Hours after anti-spam company Blue Security pulled the plug on its spam-fighting Blue Frog software and service, the spammers whose attack caused the company to wave the white flag have escalated their assault, knocking Blue Security's farewell message and thousands more Web sites offline.

Just before midnight ET, Blue Security posted a notice on its home page that it was bowing out of the anti-spam business due to concerted attacks against its Web site that took millions of other sites and blogs with it. Within minutes of that online posting, bluesecurity.com went down and remains inaccessible at the time of this writing.

According to information obtained by Security Fix, the reason is that the attackers were hellbent on taking down Blue Security's site again, but had trouble because the company had signed up with Prolexic, which specializes in protecting Web sites from "distributed denial-of-service" (DDoS) attacks.

These massive assaults harness the power of thousands of hacked PCs to swamp sites with so much bogus traffic that they can no longer accommodate legitimate visitors. Prolexic built its business catering to the sites most frequently targeted by DDoS extortion attacks -- chiefly, online gambling and betting houses. But the company also serves thousands of other businesses, including banks, insurance companies and online payment processors.

For the past nine hours, however, most of Prolexic's customers have been knocked offline by an attack that flanked its defenses. Turns out the attackers decided not to attack Prolexic, but rather UltraDNS, its main provider of domain name system (DNS) services. (DNS is what helps direct Internet traffic to its destination by translating human-readable domain names like "www.example.com" into numeric Internet addresses that are easier for computers to understand.)

UltraDNS is the authoritative DNS provider for all Web sites ending in ".org" and ".uk," and also markets its "DNS Shield" service designed to help sites defend against another, increasingly common type of DDoS -- one that targets weaknesses inherent in the DNS system. (Incidentally, UltraDNS was recently acquired by Neustar, which in turn is responsible for handling all ".biz" domain registrations, and for overseeing the nation's authoritative directory of telephone numbers.)

In this case, at least, it does not appear that the DNS Shield service worked as advertised. Earlier today, I spoke with Prolexic founder Barrett G. Lyon, who told me the attack on UltraDNS had knocked about 80 percent of his company's clients offline, or roughly 2,000 or so Web businesses. Most of those businesses also remain offline as of this writing.

According to Lyon, the unknown attackers hit a key portion of UltraDNS's network with a flood of spoofed DNS requests at a rate of around 4 to 5 gigabits per second, which is enough traffic to make just about any Web site on the Internet fall over (many Internet routers can handle only a few hundred megabits of traffic before they start to fail). But this was no normal DDoS attack-- it was a kind of DDoS on the DNS system that security experts say has become alarmingly more common over the past six to eight months.

Known as DNS amplification attacks or "reflected DNS attacks," these kinds of DDoS assaults increase the traffic hurled at a victim by orders of magnitude. In a nutshell, the attackers find a whole bunch of poorly configured DNS servers and use them to create and send spoofed DNS requests from systems they control to the DNS servers they want to cripple. Because the DNS requests appear to be coming from other trusted DNS servers, the target servers have trouble distinguishing regular, legitimate DNS lookups from ones sent by the attackers. Sustained for long enough, the attack eventually overloads the victim's DNS servers with queries and knocks them out of commission.

To put the raw power of DNS amplification into perspective, consider the attack that knocked Akamai offline in the summer of 2004. For anyone unfamiliar with this company, Akamai sells a rather pricey service that lets deep-pocketed companies like FedEx, Microsoft and Xerox mirror their Web site content at thousands of different online servers, making DDoS attacks against their sites extremely difficult.

Akamai was for a long time considered the gold standard until one day in June 2004, when a DDoS attack knocked the company's services offline for about an hour. Akamai never talked publicly about the specifics of the attack, but several sources close to the investigation told me later that the outage was the result of a carefully coordinated DNS amplification attack -- one that was stopped when the attackers decided they had made their point (which was no doubt to demonstrate to would-be buyers of their DDoS services that they could knock just about anyone off the face of the Web.)

So where am I going with all of this? Well, UltraDNS marketed its DNS Shield as a protection against exactly these same types of amplification attacks. Only in this case it doesn't appear to have worked -- though, to be fair I haven't heard UltraDNS's side of the story since they have yet to return my calls. No doubt they are busy putting out fires. At any rate, score another one for the spammers, I suppose.

Update, 7:46 p.m. ET: I heard back from Neustar. Their spokesperson, Elizabeth Penniman, declined to discuss anything about today's attacks, saying only that "we have a handle on the situation and continue to work with service providers to ensure the best possible level of service to our customers."