IPV6 link local multicasting.

Question

I'm trying to figure out how to do the equivalent of an IPV4 broadcast using IPV6.

I'm creating a non-blocking IPV6 UDP socket.

From the side broadcasting i'm literally just doing a sendto "FF02::1" on port 12346.

On the listen side I discovered I need to join the group so I did the following:

    ipv6_mreq membership;
    memset( &membership.ipv6mr_multiaddr, 0, sizeof( in6_addr ) );
    membership.ipv6mr_multiaddr.u.Word[0]   = htons( 0xff02 );
    membership.ipv6mr_multiaddr.u.Word[7]   = htons( 0x0001 );
    membership.ipv6mr_interface             = 0;

    if( enable )
    {
        if ( 0 != setsockopt( m_Socket, SOL_SOCKET, IPV6_JOIN_GROUP, (char*)&membership, sizeof( ipv6_mreq ) ) )
        {
            DisplayError();
            return false;
        }
    }

However setsockopt always returns "WSAENOPROTOOPT". Why? Can anyone help me on this one? I'm at a complete loss.

Edit: I change the level to "IPPROTO_IPV6" but now I get a "WSAEINVAL".

Answer 1

I think the problem is that you are leaving the ipv6mr_interface value at zero, which isn't good enough if you want to use a link-scope multicast address like ff02::1. You need to set the ipv6mr_interface value to the number corresponding with the local network interface you want the packets to be sent/received on. (You can find out what interface indices are available on the current computer by calling getaddrinfo() and reading the sin6_addr.s6_addr values out of the (struct sockaddr_in6 *)'s that it hands to you)

(If at this point you are thinking to yourself, wouldn't it be so much easier if interface zero acted as an "all interfaces" setting... yes, it would be. Alas, IPv6 doesn't do that for some reason :( )

Answer 2

The interface must be set for locally scoped IPv6 because the addresses are only unique to the interface. In simpler terms the address fe80::1 can belong to both eth0 and eth1 but are completely separate.

So this means you need to explicitly send a multicast packet on every up interface that supports multicast, or provide the user with a means of specifying a particular interface.

(edit) If it helps you can check out multicast code here,

http://code.google.com/p/openpgm/source/browse/trunk/openpgm/pgm/