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Rack middleware for blocking & throttling

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Rack middleware for blocking & throttling abusive requests

Rack::Attack is a rack middleware to protect your web app from bad clients.
It allows safelisting, blocklisting, throttling, and tracking based on arbitrary properties of the request.

Throttle and fail2ban state is stored in a configurable cache (e.g. Rails.cache), presumably backed by memcached or redis (at least gem v3.0.0).

See the Backing & Hacking blog post introducing Rack::Attack.

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Getting started

Install the rack-attack gem; or add it to your Gemfile with bundler:

# In your Gemfile
gem 'rack-attack'

Tell your app to use the Rack::Attack middleware.
For Rails 3+ apps:

# In config/application.rb
config.middleware.use Rack::Attack

Or for Rackup files:

# In config.ru
use Rack::Attack

Add a rack-attack.rb file to config/initializers/:

# In config/initializers/rack-attack.rb
class Rack::Attack
  # your custom configuration...

Tip: The example in the wiki is a great way to get started:
Example Configuration

Optionally configure the cache store for throttling or fail2ban filtering:

Rack::Attack.cache.store = ActiveSupport::Cache::MemoryStore.new # defaults to Rails.cache

Note that Rack::Attack.cache is only used for throttling and fail2ban filtering; not blocklisting & safelisting. Your cache store must implement increment and write like ActiveSupport::Cache::Store.

How it works

The Rack::Attack middleware compares each request against safelists, blocklists, throttles, and tracks that you define. There are none by default.

  • If the request matches any safelist, it is allowed.
  • Otherwise, if the request matches any blocklist, it is blocked.
  • Otherwise, if the request matches any throttle, a counter is incremented in the Rack::Attack.cache. If any throttle's limit is exceeded, the request is blocked.
  • Otherwise, all tracks are checked, and the request is allowed.

The algorithm is actually more concise in code: See Rack::Attack.call:

def call(env)
  req = Rack::Attack::Request.new(env)

  if safelisted?(req)
  elsif blocklisted?(req)
  elsif throttled?(req)

Note: Rack::Attack::Request is just a subclass of Rack::Request so that you
can cleanly monkey patch helper methods onto the
request object.

About Tracks

Rack::Attack.track doesn't affect request processing. Tracks are an easy way to log and measure requests matching arbitrary attributes.


Define safelists, blocklists, throttles, and tracks as blocks that return truthy values if matched, falsy otherwise. In a Rails app
these go in an initializer in config/initializers/.
A Rack::Request object is passed to the block (named 'req' in the examples).


# Always allow requests from localhost
# (blocklist & throttles are skipped)
Rack::Attack.safelist('allow from localhost') do |req|
  # Requests are allowed if the return value is truthy
  '' == req.ip || '::1' == req.ip


# Block requests from
Rack::Attack.blocklist('block') do |req|
  # Requests are blocked if the return value is truthy
  '' == req.ip

# Block logins from a bad user agent
Rack::Attack.blocklist('block bad UA logins') do |req|
  req.path == '/login' && req.post? && req.user_agent == 'BadUA'


Fail2Ban.filter can be used within a blocklist to block all requests from misbehaving clients.
This pattern is inspired by fail2ban.
See the fail2ban documentation for more details on
how the parameters work. For multiple filters, be sure to put each filter in a separate blocklist and use a unique discriminator for each fail2ban filter.

# Block suspicious requests for '/etc/password' or wordpress specific paths.
# After 3 blocked requests in 10 minutes, block all requests from that IP for 5 minutes.
Rack::Attack.blocklist('fail2ban pentesters') do |req|
  # `filter` returns truthy value if request fails, or if it's from a previously banned IP
  # so the request is blocked
  Rack::Attack::Fail2Ban.filter("pentesters-#{req.ip}", :maxretry => 3, :findtime => 10.minutes, :bantime => 5.minutes) do
    # The count for the IP is incremented if the return value is truthy
    CGI.unescape(req.query_string) =~ %r{/etc/passwd} ||
    req.path.include?('/etc/passwd') ||
    req.path.include?('wp-admin') ||


Note that Fail2Ban filters are not automatically scoped to the blocklist, so when using multiple filters in an application the scoping must be added to the discriminator e.g. "pentest:#{req.ip}".


Allow2Ban.filter works the same way as the Fail2Ban.filter except that it allows requests from misbehaving
clients until such time as they reach maxretry at which they are cut off as per normal.

# Lockout IP addresses that are hammering your login page.
# After 20 requests in 1 minute, block all requests from that IP for 1 hour.
Rack::Attack.blocklist('allow2ban login scrapers') do |req|
  # `filter` returns false value if request is to your login page (but still
  # increments the count) so request below the limit are not blocked until
  # they hit the limit.  At that point, filter will return true and block.
  Rack::Attack::Allow2Ban.filter(req.ip, :maxretry => 20, :findtime => 1.minute, :bantime => 1.hour) do
    # The count for the IP is incremented if the return value is truthy.
    req.path == '/login' and req.post?


# Throttle requests to 5 requests per second per ip
Rack::Attack.throttle('req/ip', :limit => 5, :period => 1.second) do |req|
  # If the return value is truthy, the cache key for the return value
  # is incremented and compared with the limit. In this case:
  #   "rack::attack:#{Time.now.to_i/1.second}:req/ip:#{req.ip}"
  # If falsy, the cache key is neither incremented nor checked.


# Throttle login attempts for a given email parameter to 6 reqs/minute
# Return the email as a discriminator on POST /login requests
Rack::Attack.throttle('logins/email', :limit => 6, :period => 60.seconds) do |req|
  req.params['email'] if req.path == '/login' && req.post?

# You can also set a limit and period using a proc. For instance, after
# Rack::Auth::Basic has authenticated the user:
limit_proc = proc {|req| req.env["REMOTE_USER"] == "admin" ? 100 : 1}
period_proc = proc {|req| req.env["REMOTE_USER"] == "admin" ? 1.second : 1.minute}
Rack::Attack.throttle('req/ip', :limit => limit_proc, :period => period_proc) do |req|


# Track requests from a special user agent.
Rack::Attack.track("special_agent") do |req|
  req.user_agent == "SpecialAgent"

# Supports optional limit and period, triggers the notification only when the limit is reached.
Rack::Attack.track("special_agent", :limit => 6, :period => 60.seconds) do |req|
  req.user_agent == "SpecialAgent"

# Track it using ActiveSupport::Notification
ActiveSupport::Notifications.subscribe("rack.attack") do |name, start, finish, request_id, req|
  if req.env['rack.attack.matched'] == "special_agent" && req.env['rack.attack.match_type'] == :track
    Rails.logger.info "special_agent: #{req.path}"


Customize the response of blocklisted and throttled requests using an object that adheres to the Rack app interface.

Rack::Attack.blocklisted_response = lambda do |env|
  # Using 503 because it may make attacker think that they have successfully
  # DOSed the site. Rack::Attack returns 403 for blocklists by default
  [ 503, {}, ['Blocked']]

Rack::Attack.throttled_response = lambda do |env|
  # NB: you have access to the name and other data about the matched throttle
  #  env['rack.attack.matched'],
  #  env['rack.attack.match_type'],
  #  env['rack.attack.match_data']

  # Using 503 because it may make attacker think that they have successfully
  # DOSed the site. Rack::Attack returns 429 for throttling by default
  [ 503, {}, ["Server Error\n"]]

X-RateLimit headers for well-behaved clients

While Rack::Attack's primary focus is minimizing harm from abusive clients, it
can also be used to return rate limit data that's helpful for well-behaved clients.

Here's an example response that includes conventional X-RateLimit-* headers:

Rack::Attack.throttled_response = lambda do |env|
  now = Time.now
  match_data = env['rack.attack.match_data']

  headers = {
    'X-RateLimit-Limit' => match_data[:limit].to_s,
    'X-RateLimit-Remaining' => '0',
    'X-RateLimit-Reset' => (now + (match_data[:period] - now.to_i % match_data[:period])).to_s

  [ 429, headers, ["Throttled\n"]]

For responses that did not exceed a throttle limit, Rack::Attack annotates the env with match data:

request.env['rack.attack.throttle_data'][name] # => { :count => n, :period => p, :limit => l }

Logging & Instrumentation

Rack::Attack uses the ActiveSupport::Notifications API if available.

You can subscribe to 'rack.attack' events and log it, graph it, etc:

ActiveSupport::Notifications.subscribe('rack.attack') do |name, start, finish, request_id, req|
  puts req.inspect


A note on developing and testing apps using Rack::Attack - if you are using throttling in particular, you will
need to enable the cache in your development environment. See Caching with Rails
for more on how to do this.


The overhead of running Rack::Attack is typically negligible (a few milliseconds per request),
but it depends on how many checks you've configured, and how long they take.
Throttles usually require a network roundtrip to your cache server(s),
so try to keep the number of throttle checks per request low.

If a request is blocklisted or throttled, the response is a very simple Rack response.
A single typical ruby web server thread can block several hundred requests per second.

Rack::Attack complements tools like iptables and nginx's limit_conn_zone module.


Abusive clients range from malicious login crackers to naively-written scrapers.
They hinder the security, performance, & availability of web applications.

It is impractical if not impossible to block abusive clients completely.

Rack::Attack aims to let developers quickly mitigate abusive requests and rely
less on short-term, one-off hacks to block a particular attack.


Pull requests and issues are greatly appreciated. This project is intended to be
a safe, welcoming space for collaboration, and contributors are expected to
adhere to the Code of Conduct.

Testing pull requests

To run the minitest test suite, you will need both Redis and
Memcached running locally and bound to IP on
default ports (6379 for Redis, and 11211 for Memcached) and able to be
accessed without authentication.

Install dependencies by running

bundle install

Then run the test suite by running

bundle exec rake

Mailing list

New releases of Rack::Attack are announced on
rack.attack.announce@librelist.com. To subscribe, just send an email to
rack.attack.announce@librelist.com. See the


Copyright Kickstarter, PBC.

Released under an MIT License.

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