How to defend against DDoS attacks

Computerworld - Distributed denial-of-service attacks can paralyze even the most well-structured network for days, costing millions of dollars in lost sales, freezing online services and crippling a company's reputation.
One of the most widely reported DDoS attacks was launched when The SCO Group Inc. battled an assault on its Web site from the Mydoom.B worm (see story). But DDoS attacks can be a problem for any size business in any industry. According to the 2003 CSI/FBI Computer Crime and Security Survey, DDoS attacks are the second most expensive cybercrime and the only ones to increase in 2003.
The Internet can be a dangerous place, with DDoS attacks emerging as the weapon of choice for hackers, political activists and international cyberterrorists. In addition, with ever-more-powerful tools in a hacker's arsenal, DDoS attacks are getting easier to launch. New viruses and worms take hold every month, so companies need to be prepared to fend off this ever-expanding security threat.
DDoS attacks take advantage of the openness of the Internet and its benefit of delivering packets of data from nearly any source to any destination. What makes DDoS attacks such a challenge is that illegitimate packets of data are virtually indistinguishable from legitimate ones. Typical types of DDoS attacks include bandwidth attacks and application attacks.
In a bandwidth attack, network resources or equipment are consumed by a high volume of packets. With an application attack, TCP or HTTP resources are prevented from processing transactions or requests.
So how do you protect your company's servers from the onslaught of data sent from infected PCs across the Internet? How do you keep a DDoS attack from bringing down your company's network? There are several approaches you can take to defend against a DDoS attack:
Black-holing or sinkholing: This approach blocks all traffic and diverts it to a black hole, where it is discarded. The downside is that all traffic is discarded -- both good and bad -- and the targeted business is taken off-line. Similarly, packet-filtering and rate-limiting measures simply shut everything down, denying access to legitimate users.
Routers and firewalls: Routers can be configured to stop simple ping attacks by filtering nonessential protocols and can also stop invalid IP addresses. However, routers are typically ineffective against a more sophisticated spoofed attack and application-level attacks using valid IP addresses. Firewalls can shut down a specific flow associated with an attack, but like routers, they can't perform antispoofing.

Intrusion-detection systems: IDS solutions will provide some anomaly-detection capabilities so they will recognize when valid protocols are being used as an attack vehicle. They can be used in