app: use x-forwarded-for header to get ips

app/lib/prymn_web/controllers/server_controller.ex

@@ -9,7 +9,16 @@ defmodule PrymnWeb.ServerController do
   validating their registration token.
   """
   def register(conn, %{"token" => token}) do
-    case Servers.register_server(token, conn.remote_ip) do
+    ip =
+      with [ip] <- Plug.Conn.get_req_header(conn, "x-forwarded-for"),
+           {:ok, ip} <- :inet.parse_address(to_charlist(ip)) do
+        ip
+      else
+        {:error, reason} -> raise "Could not parse ip address: #{inspect(reason)}"
+        [] -> conn.remote_ip
+      end
+
+    case Servers.register_server(token, ip) do
       {:ok, _server} ->
         json(conn, %{"connected" => true})