routes.rb

YourRailsApp::Application.routes.draw do  
  flipper_auth_app = Flipper::UI.app(Flipper.instance) do |builder|
    builder.use Rack::Auth::Basic do |username, password|
      if username == ENV['FLIPPER_USERNAME'] && password == ENV['FLIPPER_SECRET']
        true
      else
        false
      end
    end
  end

  mount flipper_auth_app, at: '/flipper'
end