# Nessus

# Nessus quick install on Kali

To fire off a quick install on the Kali side, head to the Tenable downloads page and then copy the curl command to grab the installer, which will look something like:

curl --request GET \
  --url 'https://www.tenable.com/downloads/api/v2/pages/nessus/files/Nessus-10.8.2-debian10_amd64.deb' \
  --output 'Nessus-10.8.2-debian10_amd64.deb'

# Kick off the install

sudo dpkg -i Nessus-x.deb

# Startup the Nessus service

sudo systemctl start nessusd.service

# Finish configuration

Then visit your install at https://your.nessus.ip.address:8834 and continue with the rest of the install and setup.

# Watch main nessusd.messages file

This is the file that will show things such as services starting up/down, plugin updates being applied, etc.

sudo tail -f /opt/nessus/var/nessus/logs/nessusd.messages

# Reset forgotten password

If you at least know the username you can reset the forgotten password (Windows):

cd "c:\Program Files\Tenable\Nessus"
nessus chpasswd USER-GOES-HERE

# Fix "installation expired" error

If you fire up the GUI URL and get this error:

Installation Expired
This installation has expired.  If you believe that you are are seeing this screen in error, please take one of the following corrective actions...

(and then you're presented with some contact options)

Here's how you fix (on Windows):

cd "c:\Program Files\Tenable\Nessus"
nessuscli.exe fetch --register xxx
net stop "tenable nessus"
net start "tenable nessus"

# Performance tuning

See the Nessus scan tuning guide for guidance.

What's annoying about this guide is it doesn't recommend specific values for the "max simultaneous checks per host" etc. However, during a support chat an agent offered this guidance:

For the Max Simultaneous Hosts Per Scan, the standard default for a Nessus Professional scanner is typically 30. While you mentioned seeing a suggestion of 256, this is generally a high-end cap for significantly more powerful hardware. For a 4-core system, a value of 30 is a well-balanced starting point that provides good performance without over-utilizing CPU resources.

For Max Simultaneous Checks Per Host, The recommended default is 5. This determines how many vulnerability tests are run against a single machine at once. Increasing this can speed up the scan for that specific host, but it also increases the risk of performance impact on that host or triggering network security defenses.  

To avoid overwhelming your network, ensure the "Slow down the scan when network congestion is detected" setting is enabled in your Advanced policy settings. This allows Nessus to automatically throttle itself if it detects that the network pipe is reaching capacity.  