(usagi-users 03628) Re: IPv6 multicast socket specific binding bug

[David Stevens <dlstevens@xxxxxxxxxx>]

 > If you use source filtering API and do include mode with (S,G), then I
> assume you would not receive from another source for the same group.
> Would/should you still receive multicast for other groups (if say some
> other group is joined on another socket and the bind address/port
> matches...)?

        The per-socket filters are applied by group, so an 
INADDR_ANY-bound
socket would receive from any source allowed for filters for that group.
If there is no join (& filter) on that socket, then the default (per-RFC)
is "{EXCLUDE, empty}" meaning it would be delivered.
 
> Would a possible work-around be to use source filtering API and join a
> non-existent source to prevent receiving packets on the other socket? :)
> I suppose you can't set an include mode source filter with no sources to
>  block everything.

        I expect that would work. "{INCLUDE, empty}" is identical to a
"leave group", which results in no filter at all on that socket. So,
packets would still be delivered for that group from others, if the
binding allows it. But you can always create a filter for "{INCLUDE, X}",
as you said. It's just that you have to do that for all groups that
someone on the machine might have joined and that might have traffic
to your port.
        So, that is a bit of a hack.

        These aren't the same as making a join per-socket, which is
what the original discussion was about. Because a per-socket join
(if it existed :-) ) wouldn't restrict based on source of the sender,
but on the destination. That's the same as binding to the multicast
group address. The only issue there, really, is that you can only
bind to one address, so you can't use a single socket to receive
2 different multicast groups and nothing else. But you also can't
do that with multiple unicast addresses.

        I think it comes down to asking when designing the program
what you really want. If you want only multicasts for a particular
group from any source, the you should bind to the multicast group.
That still doesn't restrict it to a particular interface, but on
Linux you can also do an SO_BINDTODEVICE to get that.
If you want all packets to the port, whether multicast or unicast,
then you should bind to INADDR_ANY. If you want unicast-only, then
you should bind to an individual unicast address (with a new socket
for each unicast address). And if you want to further restrict
the source for multicasts, then you can add source filtering to
one of the above strategies.
        These rules aren't any different than they are for unicast
addresses. Nobody is suggesting that an INADDR_ANY-bound socket
should only receive local IP unicasts if the unicast address was
added via SIOCSIFADDR on that very socket you're doing the I/O on!
A "join" is really the same, in sockets, as adding a new unicast
address to an interface; it has nothing whatever to do with what
packets that particular socket receives, aside from affecting
what packets are delivered to the machine.
        Multicast filters, unfortunately, muddy the water, because
those *are* per-socket. It matters what socket you do the filter
ioctl()'s on, but it doesn't for the joins.

                                                +-DLS