You heard about botnets briefly in Week 1, when we said that botnets are created using malware that give an attacker control over a group of computers and commonly use them to gather information from the computers (e.g., usernames and passwords), launch attacks against others. These attacks might be sending spam emails, or flooding a website with so many requests for content that the server cannot cope, which is known as a denial-of-service attack.
A single piece of malware can cause enormous damage, but when thousands, or even millions of computers run the same program, their effects can be devastating. So a botnet is a group of computers that coordinate their activity over the internet. There are a number of harmless botnets used for such purposes as the Internet Relay Chat (IRC) text messaging program, but the vast majority are created by malware.
Botnets spread through viruses and worms and once installed on the victim’s computer they use the internet to make contact with a control computer. At this point, the infected computer (often called a zombie) will do nothing more except periodically check for instructions from the control computer. Over time, more and more computers are recruited to the incipient botnet until it may contain tens of thousands of zombies, but they don’t raise suspicion as they appear to be doing nothing.
At some point in the future, the control computer will issue a command for the botnet to wake up and begin doing something. Often the people who created the botnet itself have either sold or rented the botnet to another group who want to use its capabilities.
Botnets have been used to flood the internet with spam messages, to commit fraud against advertisers and to perform so-called distributed denial-of-service attacks on companies and governments. Botnets are so large, and so widely distributed across the internet that they can be very hard to tackle and the effects of a coordinated attack on critical parts of the network can mean even very large websites struggle to remain online while the botnet targets their computers.