The only way really to respond to a DDos attack on a server, is to switch off ping response on your router. You can also switch ping response off from command level, but you will have issues with your server running, as the login gate uses Tcp Ping request to check that the gate is online.
You also need to enable to log facility on your router, as this will indicate if it is a DDos attack or not. As it may just be a response to your server, as Kaori's online checker did this.