class ApplicationController < ActionController::Base
  before_filter :correct_domain!

  private
  def correct_domain!
    unless request.host == 'www.mysite.com'
      redirect_to root_url,:status => 301  # or explicitly 'http://www.mysite.com/'
    end
  end
end

但是我不知道主机属性中是否存在尾部斜杠…