use multicast address for announce (#3241)

* use multicast address for announce

* Add explicit multicast group membership

* join multicast group on a correct side

---------

Co-authored-by: Evgeny Poberezkin <2769109+epoberezkin@users.noreply.github.com>

src/Simplex/Chat/Remote/Multicast.hsc

@@ -0,0 +1,43 @@
+module Simplex.Chat.Remote.Multicast (setMembership) where
+
+import Foreign (Ptr, allocaBytes, castPtr, pokeByteOff)
+import Foreign.C.Types (CInt (..))
+import Network.Socket
+
+#include <HsNet.h>
+
+{- | Toggle multicast group membership.
+
+NB: Group membership is per-host, not per-process. A socket is only used to access system interface for groups.
+-}
+setMembership :: Socket -> HostAddress -> Bool -> IO Bool
+setMembership sock group membership = allocaBytes #{size struct ip_mreq} $ \mReqPtr -> do
+  #{poke struct ip_mreq, imr_multiaddr} mReqPtr group
+  #{poke struct ip_mreq, imr_interface} mReqPtr (0 :: HostAddress) -- attempt to contact the group on ANY interface
+  withFdSocket sock $ \fd ->
+    (/= 0) <$> c_setsockopt fd c_IPPROTO_IP flag (castPtr mReqPtr) (#{size struct ip_mreq})
+  where
+    flag = if membership then c_IP_ADD_MEMBERSHIP else c_IP_DROP_MEMBERSHIP
+
+#ifdef mingw32_HOST_OS
+
+foreign import stdcall unsafe "setsockopt"
+    c_setsockopt :: CInt -> CInt -> CInt -> Ptr CInt -> CInt -> IO CInt
+
+c_IP_ADD_MEMBERSHIP, c_IP_DROP_MEMBERSHIP :: CInt
+c_IP_ADD_MEMBERSHIP  = 12
+c_IP_DROP_MEMBERSHIP = 13
+
+#else
+
+foreign import ccall unsafe "setsockopt"
+    c_setsockopt :: CInt -> CInt -> CInt -> Ptr CInt -> CInt -> IO CInt
+
+c_IP_ADD_MEMBERSHIP, c_IP_DROP_MEMBERSHIP :: CInt
+c_IP_ADD_MEMBERSHIP  = #const IP_ADD_MEMBERSHIP
+c_IP_DROP_MEMBERSHIP = #const IP_DROP_MEMBERSHIP
+
+#endif
+
+c_IPPROTO_IP :: CInt
+c_IPPROTO_IP = #const IPPROTO_IP