Redirect to canonical domain

config/prod.exs

@@ -7,7 +7,7 @@ config :chess, ChessWeb.Endpoint,
   root: "./assets",
   secret_key_base: "${SECRET_KEY_BASE}",
   server: true,
-  url: [host: "localhost", port: {:system, "PORT"}],
+  url: [host: "${HOST}", port: {:system, "PORT"}],
   version: Application.spec(:chess, :vsn)
 
 config :chess, Chess.Mailer,

lib/chess_web/endpoint.ex

@@ -23,6 +23,10 @@ defmodule ChessWeb.Endpoint do
     plug Phoenix.CodeReloader
   end
 
+  if Mix.env == :prod do
+    plug ChessWeb.Plugs.CanonicalDomain
+  end
+
   plug Plug.RequestId
   plug Plug.Logger
 

lib/chess_web/plugs/canonical_domain.ex

@@ -0,0 +1,28 @@
+defmodule ChessWeb.Plugs.CanonicalDomain do
+  @moduledoc false
+
+  import Plug.Conn
+
+  def init(options) do
+    options
+  end
+
+  def call(conn, _options) do
+    if conn.host != canonical_host() do
+      conn
+      |> put_status(:moved_permanently)
+      |> Phoenix.Controller.redirect(external: canonical_domain(conn))
+      |> halt()
+    else
+      conn
+    end
+  end
+
+  defp canonical_domain(conn) do
+    "//#{canonical_host()}#{conn.request_path}"
+  end
+
+  defp canonical_host() do
+    ChessWeb.Endpoint.config(:url)[:host]
+  end
+end