Most modern host firewalls are stateful, thus, they keep track of the connections made to them. Note: As with most of the configuration changes detailed in this post, lowering the number of simultaneous checks per host can increase your scan times, sometimes significantly. This is reasonable for most systems however, systems connected over low-bandwidth links, older systems, or ones with less robust networking stacks may benefit from setting this to 2 or even 1. The default for this setting is 4 or 5, depending on the scan policy used. These controls are located under the “Advanced” policy setting in Nessus: The first approach is to configure the number of vulnerability checks to run concurrently for each host. Read on to learn about multiple strategies you can use to deal with firewalls when using Nessus to perform internal or external vulnerability scans. Or, if you’re required to run a full port scan against your environment for auditing purposes, understanding the potential impact that could occur is key. However, organizations often have older equipment or infrastructure lying around which may require modifications to compensate for these types of devices. In a typical modern environment, it’s unlikely that these configurations will be needed. This approach is also ineffective against host-based firewalls: Even if you allow the scanner's IP address through the firewall, connection tracking and stateful inspection can interfere with the scan and negatively impact performance on the firewall. Scanners can sometimes be placed on network segments behind a firewall to avoid these problems, but this may not be feasible in all situations, creating an extra burden when moving a scanner around. Network-based firewalls are essential for an organization’s perimeter protection and internal network segregation, while host-based firewalls are common on both Linux and Windows systems. Of all the factors that can inhibit a successful Nessus scan - busy systems, congested networks, legacy systems, hosts with large amounts of listening services - firewalls (and other types of filtering devices) are one of the major causes of slow or inaccurate scans. Establishing the right configurations and settings can improve Nessus scan results when scanning through firewalls.
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