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Post #5 on Why Spam Filters Suck “trickle blog” series

By Desmond Liao | 2 minute read

Why Are Botnets So Difficult To Stop?

Definition: a "botnet" is commonly known as a network of infected computers used to send spam (among other actions).

The largest botnets contain hundred of thousands of "zombie" machines controlled by a "bot herder," who uses sophisticated encryption, infection and peer-to-peer (P2P) networking techniques to ensure the permanence and growth of the botnet. As the zombies are used, they become discovered and subsequently blocked. While individual zombies are constantly changing, the overall botnet and people who control them remain the same.

Because of botnets, spam does not come from a predictable set of computers rather, it comes from all over the place in a completely unpredictable manner. By leveraging the diversity of IP addresses available via botnets, spammers have rendered the blocking approach far less effective than it once was.

Further, as the number of broadband subscribers continues to grow ­ most rapidly in developing economies such as China and Eastern Europe the number of computers available to exploit for participation in botnets is expanding. As botnets increase in size and sophistication, trying to identify where the "bad stuff" is coming from is becoming less and less worthwhile.

Indeed, researchers at Georgia Tech discovered in 2006 in a survey of data from the Spamhaus black list that only 5 per cent of botnet IP addresses ever end up listed in the Spamhaus database. In another paper, the same researchers found that 85 per cent of spam zombies sent fewer than ten email messages to their honeypot server over the course of about 18 months, as shown in the above graph.

Example: A Transient Zombie

In late 2007, the zombie at (a Brazilian broadband subscriber address) began sending approximately three spams each day into one of our honey pot systems. It took 19 days for the first real-time blackhole list (RBL) to identify this IP address and cause it to be blocked. By sending only a very light trickle of email, zombies can evade detection.

While blocking continues to be a core component of the multi-layered anti-spam architecture, it makes little sense in 2008 to depend on filtering technology designed to block spam in 2001 before the advent of botnets. Approaches that seek to block spam fail to deal with the issue of unknown senders.

NEXT: Post #6 Blocking Spam in 2008
PREVIOUS: Post #4 Spamonomics: The Economics of Spamming

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