What is Nessus is For?http://www.tenable.com/products/nessus/
So we will discuss how we can install it online (if you are having internet connection)
Nessus 4.4.1 now comes pre-installed on BackTrack 5 and requires that the user activate the installation.
Here are the commands to install nessus : open terminal and type(if you don't know :P)
apt-get install nessus
After install it, create an account with adduser command like this
Registering Account For Nessus
Once you have Nessus installed on BackTrack 5, you will need to obtain a Nessus activation code. If you wish to purchase a ProfessionalFeed, you can visit the Tenable Store. If you are using Nessus at home or wish to evaluate Nessus, you can register a HomeFeed. It’s important to note that the HomeFeed is limited to 16 IP addresses per scan (whereas the ProfessionalFeed allows you to scan an unlimited number of IP addresses). The ProfessionalFeed also gives you access to features such as Configuration and Sensitive Data Auditing, SCADA plugins, Nessus Technical Support and access to the Tenable Customer Portal.
After registering they will send you activation code to your email. After you get the key, run this command, fill change xxxx-xxxx-xxxx-xxxx-xxxx with your key
/opt/nessus/bin/nessus-fetch --register xxxx-xxxx-xxxx-xxxx-xxxx
It will take some time because plugin is being updated. After the update complete, run your Nessus
Then open your browser and type this in the URL of the browser
(You can also access the Nessus Web Interface remotely/outside by using the IP address assigned to BackTrack 5 (e.g. https://192.168.37.210:8834/).)
Nessus will run on the secure channel https and on the port number 8834.
Now try and use Nessus for your own risk :-D..
Next tutorial i will post how to scan a host and get a report using nessus scanner.if you need anyhelp on any tools do post as a comment i will definitely reply on you also i also encourage everyone to use Backtrack if you wann learn more on security things