Ever wonder how phishing and malware sites manage to stay online? Through their analysis of botnets and infected hosts, the HoneyNet Project has documented an increasingly widespread technique used by online criminals: "Fast-Flux Service Networks". It's an admittedly clever and approach that makes it much harder to shut down malicious operations.
The premise behind fast-flux service networks is simple: attackers register a fully qualified domain name, and then rotate hundreds or thousands of IP addresses that are assigned to it. A DNS name may only be mapped to a particular IP for a few minutes. Each IP is an infected member of a botnet - but they are not the source of content, such as a virus or a scam web-site. Instead, they simply act as proxies, redirecting to one or more "mothership" servers that actually host the content. A more complex variation, "double-flux" service networks, implement additional misdirection by also rotating the authoritative name servers.
Like most of the HoneyNet Project's work, the whitepaper is very well-written and includes a case study with real-world examples. Definitely worth checking out if you're interested in how the more sinister side of the Internet underground operates.