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/access-control/#simple-cross-origin-request-and-actual-r

Install the gem:

gem install rack-cors

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

use Rack::Cors do |cfg|
  cfg.allow do |allow|
    allow.origins 'localhost:3000', '127.0.0.1:3000'

    allow.resource '/file/list_all/', :headers => 'x-domain-token'
    allow.resource '/file/at/*',
        :methods => [:get, :post, :put, :delete],
        :headers => 'x-domain-token'
  end

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