There is no defence against a denial-of-service attack. Just ask your security staff. They’ll hem and haw and mention some products you can buy. But then you’ll start hearing terms right out of the Vietnam War: “mitigate losses” and “acceptable casualty rate.” If you’re connected to a public network like the Internet, some bad guy will be able to devise a way to deny you service. Distributed denial-of-service attacks, in which hackers use hundreds or thousands of unwitting third-party systems to do their dirty work, are just a further iteration in the battles.