analytics/lib/plausible_web/plugs/favicon.ex

defmodule PlausibleWeb.Favicon do
  import Plug.Conn

  def init(_) do
    domains =
      case File.read(Application.app_dir(:plausible, "priv/referer_favicon_domains.json")) do
        {:ok, contents} ->
          Jason.decode!(contents)

        _ ->
          %{}
      end

    [favicon_domains: domains]
  end

  def call(conn, favicon_domains: favicon_domains) do
    case conn.path_info do
      ["favicon", "sources", source] ->
        clean_source = URI.decode_www_form(source)
        domain = Map.get(favicon_domains, clean_source, clean_source)
        res = HTTPoison.get!("https://icons.duckduckgo.com/ip3/#{domain}.ico")

        conn =
          Enum.filter(res.headers, fn {key, _val} -> key != "Transfer-Encoding" end)
          |> Enum.reduce(conn, fn {key, val}, conn ->
            put_resp_header(conn, key, val)
          end)

        send_resp(conn, 200, res.body)
        |> halt

      _ ->
        conn
    end
  end
end