How to restrict access to a node via node/12 and allow access via a path like content/sometitle?
The Global Redirect module does what you want (and more).


  ... Checks if the Clean URLs feature is enabled and then checks the
  current URL is being accessed using the clean method rather than the
  'unclean' method.


It redirects the node/xx to content/your-content etc if clean URLs feature is enabled.