Linux and denial of service - DoS

A denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a computer resource unavailable to its intended users.
We can see more on the wiki web page.

On Linux we can see if an attack is attempted with this:

netstat -ant | grep SYN_RECV | wc -l
This is an indication of a denial of service (DoS) attack against your system's X port.

It is likely one attack against a public service such as a web server or mail server.