With ordinary DDoS attacks an effective method (which Cloudflare uses) is to prompt you with an captcha before letting you pass or dropping your connection when

With ordinary DDoS attacks an effective method (which Cloudflare uses) is to prompt you with an captcha before letting you pass or dropping your connection when you fail often enough. As far as I understand it was not that hard to block this attack because it follows traffic patterns (DNS responses from open resolvers). The actual problem was that the attack was so massive that it clogged the pipes before it could reach a router (belonging to Cloudflare) that would've been able to drop the packages. Disclaimer: I am no network engineer so don't rely on my reply being factually 100% correct.