Erlang Multicast

Question

How do you use gen_udp in Erlang to do multicasting? I know its in the code, there is just no documentation behind it. Sending out data is obvious and simple. I was wondering on how to add memberships. Not only adding memberships at start-up, but adding memberships while running would be useful too.

Answer 1

Here is example code on how to listen in on Bonjour / Zeroconf traffic.

-module(zcclient).

-export([open/2,start/0]).
-export([stop/1,receiver/0]).

open(Addr,Port) ->
   {ok,S} = gen_udp:open(Port,[{reuseaddr,true}, {ip,Addr}, {multicast_ttl,4}, {multicast_loop,false}, binary]),
   inet:setopts(S,[{add_membership,{Addr,{0,0,0,0}}}]),
   S.

close(S) -> gen_udp:close(S).

start() ->
   S=open({224,0,0,251},5353),
   Pid=spawn(?MODULE,receiver,[]),
   gen_udp:controlling_process(S,Pid),
   {S,Pid}.

stop({S,Pid}) ->
   close(S),
   Pid ! stop.

receiver() ->
   receive
       {udp, _Socket, IP, InPortNo, Packet} ->
           io:format("~n~nFrom: ~p~nPort: ~p~nData: ~p~n",[IP,InPortNo,inet_dns:decode(Packet)]),
           receiver();
       stop -> true;
       AnythingElse -> io:format("RECEIVED: ~p~n",[AnythingElse]),
           receiver()
   end. 

Answer 2

Multicast sending has been answered, receipt requires subscription to the multicast group.

It (still) seems undocumented, but has been covered on the erlang-questions mailing list before. http://www.erlang.org/pipermail/erlang-questions/2003-March/008071.html

    {ok, Socket} = gen_udp:open(Port, [binary, {active, false},
                                       {reuseaddr, true},{ip, Addr}, 
                                       {add_membership, {Addr, LAddr}}]).

where the Addr is the multicast group, and LAddr is a local interface. (code courtesy of mog)

The same options used above can be passed to inet:setopts including {drop_membership, {Addr, LAddr}} to stop listening to the group.

Answer 3

I try to get this example running on my PC. What could happen, if I get always the message {error,eaddrnotavail} by opening the receive socket?

Example 1: This works:

{ok, Socket} = gen_udp:open(?PORT, [{reuseaddr,true}, {ip,?SERVER_IP},
               {multicast_ttl,4}, {multicast_loop,false}, binary]),

Example 2: Getting an runtime Error:

{ok, Socket} = gen_udp:open(?PORT, [{reuseaddr,true}, {ip,?MULTICAST_IP},
               {multicast_ttl,4}, {multicast_loop,false}, binary]),

% --> {error,eaddrnotavail}

-define(SERVER_IP, {10,31,123,123}). % The IP of the current computer
-define(PORT, 5353).
-define(MULTICAST_IP, {224,0,0,251}). 

Answer 4

Multicast is specified by IP Address

It's the same in erlang as for all languages. The IP addresses 224.0.0.0 through 239.255.255.255 are multicast addresses.

Pick an address in that range, check that you're not overlapping an already assigned address, and you are good to go.

http://www.iana.org/assignments/multicast-addresses