-module(bridge).

-export([start/2]).

-define(SIOCGIFINDEX, 16#8933).
-define(ETH_P_ALL,    16#0300).

start(Iface1, Iface2) ->
    S1 = bind(Iface1),
    S2 = bind(Iface2),
    spawn(fun() -> loop(S1, S2) end),
    spawn(fun() -> loop(S2, S1) end).

loop(Src, Dst) ->
    case socket:recv(Src) of
        {error, _} ->
            loop(Src, Dst);

        {ok, Pkt} ->
            _ = socket:send(Dst, Pkt),
            loop(Src, Dst)
    end.

%% private functions

bind(Iface) ->
    {ok, S} = socket:open(packet, raw, {raw, ?ETH_P_ALL}),
    {ok, 0} = socket:bind(S, sockaddr_ll(ifindex_of(S, Iface))),
    S.

ifindex_of(S, Iface) ->
    {ok, Return} = socket:ioctl(S, ?SIOCGIFINDEX, ifreq(Iface)),
    <<_Ifname:16/bytes, Ifr:32/native, _/binary>> = Return,
    Ifr.

sockaddr_ll(Ifr) ->
    #{
      family   => packet,
      protocol => 0,
      ifindex  => Ifr,
      hatype   => 0,
      pkttype  => 0,
      halen    => 0,
      addr     => <<0:64>>
     }.

ifreq(Dev) ->
    list_to_binary(
      [
       Dev,
       <<0:((15*8) - (length(Dev)*8)), 0:8, 0:128>>
      ]
     ).