Battle Against Fast-Flux Botnets Intensifies

First there was fast flux, and now there’s double flux: a variant of the stealthy fast-flux hosting technique used by major bot herders that rapidly shifts malicious Web servers and domain name servers (DNS) from machine to machine to evade detection.

A new advisory by the ICANN Security and Stability Advisory Committee sheds new light on the emerging double flux threat and provides proposed recommendations and best practices for Internet domain registrars, ISPs, users, and other members of the Internet community, in an effort to derail fast-flux botnets.

Fast flux is where botnet herders continuously move the location of a Web, email, or DNS server from computer to computer in an effort to keep its malicious activity -- spamming or phishing, for example -- alive and difficult to detect. IP blacklists are basically useless in finding fast flux-based botnets. The infamous Storm botnet, for instance, was one of the first to deploy this technique of preserving its botnet infrastructure and hiding from investigators. (See On the Trail of 'Fast Flux' Botnets and Attackers Hide in Fast Flux.)

“Double-flux is just another evasion technique applying two levels of... deception as opposed to one,” says David Piscitello, a member of ICANN’s SSAC and one of the authors of the paper. “It’s particularly troublesome because using domain names is a whole lot easier than using IP addresses. Before this, you could hone in on a domain server as a way of shutting down a [malicious] site. But now they [the bad guys] have one more tool in their evasion toolkit.”

With double flux, the DNS name servers that resolve the Web host names are moved from machine to machine, as are the actual hosts serving up the phony pharmaceuticals or other nefarious sites. “You’re often not even resolving to the actual Web server but to a proxy to the Web server... So you’re still one step removed from it if you get that IP,” Piscitello says. By the time investigators get on its trail, the fast-flux botnet has changed the IP address again, he says. (Plus many of these systems encrypt their communications as well, which makes it even more difficult to track them.)

“Then they repeat the whole [process] at the [DNS] name server level,” he says.

Piscitello says statistics on overall fast flux usage are tough to come by, but they appear to be increasingly on the rise -- especially by spammers and phishers -- who typically lease these types of advanced botnets from botnet herders.

Researcher Jose Nazario, senior security engineer for Arbor Networks, says he’s personally tracking about 100 fast-flux botnet domains, and other researchers he knows are tracking more than 1,000 other suspected fast-flux domains. “It’s growing... mostly among spammers and phishers, ” Nazario says. He and his colleagues are using some heuristics methods to help sniff out fast-flux domains and help shut down the bots, he says. “We are also using some of these to build DNS blacklists so we can help operators running DNS servers to kill those domains by blocking further Web apps,” he says.

Among the recommendations that ICANN proposes are that registrars uniformly: authenticate any requests for configuration changes to name servers; prevent automated changes to these configurations; and set a minimum “time to live” threshold of 30 minutes for a server so the bad guys can’t keep swapping them out every few minutes. That gives investigators and researchers a bit more breathing room to thwart double-flux, according to ICANN.

It also calls for standard quarantining of suspected fast-flux DNS domain name servers in a honeypot, for example, to gather information on the server and to track down the bots. And rather than immediately releasing a domain that was previously used illegally, use it as a poster child for abuse: “Establish and redirect visitors to a landing page explaining that this domain was suspended because it was used for illegal or objectionable activities, and inform users on ways to detect and avoid being victimized by phishing and other criminal activities,” the ICANN paper says.

And while fast-flux DNS is considered an evasion technique for botnets, it can also be a dead giveaway to the trained eye or seasoned bot hunter, fast flux experts say. The problem is getting the DNS registrars to do something about it when they spot fast flux activity; smaller and lesser-known registrars aren’t typically as responsive or proactive as the larger ones, for example.

“The thing that makes fast-flux so powerful for the bad guys in keeping their sites active is also its greatest weakness. Detection and classification is almost trivially easy, since it's all there to see in the DNS for those who know how to look,” says Rod Rasmussen, president and CTO of Internet Identity and co-chair of the Internet Policy Committee of the Anti-Phishing Working Group. “Mitigation is limited to the registrar/registry however, so that's where the challenge typically lies.”

And if and when the Internet community starts to better coordinate its efforts to fight these botnets and botnet herders feel the heat and abandon fast flux altogether, they’ll likely just adopt another technique to evade detection. “I have no misconception that if we defeat double fast-flux, the people who’ve developed such sophisticated techniques won’t come up with something new,” ICANN’s Piscitello says. “But you can’t throw up your hands and give up” either, he says.

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