OpenVAS works in client/server mode. It is an open-source result of Nessus. Nessus is a widely used tool for the purpose of identification of vulnerabilities. Even though Nessus is popular as a scanner for vulnerabilities, there are restrictions on its licensing that inhibit it from being part of the BackTrack DVD, unlike OpenVAS which is included in said DVD. For most basis enumeration purposes, Nessus and OpenVAS can be used interchangeably.
Nessus is used on almost all companies doing security testing. The latest Nessus Server and Client versions are designed for Windows, FreeBSD, Mac OS X, and most of the Linux distributions. It is easy to install and is configured in minutes. The tool is useful when different operating systems are to be enumerated on large networks with many servers positioned at various locations. For instance, a laptop with Windows XP and the Nessus Client may be used to connect to any server with Nessus of the same version.
Nessus Client starts with a window used to designate a title to the enumeration session. The Nessus server has to be running before the attacked system can be connected. The Connect button is used to connect the Nessus Client to the Nessus Server. This opens the Connection Manager dialog box and the Nessus server to be connected to can be selected here. Each of these connections has a login ID.
After making this ID, a user account is created, with a password assigned on the server. The client has to then log on to the server in questions and be authenticated using these credentials. Then, the Windows system can be used to choose the networks or hosts to be tested and the scanning policy as well. Nessus identifies the computer and the workgroup or domain.