A plug building toolkit for blocking and throttling abusive requests.

This is inspired by the Kickstarter's Rack::Attack middleware for Ruby.

If available in Hex, the package can be installed as:

1. Add plug_attack to your list of dependencies in mix.exs:

def deps do
  [{:plug_attack, "~> 0.4.2"}]
end

2. If using Elixir 1.3, ensure that plug_attack is started before your application
   (this step is no longer applicable to Elixir 1.4 ):

def application do
  [applications: [:plug_attack]]
end

We first need to construct a plug that will list various rules we want to apply:

defmodule MyApp.PlugAttack do
  use PlugAttack

  rule "allow local", conn do
    allow conn.remote_ip == {127, 0, 0, 1}
  end
end

The MyApp.PlugAttack module is now a regular plug that can be used, for example, in a phoenix endpoint.

WARNING: if you're behind a proxy, like nginx or heroku's router, you need to make sure you have a plug that respects the X-Forwarded-For headers, for example: remote_ip.

Before we implement throttling in our attack plug, we need to add a storage to our supervision tree. This can be achieved by adding following to the supervision tree:

children = [
  # other children
  worker(PlugAttack.Storage.Ets, [MyApp.PlugAttack.Storage, [clean_period: 60_000]])
]

# or using child specifications:

children = [
  {PlugAttack.Storage.Ets, name: MyApp.PlugAttack.Storage, clean_period: 60_000}
]

We've configured the table to be cleaned of stale data every minute. The usage patterns of the table by the throttling rules means no stale data will be ever accessed. This is only a measure used to control the memory usage.

Now we can add a rule to our plug allowing 10 requests every minute from a single ip address:

rule "throttle by ip", conn do
  throttle conn.remote_ip,
    period: 60_000, limit: 10,
    storage: {PlugAttack.Storage.Ets, MyApp.PlugAttack.Storage}
end

We can customize the actions taken by PlugAttack on blocked or allowed requests, by adding rate limiting headers for well behaved clients.

To do this, we can define two functions in our plug - allow_action/3 and block_action/3. Those are similar to regular plugs - accepting a connection as the first argument and opts as the last one. The middle argument represents the blocking or allowing data returned by the rule. The throttling rule returns data in the form of {:throttle, data}, where data is a keyword with various useful data we can use to construct rate limiting headers.

import Plug.Conn
def allow_action(conn, {:throttle, data}, opts) do
  conn
  |> add_throttling_headers(data)
  |> allow_action(true, opts)
end

def allow_action(conn, _data, _opts) do
  conn
end

def block_action(conn, {:throttle, data}, opts) do
  conn
  |> add_throttling_headers(data)
  |> block_action(false, opts)
end

def block_action(conn, _data, _opts) do
  conn
  |> send_resp(:forbidden, "Forbidden\n")
  |> halt # It's important to halt connection once we send a response early
end

defp add_throttling_headers(conn, data) do
  # The expires_at value is a unix time in milliseconds, we want to return one
  # in seconds
  reset = div(data[:expires_at], 1_000)
  conn
  |> put_resp_header("x-ratelimit-limit", to_string(data[:limit]))
  |> put_resp_header("x-ratelimit-remaining", to_string(data[:remaining]))
  |> put_resp_header("x-ratelimit-reset", to_string(reset))
end

Copyright 2015 Michał Muskała

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.