multicast join problems (source specific)

[Mark Geib]

Yes, this suggestion is the method to join a any-source, or legacy, multicast. And this does work correctly.

The problem I am having is trying to join a IGMPv3 source-specific multicast where the IP of the source producing the multicast must be included, which is the purpose of the {raw..} option. The normal {add_membership…} does not work with source-specific multicast.

After digging into the details of multicast by reading in "UNIX Network Programming” the problem may be that joining multicast groups is done at the IP level in the network stack and apparently all sockets on a network interface will receive the datagrams for all multicasts when there is more than one, so a bind() needs to be done to create a “filter” for each socket so that only datagrams from the configured multicast are sent to each socket. Maybe the {add_membership…} option includes this bind(), but of course the {raw…} does not.

I am thinking the way to address this problem will be using the new socket module, which provides a bind() function.

Thanks.


> On Jan 20, 2021, at 1:54 AM, Sergej Jurečko <sergej.jurecko@REDACTED> wrote:
> 
> Try connecting to multicast like so (Mc is multicast IP, Src is local IP):
> 
> {ok, SockSrc} = gen_udp:open(Port, [{reuseaddr,true},
> {ip, {0,0,0,0}}, {add_membership, {Mc, Src}},{active,true}, binary,{recbuf, 1024*1024}]);
> 
> Or maybe
> 
> {ok, SockSrc} = gen_udp:open(Port, [{reuseaddr,true},
> {ip, Src}, {multicast_ttl, 1}, {multicast_loop, false}, binary]),
> inet:setopts(SockSrc, [{add_membership, {Mc, Src}}]);
> 
> On Mon, Jan 18, 2021 at 10:20 PM Mark Geib <mark.geib.44@REDACTED <mailto:mark.geib.44@REDACTED>> wrote:
> I have a problem when joining more than one source specific multicast group. The first gen_udp:open() works as expected, I start receiving datagrams sent to the specified multicast address, and coming from the specified source. However, in another gen_server if I try to do the same, but on a different multicast and source address, I start receiving the datagrams on both the new socket as well as the first socket and from both multicast groups. The open looks like:
> 
> Port = 3100,
> Bin = << <<239,0,0,1>>/binary, <<0,0,0,0>>/binary, <<10,0,1,1>>/binary >>,
> {ok, _Socket} = gen_udp:open(Port,
>                              [binary,
>                               {active, true},
>                               {reuseaddr, true},
>                               {multicast_ttl, 30},
>                               {raw, 0, 39, Bin}]), %% 0 - IPPROTO_IP, 39 - IP_ADD_SOURCE_MEMBERSHIP
> 
> I am able to join multiple multicasts, non source specific, and receive only datagrams from the correct group by using an open like:
> 
> Port = 3100,
> {ok, _Socket} = gen_udp:open(Port,
>                              [binary,
>                               {active, true},
>                               {reuseaddr, true},
>                               {multicast_ttl, 30},
>                               {add_membership, {239,0,0,1}, {0,0,0,0}}]),
> 
> Any suggestions or help would be appreciated.
> 
> Mark.

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