How to read ruby on rails config values from within the application controller

The class method ActionController::Base.session_options returns a hash of the configuration options for the session.

So in your case you want ActionController::Base.session_options[:domain]

In Ruby on Rails 3 and above, you can access the config through Rails.application.config

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In more recent versions, you can use Rails.configuration instead. For example: Rails.configuration.time_zone. See more in the official manual on configuration