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

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);