What Is Denial Of Service (DOS) attack ?

It is an internet attack, where an attacker request a large number of connection to an internet server, through the use of an improper protocol. The attacker can leave a number of connections half open most system can handle only a small number of half open connection before they are no longer able to communicate with other system on the net, this attack completely disable the internet server.

          "OR"          You can say that, In this attack we use so much memory on the target system that it can serve the legitimate user.

Perpetrators of DoS attacks typically target sites or services hosted on high-profile web servers such as banks, credit card payment gateways, and even root name servers. This technique has now seen extensive use in certain games, used by server owners, or disgruntled competitors on games. Increasingly, DoS attacks have also been used as a form of resistance. DoS they say is a tool for registering dissent. 

Richard Stallman has stated that DoS is a form of 'Internet Street Protests’. The term is generally used relating to computer networks, but is not limited to this field; for example, it is also used in reference to CPU resource management.

One common method of attack involves saturating the target machine with external communications requests, so much so that it cannot respond to legitimate traffic, or responds so slowly as to be rendered essentially unavailable. Such attacks usually lead to a server overload. In general terms, DoS attacks are implemented by either forcing the targeted computer(s) to reset, or consuming its resources so that it can no longer provide its intended service or obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.

The list of DDoS attack victims includes some pretty major names. Microsoft suffered an attack from a DDoS called MyDoom. Crackers have targeted other major Internet players like Amazon, CNN, Yahoo and eBay. The DDoS names range from mildly amusing to disturbing :-

☻ Ping of Death - bots create huge electronic packets and sends them on to victims.

☻ Mailbomb - bots send a massive amount of e-mail, crashing e-mail servers.

☻ Smurf Attack - bots send Internet Control Message Protocol (ICMP) messages to reflectors, see above illustration.

☻ Teardrop - bots send pieces of an illegitimate packet; the victim system tries to recombine the pieces into a packet and crashes as a result.

Once an army begins a DDoS attack against a victim system, there are few things the system administrator can do to prevent catastrophe. He could choose to limit the amount of traffic allowed on his server, but this restricts legitimate Internet connections and zombies alike. If the administrator can determine the origin of the attacks, he can filter the traffic. Unfortunately, since many zombie computers disguise (or spoof) their addresses, this isn't always easy to do.