add IP stack simulator

src/DNSServer.hh

@@ -21,6 +21,11 @@ public:
   void listen(int port);
   void add_socket(int fd);
 
+  static std::string response_for_query(
+      const void* vdata, size_t size, uint32_t resolved_address);
+  static std::string response_for_query(
+      const std::string& query, uint32_t resolved_address);
+
 private:
   std::shared_ptr<struct event_base> base;
   std::unordered_map<int, std::unique_ptr<struct event, void(*)(struct event*)>> fd_to_receive_event;