A "denial-of-service" attack is characterized by an explicit attempt by attackers to prevent legitimate users of a service from using that service. Examples include:


Attempts to "flood" a network, thereby preventing legitimate network traffic,
Attempts to disrupt connections between two machines, thereby preventing access to a service,
Attempts to prevent a particular individual from accessing a service,
Attempts to disrupt service to a specific system or person.

Not all service outages, even those that result from malicious activity, are necessarily denial-of-service attacks. Other types of attack may include a denial of service as a component, but the denial of service may be part of a larger attack.

Illegitimate use of resources may also result in denial of service. For example, an intruder may use your anonymous ftp area as a place to store illegal copies of commercial software, consuming disk space and generating network traffic

 Denial-of-service attacks can essentially disable your computer or your network. Depending on the nature of your enterprise, this can effectively disable your organization.

Some denial-of-service attacks can be executed with limited resources against a large, sophisticated site. This type of attack is sometimes called an "asymmetric attack." For example, an attacker with an old PC and a slow modem may be able to disable much faster and more sophisticated machines or networks.

Denial-of-service attacks come in a variety of forms and aim at a variety of services. There are three basic types of attack:


Consumption of scarce, limited, or non-renewable resources
destruction or alteration of configuration information
physical destruction or alteration of network components.

 
Computers and networks need certain things to operate: network bandwidth, memory and disk space, CPU time, data structures, access to other computers and networks, and certain environmental resources such as power, cool air, or even water.

Denial-of-service attacks are most frequently executed against network connectivity. The goal is to prevent hosts or networks from communicating on the network. An example of this type of attack is the "SYN flood" attack described in

http://www.cert.org/advisories/CA-1996-21.html

In this type of attack, the attacker begins the process of establishing a connection to the victim machine, but does it in such a way as to prevent the ultimate completion of the connection. In the meantime, the victim machine has reserved one of a limited number of data structures required to complete the impending connection. The result is that legitimate connections are denied while the victim machine is waiting to complete bogus "half-open" connections.

You should note that this type of attack does not depend on the attacker being able to consume your network bandwidth. In this case, the intruder is consuming kernel data structures involved in establishing a network connection. The implication is that an intruder can execute this attack from a dial-up connection against a machine on a very fast network.

An intruder can also use your own resources against you in unexpected ways. One example is described in

http://www.cert.org/advisories/CA-1996-01.html

In this attack, the intruder uses forged UDP packets to connect the echo service on one machine to the chargen service on another machine. The result is that the two services consume all available network bandwidth between them. Thus, the network connectivity for all machines on the same networks as either of the targeted machines may be affected.

An intruder may also be able to consume all the available bandwidth on your network by generating a large number of packets directed to your network. Typically, these packets are ICMP ECHO packets, but in principle they may be anything. Further, the intruder need not be operating from a single machine; he may be able to coordinate or co-opt several machines on different networks to achieve the same effect.

In addition to network bandwidth, intruders may be able to consume other resources that your systems need in order to operate. For example, in many systems, a limited number of data structures are available to hold process information (process identifiers, process table entries, process slots, etc.). An intruder may be able to consume these data structures by writing a simple program or script that does nothing but repeatedly create copies of itself. Many modern operating systems have quota facilities to protect against this problem, but not all do. Further, even if the process table is not filled, the CPU may be consumed by a large number of processes and the associated time spent switching between processes. Consult your operating system vendor or operating system manuals for details on available quota facilities for your system.

An intruder may also attempt to consume disk space in other ways, including

generating excessive numbers of mail messages. For more information, please see

http://www.cert.org/tech_tips/email_bombing_spamming.html

intentionally generating errors that must be logged
placing files in anonymous ftp areas or network shares, For information on proper configuration for anonymous ftp, please see

http://www.cert.org/tech_tips/anonymous_ftp_config.html

In general, anything that allows data to be written to disk can be used to execute a denial-of-service attack if there are no bounds on the amount of data that can be written.

Also, many sites have schemes in place to "lockout" an account after a certain number of failed login attempts. A typical set up locks out an account after 3 or 5 failed login attempts. An intruder may be able to use this scheme to prevent legitimate users from logging in. In some cases, even the privileged accounts, such as root or administrator, may be subject to this type of attack. Be sure you have a method to gain access to the systems under emergency circumstances. Consult your operating system vendor or your operating systems manual for details on lockout facilities and emergency entry procedures.

An intruder may be able to cause your systems to crash or become unstable by sending unexpected data over the network. An example of such an attack is described in

http://www.cert.org/advisories/CA-1996-26.html

If your systems are experiencing frequent crashes with no apparent cause, it could be the result of this type of attack.

An improperly configured computer may not perform well or may not operate at all. An intruder may be able to alter or destroy configuration information that prevents you from using your computer or network.

For example, if an intruder can change the routing information in your routers, your network may be disabled. If an intruder is able to modify the registry on a Windows NT machine, certain functions may be unavailable.

For information on configuring UNIX machines, see

http://www.cert.org/tech_tips/unix_configuration_guidelines.html

For information on configuring Microsoft Windows NT machines, please see

http://www.microsoft.com/security/

Well, hopefully this text has given you a little bit of what the infamous "DoS" attack is.

I'll be adding some DoS applications soon for all who wants to...well you know....see above.