A Denial of Service (DoS) attack is an attack that aims to make some service unavailable. In the last few years, usually we speak about DDoS, since those are very effective and cheap and for those reasons have become the most used DoS attack. DDoS attacks consist in multiple machines trying to overload a server or its connection to make the services that are running on that server unavailable.

The good part about DoS attacks is that in majority of the cases, as soon as they end it, all is back to normal. When this is not true, small actions have to be executed by system administrators, such as restarting a service or rebooting a machine.