What is a Denial of Service?

Denial of Service Attacks & Why They Happen

Jake Davis (Topiary) Charged With DDos Crimes (2011)
Jake Davis (Topiary) Charged With DDos Crimes (2011). Peter Macdiarmid / Getty Images

The term Denial of Service (DoS) refers to events that render systems on a computer network temporarily unusable.  Denials of service can happen accidentally as the result of actions taken by network users or administrators, but often they are malicious DoS attacks.

One of the more recent DDoS attacks (more on these below) occurred on Friday, October 21, 2016, and rendered many popular websites completely unusable for most of the day.

Denial of Service Attacks

DoS attacks exploit various weaknesses in computer network technologies. They may target serversnetwork routers, or network communication links. They can cause computers and routers to shut down ("crash") and links to bog down. They usually do not cause permanent damage.

Perhaps the most famous DoS technique is Ping of Death. The Ping of Death attack works by generating and sending special network messages (specifically, ICMP packets of non-standard sizes) that cause problems for systems that receive them. In the early days of the Web, this attack could cause unprotected Internet servers to crash quickly.

Modern Web sites have generally all been safeguarded against DoS attacks but they're certainly not immune.

Ping of Death is one kind of buffer overflow attack. These attacks overrun a target computer's memory and break its programming logic by sending things of larger sizes than it was designed to handle.

Other basic types of DoS attacks involve

  • flooding a network with useless activity so that genuine traffic cannot get through. The TCP/IP SYN and smurf  attacks are two common examples.
  • remotely overloading a system's CPU so that valid requests cannot be processed.
  • changing permissions or breaking authorization logic to prevent users from logging into a system. One common example involves triggering a rapid series of false login attempts that lock out accounts from being able to log in.
  • deleting or interfering with specific critical applications or services to prevent their normal operation (even if the system and network overall are functional).

DoS attacks are most common against Web sites that provide controversial information or services. The financial cost of these attacks can be very large. Those involved in planning or executing attacks are subject to criminal prosecution as in the case of Jake Davis (pictured) of the hacking group Lulzsec.

DDoS - Distributed Denial of Service

Traditional denial of service attacks are triggered by just one person or computer. In comparison, a distributed denial of service (DDoS) attack involves multiple parties.

Malicious DDoS attacks on the Internet, for example, organize large numbers of computers into a coordinated group called a botnet that are then capable of flooding a target site with immense amounts of network traffic.

Accidental DoS

Denials of service can also be triggered unintentionally in several ways:

  • many users suddenly trying to access a network or server at the same time, such as visiting a public Web site where a major social event is happening
  • network administrators accidentally unplugging a cable, or incorrectly configuring routers
  • a system becoming infected with a computer virus or worm