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An HTTP proxy that gsubs the contents of the requests and responses in order to make a mostly transparent proxy.

It aids in the development and testing of integration with external services, especially when the browser has to interact with those services directly. Often, these services will only redirect to certain, predefined URLs, but your test suite spins up its services dynamically. This includes services like OAuth/OpenId and payment gateways (e.g. Stripe, Cybersource).


Inherit from Rack::TranslatingProxy and implement #target_host and #request_mapping hook methods to customize your translating proxy Rack app.

class MyTranslatingProxy < Rack::TranslatingProxy
  def target_host
    # ...

  def request_mapping
    # ...

Target host

def target_host

Implement this method to specify which host the proxy will send all the requests to. If the app is running on http://localhost:5555, then if you call http://localhost:5555/some/path you will actually receive the contents of https:///

The scheme (http:// vs http://) does not need to match between the proxy and target host.

There is no need to memoize this method.

Request mapping

def request_mapping
    'some string'       => 'another string',
    'some other string' => 'another another string'

Implement this method to specify which strings will be translated and to what. On requests, the keys will be translated to the values, and in the responses, the values will be translated into the keys. In this example, in the request, any instance or some string will be translated to another string before the proxy makes the request to the target host. When the target host responds, the proxy will translate any instance of another string into some string.

On the request, the proxy will gsub the path, the query string, the location header(s) and the request body. On the response, the proxy will gsub the response body.

You will often want to include a rule for the target host to rewrite the proxy's current URL to the target host's URL. This is useful particularly for handling redirects. For example, if you have a mapping rule like 'http://localost:5555' => '', when a service redirects to, the proxy will rewrite that to http://localhost:555/payment-accepted.

There is no need to memoize this method.

Example: OAuth service

Here is an example of a translating proxy for an OAuth service.

class OAuthProxy < Rack::TranslatingProxy
  # the host to proxy
  def target_host

  def request_mapping
      # the URI of this proxy => URI of the target
      'http://localhost:5555' => target_host,

      # our dev app server    => URI that the OAuth implementation has on
      #                          file and wants to redirect to
      'http://localhost:3000' => '',

Assuming you run the proxy on localhost:5555, you can configure your OAuth-enabled app to point to http://localhost:5555. Your test suite and your browser can hit the real OAuth server.


Add this line to your application's Gemfile:

gem 'rack-translating_proxy'

And then execute:

$ bundle

Or install it yourself as:

$ gem install rack-translating_proxy


Make sure you are running the test servers:


Then, run


The test servers will update automatically because they are using shotgun. If you don't need them to reload on every request, run one or both of them with the fast parameter, like

./spec/script/example_target_host fast


  1. Fork it ( )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request


Proxy that translates strings




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