Understanding Denial-of-Service Attacks

You may have heard of denial-of-service attacks launched against websites,
but you can also be a victim of these attacks. Denial-of-service attacks can
be difficult to distinguish from common network activity, but there are some
indications that an attack is in progress.

What is a denial-of-service (DoS) attack?

In a denial-of-service (DoS) attack, an attacker attempts to prevent
legitimate users from accessing information or services. By targeting your
computer and its network connection, or the computers and network of the
sites you are trying to use, an attacker may be able to prevent you from
accessing email, websites, online accounts (banking, etc.), or other
services that rely on the affected computer.

The most common and obvious type of DoS attack occurs when an attacker
"floods" a network with information. When you type a URL for a particular
website into your browser, you are sending a request to that site's computer
server to view the page. The server can only process a certain number of
requests at once, so if an attacker overloads the server with requests, it
can't process your request. This is a "denial of service" because you can't
access that site.

An attacker can use spam email messages to launch a similar attack on your
email account. Whether you have an email account supplied by your employer
or one available through a free service such as Yahoo or Hotmail, you are
assigned a specific quota, which limits the amount of data you can have in
your account at any given time. By sending many, or large, email messages to
the account, an attacker can consume your quota, preventing you from
receiving legitimate messages.

What is a distributed denial-of-service (DDoS) attack?

In a distributed denial-of-service (DDoS) attack, an attacker may use your
computer to attack another computer. By taking advantage of security
vulnerabilities or weaknesses, an attacker could take control of your
computer. He or she could then force your computer to send huge amounts of
data to a website or send spam to particular email addresses. The attack is
"distributed" because the attacker is using multiple computers, including
yours, to launch the denial-of-service attack.

How do you avoid being part of the problem?

Unfortunately, there are no effective ways to prevent being the victim of a
DoS or DDoS attack, but there are steps you can take to reduce the
likelihood that an attacker will use your computer to attack other
* Install and maintain anti-virus software (see Understanding Anti-Virus
Software for more information).
* Install a firewall, and configure it to restrict traffic coming into and
leaving your computer (see Understanding Firewalls for more
* Follow good security practices for distributing your email address (see
Reducing Spam for more information). Applying email filters may help you
manage unwanted traffic.

How do you know if an attack is happening?

Not all disruptions to service are the result of a denial-of-service attack.
There may be technical problems with a particular network, or system
administrators may be performing maintenance. However, the following
symptoms could indicate a DoS or DDoS attack:
* unusually slow network performance (opening files or accessing websites)
* unavailability of a particular website
* inability to access any website
* dramatic increase in the amount of spam you receive in your account

What do you do if you think you are experiencing an attack?

Even if you do correctly identify a DoS or DDoS attack, it is unlikely that
you will be able to determine the actual target or source of the attack.
Contact the appropriate technical professionals for assistance.
* If you notice that you cannot access your own files or reach any
external websites from your work computer, contact your network
administrators. This may indicate that your computer or your
organization's network is being attacked.
* If you are having a similar experience on your home computer, consider
contacting your internet service provider (ISP). If there is a problem,
the ISP might be able to advise you of an appropriate course of action.

Author: Mindi McDowell

Produced 2004 by US-CERT, a government organization.

Note: This tip was previously published and is being re-distributed to increase awareness.

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