Nessus is a computer program originally written by Renaud Deraison, which is used to monitor computer and computer security. Originally, Nessus was an open source software package, but in 2005, the software was closed, which means that not everyone can view or edit their source code.
It consists of a server program, which provides the actual scans, and a client program, nessus, which connects to the server program. A web interface is available that simplifies working with Nessus.
A security scan always begins searching for the open TCP / IP ports on the machine (s) to be attacked, a so-called port scan. For this purpose, Nessus uses built-in scanners, but it can also use third party scanners such as the nmap scanner.
The package is available for both Windows and Linux. There are two licenses available online: one for non-commercial use and one for commercial use. There are dozens of thousands of plug-ins available for the package, written in the NASL language. This language is specific to Nessus, files written in the language have the extension .nasl. NASL stands for: Nessus Attack Scripting Language, so it's a script language for attacks with Nessus.
A fork (tagging of code) was created from the open source version of Nessus under the name of OpenVAS. This is still being developed. OpenVAS can also interpret NASL scripts. Also see Externe link
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