What is a Denial of Service (DoS) Attack? - Definition, Types & Examples

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  • 0:01 An Unpleasant Surprise
  • 0:49 What Is a Denial of…
  • 1:47 Target Categories
  • 3:00 What Does an Attack Look Like?
  • 4:08 How to Defend
  • 5:20 Lesson Summary
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Lesson Transcript
Instructor: Kent Beckert

Kent is an adjunct faculty member for the College of Business at Embry-Riddle Aeronautical University and has a Master's degree in Technical Management.

In this lesson, we will describe a Denial of Service (DoS) and a Distributed Denial of Service (DDoS) attack and identify how these attacks occur. We will also discuss the objectives of an attack and some guidelines on how to prevent or limit the impacts of a DoS or DDoS attack.

An Unpleasant Surprise

Imagine that you get home from a hard day at work or school, have a bite to eat, and then sit at your computer. After you've checked your e-mail, you're ready to play an online game. Excitement begins to build, but then you notice your browser was denied access to your network, preventing you from connecting to the Internet. In desperation, you turn your computer's power off and restart it. After the reboot, your computer still cannot reach any networked service. Since only your computer was involved, you think it may be a Denial of Service attack. However, days later you learn that the attack you experienced was a Distributed Denial of Service, an attack involving numerous computers that flooded the game servers and prevented anyone from gaining access.

What Is a Denial of Service Attack?

By definition, Denial of Service or Distributed Denial of Service attacks are organized electronic incursions. Their purpose is to disrupt an organization's network operations by denying access to its users. In our fictional scenario, the attacks were designed to severely limit the access and use of online games and e-mail, but they can affect any online activity.

Making matters worse, the culprit, or attacker, can take control of your computer or terminal and use it to infect thousands of other computers, referred to as zombies. Zombies are computers that are infected and taken over by the attacker. The attacker then uses these zombies to generate millions of data packets, or requests for service, which eventually overload e-mail, web, and network servers. In this scenario, because more than one computer was involved and infected zombies were used, this type of attack is considered a distributed attack.

Target Categories

There are three basic target categories: networks, systems, and applications. Networks and systems are typically grouped together, both because systems are comprised of networks and the intended targets are usually embedded within one or the other. Applications, on the other hand, are treated differently, in that one or more applications are kept busy handling junk requests, and eventually become unable to respond to legitimate requests for service. For example, when you sat down at your computer and tried to access an online game, the DoS or DDoS attack had already saturated your browser application, rendering it useless and unable to connect to the Internet.

Keeping the three target categories in mind, DoS and DDoS attacks are designed to accomplish multiple objectives. The first objective is to consume as much bandwidth as possible, creating a bottleneck in the network and causing severe slowdowns. The second objective is to focus on network interface services, like the NetworkManager service, which notifies administrators of changes in the network. The third objective concentrates on specific computing or networking resources, such as a router, or a peripheral device, like a network printer.

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