prop/lib/microwaveprop_web/plugs/remote_ip.ex

defmodule MicrowavepropWeb.Plugs.RemoteIp do
  @moduledoc """
  Extracts the client IP from `Cf-Connecting-Ip` / `X-Forwarded-For` headers,
  but only when the immediate peer (`conn.remote_ip`) is a trusted proxy.

  Deployment: the app runs as pods in the k8s `prop` namespace behind the
  cluster ingress, which itself sits behind Cloudflare. The only hosts that
  should ever terminate TCP to a pod are ingress replicas on the cluster
  pod network, so we only honour forwarded-IP headers when the raw socket
  peer falls inside a configured CIDR list.

  Trusted proxies are configured via `config :microwaveprop, :trusted_proxies`
  (a list of CIDR strings). The default list covers loopback and the RFC1918
  / ULA ranges used by k8s cluster pod networks. `config/runtime.exs` reads
  `TRUSTED_PROXY_CIDRS` (comma-separated) in production for tuning without
  a rebuild.

  Rationale: without this guard any client that reaches a pod directly
  (cluster-internal, `kubectl port-forward`, a misconfigured Service) could
  spoof `Cf-Connecting-Ip` to set `conn.remote_ip`, which flows into session
  storage and logs.
  """

  @behaviour Plug

  import Bitwise

  require Logger

  @ip_headers ["cf-connecting-ip", "x-forwarded-for"]

  # RFC1918 + CGNAT + loopback + link-local + IPv6 loopback + ULA.
  # Matches typical k8s pod/service CIDRs and the ingress mesh.
  @default_trusted_proxies [
    "127.0.0.0/8",
    "10.0.0.0/8",
    "172.16.0.0/12",
    "192.168.0.0/16",
    "100.64.0.0/10",
    "169.254.0.0/16",
    "::1/128",
    "fc00::/7",
    "fe80::/10"
  ]

  @impl true
  def init(opts) do
    cidrs =
      Keyword.get_lazy(opts, :trusted_proxies, fn ->
        Application.get_env(:microwaveprop, :trusted_proxies, @default_trusted_proxies)
      end)

    parsed =
      cidrs
      |> Enum.map(&parse_cidr/1)
      |> Enum.reject(&is_nil/1)

    %{trusted_proxies: parsed}
  end

  @impl true
  def call(conn, %{trusted_proxies: trusted_proxies}) do
    conn =
      if peer_trusted?(conn.remote_ip, trusted_proxies) do
        case extract_forwarded_ip(conn) do
          nil -> conn
          ip -> %{conn | remote_ip: ip}
        end
      else
        conn
      end

    Logger.metadata(
      remote_ip: format_ip(conn.remote_ip),
      method: conn.method,
      request_path: conn.request_path
    )

    conn
  end

  defp extract_forwarded_ip(conn) do
    Enum.find_value(@ip_headers, fn header ->
      case Plug.Conn.get_req_header(conn, header) do
        [value | _] ->
          value
          |> String.split(",")
          |> hd()
          |> String.trim()
          |> parse_ip()

        _ ->
          nil
      end
    end)
  end

  defp parse_ip(str) do
    case :inet.parse_address(String.to_charlist(str)) do
      {:ok, ip} -> ip
      _ -> nil
    end
  end

  # Parses "10.0.0.0/8" or "fc00::/7" into {ip_integer, mask_integer, bit_width}.
  # Invalid entries are logged and dropped at init-time.
  defp parse_cidr(cidr) when is_binary(cidr) do
    case String.split(cidr, "/", parts: 2) do
      [ip_str, prefix_str] ->
        with {prefix, ""} <- Integer.parse(prefix_str),
             {:ok, ip} <- :inet.parse_address(String.to_charlist(ip_str)),
             bit_width = bit_width_for(ip),
             true <- prefix >= 0 and prefix <= bit_width do
          mask = cidr_mask(prefix, bit_width)
          {ip_to_integer(ip), mask, bit_width}
        else
          _ ->
            Logger.warning("RemoteIp: could not parse CIDR #{inspect(cidr)}")
            nil
        end

      _ ->
        Logger.warning("RemoteIp: CIDR missing /prefix: #{inspect(cidr)}")
        nil
    end
  end

  defp parse_cidr(_), do: nil

  defp bit_width_for(ip) when tuple_size(ip) == 4, do: 32
  defp bit_width_for(ip) when tuple_size(ip) == 8, do: 128

  # Build a prefix-bit mask: e.g. prefix=24, bit_width=32 -> 0xFFFFFF00.
  defp cidr_mask(0, _bit_width), do: 0

  defp cidr_mask(prefix, bit_width) do
    host_bits = bit_width - prefix
    all_ones = (1 <<< bit_width) - 1
    all_ones - ((1 <<< host_bits) - 1)
  end

  defp peer_trusted?(peer_ip, trusted) when tuple_size(peer_ip) in [4, 8] do
    peer_width = bit_width_for(peer_ip)
    peer_int = ip_to_integer(peer_ip)

    Enum.any?(trusted, fn {cidr_int, mask, bit_width} ->
      bit_width == peer_width and band(peer_int, mask) == band(cidr_int, mask)
    end)
  end

  defp peer_trusted?(_, _), do: false

  defp ip_to_integer({a, b, c, d}) do
    (a <<< 24) + (b <<< 16) + (c <<< 8) + d
  end

  defp ip_to_integer({a, b, c, d, e, f, g, h}) do
    (a <<< 112) + (b <<< 96) + (c <<< 80) + (d <<< 64) +
      (e <<< 48) + (f <<< 32) + (g <<< 16) + h
  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