Nessus was released in 1998. It was initially developed as a free remote security scanner but later became proprietary when Renaud Deraison--the developer of Nessus--co-founded a company called Tenable Network Security. For personal use, the product is freely available, but for commercial use, it is not. It costs $1200/year subscription fee for a single scanner. For the latest on their pricing check out their online store.
Nessus' primary function is to provide security scanning capabilities to its user. Like Nmap, it can do network discovery, but unlike Nmap, it is designed to scan systems to determine their vulnerabilities. Some vulnerability checks, if safe mode is disabled, can crash a system! As such, administrators expose a system to the scans prior to putting them in production.
Because of what Nessus can do, it can be applied in the following ways:
- Security audits
- Asset profiling
- Sensitive data discovery
- Vulnerability analysis
All these can probably done with Nmap, but it would take a lot of work.
Tenable Network Security develops many security check plugins. Between all these plugins, Nessus is able to check for over 12,000 CVEs (Common Vulnerability Exposures).
Nessus is really a different class of tool compared to Nmap. It is more of an enterprise scanner. Nessus uses a web interface for its user interface, and it allows the Nessus administrator to create multiple users. The one interesting thing about Nessus is the ability to create policies which are composed of scanning specifications--to include what security plugins are to be used, the types of ports to scan, any credentials that may be needed, and so on. A scan can then be created using that policy. It is in scans where targets are specified.
Through the user interface, one can view scan results even as the scan is in progress. Reports are generated invarious formats. This complete approach to vulnerability scanning puts Nessus on a higher level than Nmap.