I asked Paul what the average entity could do to mitigate the damage from a large DDoS attack. He said, "The first thing you need to do is decide whether you want to stay in business during the attack or not. If you want to stay in business, you'll need to absorb all the attack traffic along with your legitimate traffic. This means your broadband connection, routers, firewall, Web servers, and back-end databases have to be able to deal with the attack. Talk to your ISP: at many ISPs, traffic levels above 2Gbps will get you billed at the 95th percentile. If our ISP had committed us to that rate, CastleCops would have incurred a $33,000 bill for a few days of DDoS traffic, which would have put our mostly volunteer organization out of business. Find out ahead of time how your ISP will handle DDoS events, and how you will be billed."
Paul continued, "I think many sites will need to scale to handle 10Gbps-to-30Gbps traffic loads." DDoS attackers will often start at lower traffic levels and attempt to increase the pain until your ISP, an upstream neighbor, or your servers fold. Although DDoS attacks above 10Gbps to 30Gbps occur, they are rare.
I asked Paul what administrators could do next to mitigate the attack. He offered, "Turn on your server and equipment logging to give you as much detail as possible. For example, Apache Web servers log connections by default, but not in enough detail. You'll want to get direct and forwarded IP addresses (to catch proxied connections), content details, user agents, and referral addresses." Then, using the information you collect, you can normally find a pattern to the DDoS traffic and place a filter on intervening devices to drop the malicious packets. Determined DDoS hackers will often change the pattern, so you will need to be on your feet and realize that implementing your first filter doesn't mean the DDoS is over.
Riding the DDoS storm
You can use an anti-DDoS mitigation service, such as Prolexic, to help out. Essentially, you change your Web server's public IP address (the DNS A record) to point to the mitigation service's IP address. They will scrub out the bad traffic and pass back the good traffic. You can also buy routers and network defense devices (such as Cisco or Juniper) built to take down DDoS attacks. Some of the devices will start working right away, putting down malicious traffic from the start, while others have to be plugged in a few weeks ahead of time to learn the difference between legitimate and anomalous traffic.
If you are ready to pursue possible criminal charges against the attacker, collect the best evidence you can and call your local or national authorities tasked with following up on Internet crime. In the United States, contact your local FBI field office. They will direct you to the appropriate division. It makes sense to have the appropriate numbers researched and documented ahead of time. From the time that you make the call, follow the recommendations of law enforcement.