Botnet

A botnet, from "robot network", is a set of compromised machines, called zombies, which can collectively perform tasks for the bot herder.

Machines may be taken over in a variety of ways. Trojan horse programs in games or pornography are used. Some viruses make the victim computer part of a botnet. A famous example was the Storm botnet [1], spread mainly by email messages. Some web sites include malicious code which attempts to subvert visitors' machines.

However, the larger botnets are built by actively scanning for insecure machines. Any computer can be attacked in this way, but one target is common enough and often insecure enough to be the favorite. The attacker looks up the IP address ranges used for ADSL or other broadband Internet service; any machine he can compromise there will have a good enough net connection to be quite useful to him. He then scans those IP addresses looking for Windows machines that have not installed Microsoft's security updates and are therefore vulnerable to known attacks. Several simple defenses can greatly reduce the risk of such attacks: doing Windows updates prevents most, a cheap off-the-shelf router provides some firewall protection between your machine and the net, and checking your Windows network settings to ensure that you are not allowing shares to arbitrary people can block some attacks.

The main applications of botnets are to perpetrate various sorts of net.evil, such as conducting distributed denial of service attacks and sending spam.