The idea of DOS attack is to reduce the quality of service offered by server, or to crash server with heavy work load. DoS (Denial of Service) attack does not involve breaking into the target server. This is normally achieved by either overloading the target network or target server, or by sending network packets that that may cause extreme confusion at target network or target server.
A "denial-of-service" attack is characterized by an explicit attempt by attackers to prevent legitimate users of a service from using that service. Some of the examples are
• 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.
One simple DoS (Denial of Service) attack was called the "Ping of Death." The Ping of Death was able to exploit simple TCP/IP troubleshooting ping tool. Using ping tool, hackers would flood a network with large packet requests that may ultimately crash the target server.