Rack::Cors provides support for Cross-Origin Resource Sharing (CORS) for Rack compatible web applications. The CORS spec allows web applications to make cross domain AJAX calls without using workarounds such as JSONP. For a thorough write up on CORS, see this blog post:

www.nczonline.net/blog/2010/05/25/cross-domain-ajax-with-cross-origin-resource-sharing/

Or for all the gory details, you can read the spec here:

www.w3.org/TR/cors/

Install the gem:

gem install rack-cors

In your Gemfile:

gem 'rack-cors', :require => 'rack/cors'

You configure Rack::Cors by passing a block to the use command:

use Rack::Cors do
  allow do
    origins 'localhost:3000', '127.0.0.1:3000',
            /http:\/\/192\.168\.0\.\d{1,3}(:\d )?/
            # regular expressions can be used here

    resource '/file/list_all/', :headers => 'x-domain-token'
    resource '/file/at/*',
        :methods => [:get, :post, :put, :delete, :options],
        :headers => 'x-domain-token',
        :expose  => ['Some-Custom-Response-Header'],
        :max_age => 600
        # headers to expose
  end

  allow do
    origins '*'
    resource '/public/*', :headers => :any, :methods => :get
  end
end

Put something like the code below in “config/application.rb” of your Rails application. For example, this will allow GET, POST or OPTIONS requests from any origin on any resource.

module YourApp
  class Application < Rails::Application

    # ...

    config.middleware.use Rack::Cors do
      allow do
        origins '*'
        resource '*', :headers => :any, :methods => [:get, :post, :options]
      end
    end

  end
end

See guides.rubyonrails.org/rails_on_rack.html for more details on rack middlewares or railscasts.com/episodes/151-rack-middleware.