Concepts of Denial-of-Service Attack & Distributed Denial of Service

For better understanding of Denial-of-Service Attack & Distributed Denial of Service (DoS/DDoS) attacks, one must be familiar with their concepts beforehand. This module discusses about what a DoS attack is, what a DDoS attack is, and how the DDoS attacks work.

What is a Denial-of-Service Attack?

DoS is an attack on a computer or network that reduces, restricts, or prevents accessibility of system resources to its legitimate users. In a DOS attack, attackers flood a victim’s system with non-legitimate service requests or traffic to overload its resources, bringing the system down, leading to unavailability of the victim’s website or at least significantly slowing the victim’s system or network performance. The goal of a DoS attack is not to gain unauthorized access to a system or to corrupt data; it is to keep the legitimate users away from using the system.

Following are the examples of types of DoS attacks:

• Flooding the victim’s system with more traffic than can be handled
• Flooding a service (e.g., internet relay chat (IRC)) with more events than it can handle
• Crashing a transmission control protocol (TCP/Internet protocol OP) stack by sending corrupt packets
• Crashing a service by interacting with it in an unexpected way
• Hanging a system by causing it to go into an infinite loop

In general,Denial-of-Service Attack DoS attacks target network bandwidth or connectivity. Bandwidth attacks overflow the network with a high volume of traffic using existing network resources, thus depriving legitimate users of these resources, Connectivity attacks overflow a computer with a large amount of connection requests, consuming all available resources of the OS so that the computer cannot process legitimate users’ requests.

Imagine a pizza delivery company, which does much of its business over the phone. If an attacker wanted to disrupt this business, he could figure out a way to tie up the company’s phone lines, making it impossible for the company to do business. That is how a DoS attack works—the attacker uses up all the ways to connect to the system, making legitimate business impossible, DoS attacks are a kind of security break that does not generally result in the theft of information. However, these attacks can harm the target in terms of time and resources. However, failure might mean the loss of a service such as email. In a worst-case scenario, a DOS attack can mean the accidental destruction of the files and programs of millions of people who happen to be surfing the Web at the time of attack.

A Distributed Denial of Service DDoS attack is a large-scale, coordinated attack on the availability of services on a victim’s system or network resources, launched indirectly through many compromised computers (botnets) on the Internet.

How Distributed Denial-of-Service Attacks Work?

In a Distributed Denial of Service DDoS attack, many applications found the target browser or network with fake exterior requests that make the system, network, browser, or site slow, useless, and disabled or unavailable.

The attacker initiates the DDoS attack by sending a command to the zombie agents. These zombie agents send a connection request to a large number of reflector systems with the spoofed IP address of the victim. The reflector systems see these requests as coming from the victim’s machine instead of the zombie agents due to spoofing of source IP address. Hence, they send the requested information (response to connection request) to the victim. The victim’s machine is flooded with unsolicited responses from several reflector computers at once. This either may reduce the performance or may cause the victim’s machine to shut down completely.

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Module Objectives

Denial-of-Service (DOS) and Distributed Denial-of-Service (DDoS) attacks became a serious threat to computer networks. These attacks attempt to make a machine or network resource unavailable to its authorized users. Usually DoS/DDoS attacks exploit vulnerabilities within the implementation of TCP/IP model protocol or bugs in a specific OS.

This module starts with a summary of DoS and DDoS attacks. It provides an insight into different DoS/DDoS attack techniques. Later, it discusses about botnet network, DoS/DDoS attack tools, techniques to detect DoS/DDoS attacks, and DoS/DDoS countermeasures. The module ends with a summary of penetration testing steps an ethical hacker should follow to perform a security assessment of the target.

At the end of this module, you’ll be ready to perform the following:

• Describe the DoS/DDoS concepts
• Perform DoS/DDoS using various attack techniques
• Describe Botnets
• Describe DoS/DDoS case studies
• Explain different DoS/DDoS attack tools
• Apply best practices to mitigate Do5IDD05 attacks
• Perform DoS/DDoS penetration testing

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