Handle only 404s dynamically. It uses a normal controller and route for 404s, letting everything else go to the Rails default /public error pages. In my case it was to use the subdomain logic in my ApplicationController.

application.rb

module MyApp
  class Application < Rails::Application
    require Rails.root + 'lib/custom_public_exceptions'
    config.exceptions_app = CustomPublicExceptions.new Rails.public_path
  end
end

custom_public_exceptions.rb

class CustomPublicExceptions < ActionDispatch::PublicExceptions

  def call(env)
    status      = env["PATH_INFO"][1..-1]

    if "404" == status
      # handle just 404 in our routes
      MyApp::Application.routes.call env
    else
      super env
    end
  end
  
end

errors_controller.rb

class ErrorsController < ApplicationController
  skip_before_filter :authenticate_user! # if using Devise

  def not_found
  end

end

routes.rb

Geknowm::Application.routes.draw do
  match "/404", :to => "errors#not_found"
end

Pretty great, one simple change is to use Rails.application.routes.call env instead of MyApp::Application.routes.call env so that you don't have to change that line to your application

class CustomPublicExceptions < ActionDispatch::PublicExceptions

  def call(env)
    status      = env["PATH_INFO"][1..-1]

    if "404" == status
      # handle just 404 in our routes
      MyApp::Application.routes.call env
    else
      super env
    end
  end

end

Can you write spec for this?

how, if I want to call the page can not be found on another controller to provide the conditions?

if no_session.nil?
  # how to call 404 page
  return false
else
  return true
end