discourse/app/controllers/permalinks_controller.rb

class PermalinksController < ApplicationController
  skip_before_filter :check_xhr, :preload_json

  def show
    url = request.fullpath[1..-1]
    permalink = Permalink.find_by_url(url)
    if permalink && permalink.target_url
      redirect_to "#{Discourse::base_uri}#{permalink.target_url}", status: :moved_permanently
    else
      raise Discourse::NotFound
    end
  end
end