NOTE: Rack v3.0.0 was recently released. Please check the Upgrade Guide for more details about migrating your existing servers, middlewares and applications. For detailed information on specific changes, check the Change Log.

Rack provides a minimal, modular, and adaptable interface for developing web applications in Ruby. By wrapping HTTP requests and responses in the simplest way possible, it unifies and distills the bridge between web servers, web frameworks, and web application into a single method call.

The exact details of this are described in the Rack Specification, which all Rack applications should conform to.


Add the rack gem to your application bundle, or follow the instructions provided by a supported web framework:

# Install it generally:
$ gem install rack

# or, add it to your current application gemfile:
$ bundle add rack --version 3.0.0

If you need features from Rack::Session or bin/rackup please add those gems separately.

$ gem install rack-session rackup


Create a file called config.ru with the following contents:

run do |env|
  [200, {}, ["Hello World"]]

Run this using the rackup gem or another supported web server.

$ gem install rackup
$ rackup
$ curl http://localhost:9292
Hello World

Supported web servers

Rack is supported by a wide range of servers, including:

You will need to consult the server documentation to find out what features and limitations they may have. In general, any valid Rack app will run the same on all these servers, without changing anything.


Rack provides a separate gem, rackup which is a generic interface for running a Rack application on supported servers, which include WEBRick, Puma, Falcon and others.

Supported web frameworks

These frameworks and many others support the Rack Specification:

Available middleware shipped with Rack

Between the server and the framework, Rack can be customized to your applications needs using middleware. Rack itself ships with the following middleware:

All these components use the same interface, which is described in detail in the Rack Specification. These optional components can be used in any way you wish.

Convenience interfaces

If you want to develop outside of existing frameworks, implement your own ones, or develop middleware, Rack provides many helpers to create Rack applications quickly and without doing the same web stuff all over:


Rack exposes several configuration parameters to control various features of the implementation.


Rack::Utils.param_depth_limit = 32 # default

The maximum amount of nesting allowed in parameters. For example, if set to 3, this query string would be allowed:


but this query string would not be allowed:


Limiting the depth prevents a possible stack overflow when parsing parameters.


Rack::Utils.multipart_file_limit = 128 # default

The maximum number of parts with a filename a request can contain. Accepting too many parts can lead to the server running out of file handles.

The default is 128, which means that a single request can't upload more than 128 files at once. Set to 0 for no limit.

Can also be set via the RACK_MULTIPART_FILE_LIMIT environment variable.

(This is also aliased as multipart_part_limit and RACK_MULTIPART_PART_LIMIT for compatibility)


The maximum total number of parts a request can contain of any type, including both file and non-file form fields.

The default is 4096, which means that a single request can't contain more than 4096 parts.

Set to 0 for no limit.

Can also be set via the RACK_MULTIPART_TOTAL_PART_LIMIT environment variable.




See CONTRIBUTING.md for specific details about how to make a contribution to Rack.

Please post bugs, suggestions and patches to GitHub Issues.

Please check our Security Policy for responsible disclosure and security bug reporting process. Due to wide usage of the library, it is strongly preferred that we manage timing in order to provide viable patches at the time of disclosure. Your assistance in this matter is greatly appreciated.

See Also


The plethora of useful middleware created the need for a project that collects fresh Rack middleware. rack-contrib includes a variety of add-on components for Rack and it is easy to contribute new modules.


Provides convenient session management for Rack.


The Rack Core Team, consisting of

and the Rack Alumni

would like to thank:


Rack is released under the MIT License.