routes.rb

require 'valid_subdomain'
# config/routes
Rails::Application.routes.draw do
  constraints(::ApplicationController::ValidSubdomain) do
    resource :lol
  end
end

valid_subdomain.rb

# app/controllers/application
class ApplicationController
  class ValidSubdomain
    def self.matches?(request)
      return request.subdomain =~ /tacnol/  
    end
  end
end