DoS (Denial of Service)

A denial-of-service attack (DoS attack) is an attempt to make a computer resource unavailable to its intended users. Although the means to, motives for and targets of a DoS attack may vary, it generally comprises the concerted, malevolent efforts of a person or persons to prevent an Internet site or service from functioning efficiently or at all, temporarily or indefinitely.
Perpetrators of DoS attacks typically — but not exclusively — target sites or services hosted on high-profile web servers; a pair of DNS Backbone DDoS Attacks, on October 22, 2002 and February 6, 2007, targeted DNS root servers, in an apparent attempt to "disable the Internet" itself by taking away an option of addressing Internet servers by their human-friendly names.
One common method of attack involves saturating the target (victim) machine with external communications requests, such that it cannot respond to legitimate traffic, or responds so slowly as to be rendered effectively unavailable. In general terms, DoS attacks are implemented by:
forcing the targeted computer(s) to reset, or consume its resources such that it can no longer provide its intended service; and/or,
obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.
Denial-of-service attacks are considered violations of the IAB's Internet proper use policy. They also commonly constitute violations of the laws of individual nations.

Methods of attack

A "denial-of-service" attack is characterized by an explicit attempt by attackers to prevent legitimate users of a service from using that service. Examples include:
-flooding a network, thereby preventing legitimate network traffic;
-disrupting a server by sending more requests than it can possibly handle, thereby preventing access to a service;
-preventing a particular individual from accessing a service;
-disrupting service to a specific system or person.

Clones denying entrance into a Yahoo! chatroom.
Attacks can be directed at any network device, including attacks on routing devices and Web, electronic mail, or Domain Name System servers.

A DoS attack can be perpetrated in a number of ways. There are five basic types of attack:
-consumption of computational resources, such as bandwidth, disk space, or CPU time;
-disruption of configuration information, such as routing information;
-disruption of state information, such as unsolicited resetting of TCP sessions;
-disruption of physical network components.
-obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.

A DoS attack may include execution of malware intended to:
-max out the CPU's usage, preventing any work from occurring;
-trigger errors in the microcode of the machine;
-trigger errors in the sequencing of instructions, so as to force the computer into an unstable state or lock-up;
-exploits errors in the operating system to cause resource starvation and/or thrashing, i.e. to use up all available facilities so no real work can be accomplished;
-crash the operating system itself.