A denial-of-service attack (DoS attack) or distributed denial-of-service attack (DdoS attack) is the process where the computer (so called victim) consisting important data becomes unreachable to its users. Usually the performers of DoS attacks target sites or services hosted on high-profile web servers such as banks, credit card payment gateways, and even root nameservers.
DoS attacks usually have the aim of reseting the targeted computer, or consuming its resources in order to make it fail supporting its intended service or to make the communication between the victim and its users inadequately.
For establishing a connection the user sends request to the server for authentication, and then the server responds with authentication approval to the user. Finally after the user acknowledges the server’s approval, the connection is established.
Upon a DoS attack, the user sends a number of authentication requests in order to fill up the server. All requests have false returns addresses so that the server can’t find the user for sending him the authentication approval. When this happens, the server waits for little time, sometimes more than one minute, and then closes the connection. If the connection is closed, the attacker sends new amount of requests keeping the service indefinitely.