make dns server configurable.

config/config.exs

@@ -2,29 +2,8 @@
 # and its dependencies with the aid of the Mix.Config module.
 use Mix.Config
 
-# This configuration is loaded before any dependency and is restricted
-# to this project. If another project depends on this project, this
-# file won't be loaded nor affect the parent project. For this reason,
-# if you want to provide default values for your application for
-# 3rd-party users, it should be done in your "mix.exs" file.
+config :dough,
+  # default, an opennic server.
+  dns_server: "37.235.1.174"
 
-# You can configure your application as:
-#
-#     config :dough, key: :value
-#
-# and access this configuration in your application as:
-#
-#     Application.get_env(:dough, :key)
-#
-# You can also configure a 3rd-party app:
-#
-#     config :logger, level: :info
-#
-
-# It is also possible to import configuration files, relative to this
-# directory. For example, you can emulate configuration per environment
-# by uncommenting the line below and defining dev.exs, test.exs and such.
-# Configuration from the imported file will override the ones defined
-# here (which is why it is important to import them last).
-#
 #     import_config "#{Mix.env}.exs"

lib/dough/dohplug.ex

@@ -83,8 +83,9 @@ defmodule Dough.DoHPlug do
   end
 
   def handoff_dns(dns_message) do
+    dns_server = Application.get_env(:dough, :dns_server)
     client = Socket.UDP.open!()
-    send!(client, dns_message, {"8.8.8.8", 53})
+    send!(client, dns_message, {dns_server, 53})
     {data, _server} = recv!(client)
     data
   end