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Understanding Denial-of-Service Attacks
You may have heard of denial-of-service attacks launched against web
sites, 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.
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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, web sites, 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 web site 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 web site 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
computers:
- 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 information).
- 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 web sites)
- unavailability of a particular web site
- inability to access any web site
- 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 web sites 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
Copyright 2004 Carnegie Mellon University. Terms of use
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Last
updated
August 1, 2007
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