What is a denial of service ?

How is a Denial of Service visible ?

A "Denial Of Service" (DOS) is a method in which an attacker seeks to make a machine or network unavailable. Any attack method which makes the system unavailable for use by legit users can be classified as a Denial of service. Systems that suffer from a Denial Of Service may exhibit different symptoms based on the type of resource that is being abused. System resources such as disk, memory, network bandwidth may be consumed or disabled in an attack to disrupt a system or network in parts of a larger attack.

What is a distributed denial-of-service (DDoS) attack?

The incoming traffic flooding the victim originates from many different sources. This effectively makes it impossible to stop the attack simply by blocking a single source.