How to defend against DDoS attacks

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 conjunction with firewalls to automatically block traffic. On the downside, they're not automated, so they need manual tuning by security experts, and they often generate false positives.

Servers: Proper configuration of server applications is critical in minimizing the effect of a DDoS attack. An administrator can explicitly define what resources an application can use and how it will respond to requests from clients. Combined with a DDoS mitigation appliance, optimized servers stand a chance of continued operations through a DDoS attack.

DDoS mitigation appliances: Several companies either make devices dedicated to sanitizing traffic or build DDoS mitigation functionality into devices used primarily for other functions such as load balancing or firewalling. These devices have varying levels of effectiveness. None is perfect. Some legitimate traffic will be dropped, and some illegitimate traffic will get to the server. The server infrastructure will have to be robust enough to handle this traffic and continue to serve legitimate clients.

Over-provisioning: or buying excess bandwidth or redundant network devices to handle spikes in demand can be an effective approach to handling DDoS attacks. One advantage of using an outsourced service provider is that you can buy services on demand, such as burstable circuits that give you more bandwidth when you need it, rather than making an expensive capital investment in redundant network interfaces and devices.

For the most part, companies don't know in advance that a DDoS attack is coming. The nature of an attack will often change midstream, requiring the company to react quickly and continuously over several hours or days. Since the primary effect of most attacks is to consume your Internet bandwidth, a well-equipped managed hosting provider has both the bandwidth and appliances to mitigate the effects of an attack.


DDoS attacks are destructive stealth weapons that can shutter a business. Our reliance on the Internet continues to grow, and the threat of DDoS attacks continues to expand. Organizations need to ensure operational continuity and resource availability with a vigilant DDoS mitigation approach if they want to conduct "business as usual."

Paul Froutan is vice president of engineering at Rackspace Managed Hosting, a provider of managed hosting services in San Antonio. An expert in traffic analysis and server scalability, he also holds a U.S. patent for his IDS, part of Rackspace's PrevenTier three-tier security system designed to help identify and mitigate the effects of DDoS attacks.

Copyright © 2004 IDG Communications, Inc.

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