Respond to multicast address for multicast queries

Without this fix the standard linux avahi-daemon won't accept
the responses from this library.

Signed-off-by: Miguel Angel Ajo <majopela@redhat.com>

Author: mangelajo

server.go

@@ -269,19 +269,24 @@ func (s *Server) handleQuestion(q dns.Question) (multicastRecs, unicastRecs []dn
 
 // sendResponse is used to send a response packet
 func (s *Server) sendResponse(resp *dns.Msg, from net.Addr, unicast bool) error {
-	// TODO(reddaly): Respect the unicast argument, and allow sending responses
-	// over multicast.
 	buf, err := resp.Pack()
 	if err != nil {
 		return err
 	}
 
 	// Determine the socket to send from
 	addr := from.(*net.UDPAddr)
+
 	if addr.IP.To4() != nil {
+		if !unicast {
+			addr.IP = net.ParseIP(ipv4mdns)
+		}
 		_, err = s.ipv4List.WriteToUDP(buf, addr)
 		return err
 	} else {
+		if !unicast {
+			addr.IP = net.ParseIP(ipv6mdns)
+		}
 		_, err = s.ipv6List.WriteToUDP(buf, addr)
 		return err
 	}