rescue_from ActionController::RoutingError doesn't work

The ActionController::RoutingError is raised when Rails tries to match the request with a route. This happens before Rails even initializes a controller - thus your ApplicationController never has a chance to rescue the exception.

Instead the Rails default exceptions_app kicks in - note that this is an app in the Rack sense - it takes a ENV hash with a request and returns a response - in this case the static /public/404.html file.

What you can do is have your Rails app handle rendering the error pages dynamically instead:

# config/application.rb
config.exceptions_app = self.routes # a Rack Application

# config/routes.rb
match "/404", :to => "errors#not_found", :via => :all
match "/500", :to => "errors#internal_server_error", :via => :all

You would then setup a specific controller to handle the error pages - don't do this in your ApplicationController class as you would be adding a not_found and internal_server_error method to all your controllers!

class ErrorsController < ActionController::Base
  protect_from_forgery with: :null_session

  def not_found
    render(status: 404)
  end

  def internal_server_error
    render(status: 500)
  end
end

Code borrowed from Matt Brictson: Dynamic Rails Error Pages - read it for the full rundown.

There is a better way to do it:

routes.rb

Rails.application.routes.draw do
  match '*unmatched', to: 'application#route_not_found', via: :all
end

application_controller.rb

class ApplicationController < ActionController::Base
  def route_not_found
    render file: Rails.public_path.join('404.html'), status: :not_found, layout: false
  end
end

To test locally, set the following and restart server.

config/development.rb

config.consider_all_requests_local = false

Tested with Rails 6.