Log client IP from X-Forwarded-For in request logs

Add RemoteIp plug that extracts real client IP from X-Forwarded-For
header (set by nginx/dokku proxy) and adds it to Logger metadata.

config/config.exs

@@ -20,7 +20,7 @@ config :esbuild,
 # Configure Elixir's Logger
 config :logger, :default_formatter,
   format: "$time $metadata[$level] $message\n",
-  metadata: [:request_id]
+  metadata: [:request_id, :remote_ip]
 
 # Configure the mailer
 #

lib/microwaveprop_web/endpoint.ex

@@ -41,6 +41,7 @@ defmodule MicrowavepropWeb.Endpoint do
     cookie_key: "request_logger"
 
   plug Plug.RequestId
+  plug MicrowavepropWeb.Plugs.RemoteIp
   plug Plug.Telemetry, event_prefix: [:phoenix, :endpoint]
 
   plug Plug.Parsers,

lib/microwaveprop_web/plugs/remote_ip.ex

@@ -0,0 +1,48 @@
+defmodule MicrowavepropWeb.Plugs.RemoteIp do
+  @moduledoc """
+  Extracts the client IP from X-Forwarded-For (set by nginx/dokku proxy)
+  and assigns it to conn.remote_ip and Logger metadata.
+  """
+
+  @behaviour Plug
+
+  @impl true
+  def init(opts), do: opts
+
+  @impl true
+  def call(conn, _opts) do
+    ip =
+      case Plug.Conn.get_req_header(conn, "x-forwarded-for") do
+        [forwarded | _] ->
+          forwarded
+          |> String.split(",")
+          |> hd()
+          |> String.trim()
+          |> parse_ip()
+
+        _ ->
+          nil
+      end
+
+    conn =
+      if ip do
+        %{conn | remote_ip: ip}
+      else
+        conn
+      end
+
+    Logger.metadata(remote_ip: format_ip(conn.remote_ip))
+    conn
+  end
+
+  defp parse_ip(str) do
+    case :inet.parse_address(String.to_charlist(str)) do
+      {:ok, ip} -> ip
+      _ -> nil
+    end
+  end
+
+  defp format_ip({a, b, c, d}), do: "#{a}.#{b}.#{c}.#{d}"
+  defp format_ip({a, b, c, d, e, f, g, h}), do: "#{a}:#{b}:#{c}:#{d}:#{e}:#{f}:#{g}:#{h}"
+  defp format_ip(_), do: "unknown"
+end