How does a DOS attack works - Behind the Scenes

There are two main approaches to denying a service:


a FLOODING ATTACK, sending a vast number of seemingly legitimate messages.
&
a VULNERABILITY ATTACK, exploiting a vulnerability present on the target

FLOODING ATTACK : Flooding or Bandwidth attacks are attempts to consume resources, such as network bandwidth or equipment throughput. High-data-volume attacks can consume all available bandwidth between an ISP and your site. The link fills up, and legitimate traffic slows down. Timeouts may occur, causing retransmission, generating even more traffic.

Flooding attacks work by sending a vast number of messages whose processing requires the server to allocate some key resource at the target. Once the server allocates its key resource to the attack, legitimate users cannot receive service. The crucial feature of flooding attacks is that their strength lies in the volume, so the flow of traffic must be so large as to consume the target's resources. If the attacker engages more than one machine to send out the attack traffic, then it is known as a DDoS attack.

Techniques : SYN Flood, Smurf, Fraggle


VULNERABILITY ATTACKS : Malicious messages by the attacker represent an unexpected input that the application programmer did not foresee. The messages cause the target application to go into an infinite loop; to severely slow down, crash, freeze, or reboot a machine; or to consume a vast amount of memory and deny service to legitimate users.
Techniques : teardrop, land, ping of death, Naptha