A Free Educational Resource Created by Carnegie Mellon University to Empower You to Secure Your Part of Cyberspace

Denial of Service (DoS)

A type of attack used to prevent legitimate users from accessing services or resources

Imagine a scenario in a supermarket where a single customer brings in a 100,000 items to the counter. If there are other customers waiting in the line, they are virtually denied service as the single customer will occupy the counter for a very long time.

In a denial of service attack, a hacker can prevent authorized or intended users from accessing resources and services. The hacker can target the computers or network connections. By carrying out the attack, the hacker can prevent users from accessing several websites, using email, video conferencing, banking services and online shopping. In effect, a denial of service attack prevents users from accessing any content from computers and networks that are affected by the attack.

One of the most common ways of performing a denial of service attack on a website is to flood the website with a huge number of information requests. This will prevent other users from accessing it, as each website can accept only a limited number of requests. If you get a "denial of service," it is because the site is inaccessible.

Detecting Denial of Service Attacks

There may be several disruptions in a network. All of them are not caused by denial of service attacks. They may be due to network maintenance or technical problems. But there are several symptoms of denial of service attacks. US Computer Emergency Response Team  (US-CERT) has identified the following as some of the symptoms:

  • Slower network performance or delays in accessing files
  • Inability to access particular websites
  • A very high increase in spam content
  • Unusually slow access to a website

However, a slow website is not necessarily under attack. For example, at times CNN has breaking news that grabs the attention of millions of users. The CNN website may have the capacity to accept one million users at a time. If there are 50 million users trying to access content simultaneously, the access will definitely be very slow. Though there is a symptom of a denial of service attack, there is no such attack in reality.

Protective Measures

There are no sure shot ways of preventing a denial of service attack. According to US-CERT , some of the methods that can be used to reduce the chances of a denial of service attack are:

  • Installing and enabling firewall: A firewall helps in limiting the type of traffic that can enter or exit a computer and can be helpful in filtering out unwanted content.
  • Distributing email addresses only to trusted parties: Care must be exercised to distribute email address. If an email address goes to a spammer, the email inbox could be flooded with spam messages and making it difficult for users to read legitimate messages.
  • Installation and regular updates of antivirus software: There are several malware that can slow down computers and result in a denial of service attack. Regularly updating antivirus software will help prevent most of the known malware.

Legal Issues

Ethical

The Internet Architecture Board considers denial of service attacks as violations of proper internet usage policies. Denial of service attacks also violate the usage policies of most Internet service providers.

Legal

US Cyber laws also have provisions against denial of service attacks. Attackers are punishable under The Computer Fraud and Abuse Act  and punishment may even include prison terms.

References

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