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Fast flux is a DNS technique used by botnets to hide phishing and malware delivery sites behind an ever-changing network of compromised hosts acting as proxies. It can also refer to the combination of peer-to-peer networking, distributed command and control, web-based load balancing and proxy redirection used to make malware networks more resistant to discovery and counter-measures. The Storm Worm is one of the recent malware variants to make use of this technique.
The basic idea behind Fast flux is to have numerous IP addresses associated with a single fully qualified domain name, where the IP addresses are swapped in and out with extremely high frequency, through changing DNS records.
Internet users may see fast flux used in phishing attacks linked to criminal organizations, including attacks on MySpace.
While security researchers have been aware of the technique since at least November 2006, the technique has only received wider attention in the security trade press starting from July 2007.
Single-flux and double-flux 
The simplest type of fast flux, referred to itself as "single-flux", is characterized by multiple individual nodes within the network registering and de-registering their addresses as part of the DNS A (address) record list for a single DNS name. This combines round robin DNS with very short—usually less than five minutes (300s) -- TTL (time to live) values to create a constantly changing list of destination addresses for that single DNS name. The list can be hundreds or thousands of entries long.
A more sophisticated type of fast flux, referred to itself as "double-flux", is characterized by multiple nodes within the network registering and de-registering their addresses as part of the DNS Name Server record list for the DNS zone. This provides an additional layer of redundancy and survivability within the malware network.
Within a malware attack, the DNS records will normally point to a compromised system that will act as a proxy server. This method prevents some of the traditionally best defense mechanisms from working — e.g., IP-based access control lists (ACLs). The method can also mask the systems of attackers, which will exploit the network through a series of proxies and make it much more difficult to identify the attackers' network. The record will normally point to an IP where bots go for registration, to receive instructions, or to activate attacks. Because the IPs are proxified, it is possible to disguise the originating source of these instructions, increasing the survival rate as IP-based block lists are put in place.
See also 
Domain Generation Algorithm - A malware control technique where multiple domain names are generated by victim hosts.
- Danford; Salusky (2007). "The Honeynet Project: How Fast-Flux Service Networks Work". Retrieved 2010-08-23.
- Spamhaus explanation of Fast Flux hosting
- Phishing by proxy SANS Internet Storm Center diary from 2006-11-28 describes use of compromised hosts within botnets making use of fast flux techniques to deliver malware.
- MySpace Phish and Drive-by attack vector propagating Fast Flux network growth SANS Internet Storm Center diary from 2007-06-26 with technical details on FluxBot and fast flux techniques (warning: contains links to malicious code).
- Know Your Enemy: Fast-Flux Service Networks; An Ever Changing Enemy honeynet.org technical article from July 2007 and additional information on fast flux, including "single-flux" and "double-flux" techniques.
- Fast flux foils bot-net takedown SecurityFocus article from 2007-07-09 describing impact of fast flux on botnet counter-measures.
- Attackers Hide in Fast Flux darkreading article from 2007-07-17 on the use of fast flux by criminal organizations behind malware.
- .Asia registry to crack down on phishy domains article from 2007-10-12 mentions the use of fast flux in phishing attacks.
- .Asia registry to crack down on phishy domains alternate source for article above.
- CRYPTO-GRAM October 15, 2007 issue mentions fast flux as a DNS technique utilized by the Storm Worm.
- ATLAS Summary Report - Real-time global report of fast flux activity.
- Spam Trackers Wiki Entry on Fast Flux
- SAC 025 SSAC Advisory on Fast Flux Hosting and DNS
- GNSO Issues Report on Fast Flux Hosting
- FluXOR project from Computer and Network Security Lab (LaSeR) @ Università degli Studi di Milano (down as 07/27/2012)
- abuse.ch FastFlux Tracker
- RemovingMalware's Guide to Fast Flux DNS - How Criminals are using Fast Flux DNS to stay hidden