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On the Trail of 'Fast Flux' Botnets

Stealth botnet method makes botnets hard to kill, but not impossible

Fast-flux botnets are multiplying and making it tougher to trace botnets, but there are ways to sniff out these wily networks run by the dark side, researchers say. (See Attackers Hide in Fast Flux.)

"I don't think there's any question that the FFers know they are being monitored," says Nicholas Bourbaki, an independent researcher and fast-flux expert. That poses a few obvious challenges to an investigation, including the perpetrators turning on you: Bourbaki goes only by this pseudonym for fear of reprisal from botnet operators that he tracks and fights. "If you are not a Trend Micro, Symantec, [or] McAfee, you are in the position of being wiped off of the map anytime you get too annoying to these criminals."

Fast flux is an advanced method being used by determined botnet operators to hide and preserve their malicious Websites and botnet infrastructures. The bad guys behind Warezov/Stration and Storm, for instance, have separately moved their infrastructures to fast-flux service networks, according to members of the Honeynet Project & Research Alliance, who monitor fast-flux behavior via their honeypots.

With fast flux, infected bot machines serve as proxies or hosts for malicious Websites and get rotated regularly, changing DNS records to evade discovery. IP blacklists are basically useless in finding fast flux-based botnets. The bad guys behind these networks can easily hide their fake online pharmacies, pornography, phishing sites, and other malicious content servers using this round-robin process.

Researchers have witnessed a major jump in the use of this method. Bourbaki says, in December of 2006, he and other researchers logged about 2,500 unique host names "fast fluxing" on over 18,000 unique IPs, and so far in August, there are over 14,000 host names on over 36,000 unique IPs. And this represents just a snapshot of data on fast flux.

Bourbaki, who was originally scheduled to talk about tracking fast flux on a panel at this week's closed-door Internet Security Operations and Intelligence III (ISOI3) summit but did not due to a scheduling conflict, says one underlying problem is that most network equipment does not filter by host names. "Our defense mechanisms need to be able to segment traffic and treat certain traffic with more scrutiny than others. And they need to be able to block, or just do reputation-based [filtering] based on name server host name, not just IP," he says. (See 'Dream Team' Takes on Black Hats.)

Still, IP blacklists are essential to the fight against botnets because they keep botnets on the move, and their movement is one way to detect them. "They either move, or they are on a blacklist," he says. "And if they move, that movement is detectable, and the difference between the way malicious resources move versus the way legitimate [ones do] can be used to protect the enterprise and the user."

Bourbaki was careful not to disclose too much detail on how this detection process works for fear of tipping off the cybercriminals behind these botnets. But he says he uses this "reputation system" approach in his small hosting business, and it's been effective. And some ISPs also use it, he notes.

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