Vfrdtyky

(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] = '';



    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] = '';// 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 ?

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);