A method that hackers use to prevent or deny legitimate users access to a computer. DoS attacks are typically executed using the DoS tools that send many request packets to a targeted Internet server (usually Web, FTP, or Mail server). The attack floods the server's resources and makes the system unusable. Any system that is connected to the Internet and is equipped with TCP-based network services is subject to attack.
For example, imagine a hacker creates a program that calls a local pizza store. The pizza store answers the telephone, but learns that it is a prank call. If the program repeats this task continuously, it prevents legitimate customers from ordering pizza because the telephone line is busy. This technique is called denial of service, and analogous to a denial-of-service attack. Many denial-of-service attack tools are capable of a distributed denial-of-service attack. For example, imagine the hacker secretly plants a program onto many computers on the Internet. This attack has a bigger impact because there are more computers that call the same pizza store. Since the program is not running from the attacker's computer, it is difficult to trace the attacker. The attacker has controlled only the computer that secretly had the program installed. DoS tools such as TFN, TFN2K, and Trinoo are distributed denial-of-service attack tools. The denial-of-service attack tools can be secretly installed onto a large number of systems. Systems that unknowingly have denial-of-service attack tools installed are called zombie agents or drones.