I recently had reason to spend a while working with Nessus on Windows XP (Service Pack 2). Usually, I use a Nessus Server running on Linux, either running locally if I am onsite, or one installed on our company infrastructure for scanning from the Internet. In fact, you read the documentation don’t you?, Tenable specifically recommends in the Nessus Installation Guide that you _not_ run Nessus on XP, and instead use a Windows Server product, such as Windows Server 2003.
The reason for this is that in Windows XP Service Pack 2, Microsoft introduced a number of Network Protection Technologies for mitigating the spread of malware. One of these limits the number of simultaneous incomplete outbound TCP connection attempts to 10, with additional attempts being queued and potentially dropped. This impacts the reliability of at least port scanning, and possibly other security checks.
Unfortunately the scenario I was working with required me to be running Nessus through a VPN client (never ideal), in reality requiring me to be on XP. Tenable does, however, have some recommendations for running Nessus as reliably as possible on XP:
- Max number of hosts: 10
- Max number of security checks: 4
- Max number of packets per second for port scan: 50
The maximum hosts/security checks setting is standard in all of the Nessus clients I’ve used, however the packets per second setting seems to only be available within the client shipped with the Windows Nessus server. If you, like me, are using the new NessusClient 3.0 beta for Windows, you need to make the following change to the Nessus server’s configuration to ensure that 50 is the default value:
- Go to the “config” directory in your Nessus server installation. By default this is C:\Program Files\Tenable\Nessus\config
- Open config.default.xml for editing - just use Notepad if you don’t have an XML editor
- Find the SYN Scan:Max number of packets per second for port scan node, and edit the value (the CDATA bit) from 500 to 50
This value should now be the default for all new scans.
This worked well for me, however needless to say that running a Nessus scan in VMWare (slowdown factor one), over a VPN link (slowdown factor two), over a transatlantic Internet connection (slowdown factor three), the scan took quite a while to complete…