Handle X-Forwarded-Prefix with Plug.Traefik

plug_traefik.ex

defmodule Plug.Traefik do
  use Plug.Builder

  @assets_dir System.cwd <> "/priv/web/assets/"

  plug Plug.Static, at: "/assets", from: @assets_dir
  plug :x_forwarded_prefix

  def x_forwarded_prefix(conn, _) do
    register_before_send(conn, fn(conn) ->
      case get_req_header(conn, "x-forwarded-prefix") do
        [prefix] -> resp(conn, conn.status, replace(conn.resp_body, prefix))
        []       -> conn
      end
    end)
  end

  defp replace(source, prefix) when is_bitstring(source) do
    case String.starts_with?(source, "/") do
      true ->
        String.replace(source, "/", prefix <> "/", global: false)
      false ->
        source
        |> String.replace("src=\"/", "src=\"" <> prefix <> "/")
        |> String.replace("href=\"/", "href=\"" <> prefix <> "/")
    end
  end

  defp replace(source, prefix) when is_list(source) do
    source
    |> Enum.map(fn item -> replace(item, prefix) end)
  end

  defp replace(source, _prefix) do
    source
  end
end