Rustock botnet gets eight hours sleep a night

Security researchers have said that one of the biggest 'botnets' - an army of infected computers - takes an eight-hour break every day.

Though most botnets churn out spam and sometimes even malware around the clock, Rustock has predictable patterns and at 0000BST it shuts down for eight hours, resuming its activities at 0800BST the following morning.

Rustock is estimated to consist of between 1.3 million and 1.9 million infected PCs, and is responsible for around 10 per cent of all spam email, according to figures from MessageLabs.

Another botnet, Grum, is only around half the size of Rustock, but responsible for more than 23 per cent of all spam. Bobax, responsible for 15.7 per cent of spam is smaller again.

Between them, Rustock, Grum and Bobax are responsible for the bulk of spam activity online, taking over from the Cutwail botnet, which at its peak generated nearly half of all spam email.

"Over the past year, we have seen a number of ISPs taken offline for hosting botnet activity resulting in a case of sink or swim and an ensuing shift in botnet power," said Paul Wood of MessageLabs.

"This has undermined the power of the more dominant botnets like Cutwail," he continued.