaddr*)&groupSock, sizeof groupSock)) {
perror("Binding datagram socket error");
close(sd);
return 1;
} else {
cout << "Binding datagram socket...OK." << endl;
}
/* Join the multicast group ff0e::/16 on the local */
/* interface. Note that this IP_ADD_MEMBERSHIP option must be */
/* called for each local interface over which the multicast */
/* datagrams are to be received. */
struct ipv6_mreq group;
inet_pton (AF_INET6, "ff0e::", &group.ipv6mr_multiaddr.s6_addr);
group.ipv6mr_interface = ifindex;
if(setsockopt(sd, IPPROTO_IPV6, IPV6_ADD_MEMBERSHIP, (char *)&group, sizeof group) < 0) {
perror("Adding multicast group error");
close(sd);
return 1;
} else {
cout << "Adding multicast group...OK." << endl;
}
if (read(sd, databuf, datalen) < 0) {
perror("Error in read");
} else {
databuf[sizeof databuf - 1] = '\0';// just for safety
cout << "Read Okay" << endl;
cout << "Message is : " << databuf << endl;
}
}
return 0;
}
Here the address is ff0e:: but I have try with ff01:: and ff02::.
I need help, I have not found any simple documentation about that. Thanks in advance for any answer.
Edit :
Thanks Ron Maupin and Jeremy Friesner for these comments, it helps me.
Edit :
THANKS Jeremy ! Your advice to use ff12::blah:blah(...) instead of ff0e:: works ! Should I write answer to my question to close the thread ?

Answer 1

This code below is right:
The only thing wrong is the address used for the multicast.
Like Jeremy said it, ff0e:: is not correct, I used instead ff12::feed:a:dead:beef and it works.
It is possible to get the name and index of the available interface by using if_nameindex().
Update : I try to remove some code to see if it work without it and I manage to get this :
server :
// OPEN
int fd = socket(AF_INET6, SOCK_DGRAM, 0);
// BIND
struct sockaddr_in6 address = {AF_INET6, htons(4321)};
bind(fd, (struct sockaddr*)&address, sizeof address);
// JOIN MEMBERSHIP
struct ipv6_mreq group;
group.ipv6mr_interface = 0;
inet_pton(AF_INET6, "ff12::1234", &group.ipv6mr_multiaddr);
setsockopt(fd, IPPROTO_IPV6, IPV6_ADD_MEMBERSHIP, &group, sizeof group);
// READ
char buffer[128];
read(fd, buffer, sizeof buffer);
client :
// OPEN
int fd = socket(AF_INET6, SOCK_DGRAM, 0);
// ADDRESS
struct sockaddr_in6 address = {AF_INET6, htons(4321)};
inet_pton(AF_INET6, "ff12::1234", &address.sin6_addr);
// SEND TO
char buffer[128];
strcpy(buffer, "hello world!");
sendto(fd, buffer, sizeof buffer, 0, (struct sockaddr*)&address, sizeof address);