The nessusd.rules the file is an advanced configuration file in Nessus that allows you to fine-tune scan behaviour. It’s a powerful way to control which plugins, targets, or ports are included or excluded from scans. This file is particularly useful for global rules that affect all scans on the Nessus server.
What is the nessusd.rules File?
The nessusd.rules file allows administrators to:
- Exclude specific hosts or IP ranges from scans.
- Disable certain plugins globally or for specific targets.
- Prevent scanning of particular ports or protocols.
This file acts as a filter, enabling you to enforce rules before scans begin, ensuring compliance with organizational policies or avoiding sensitive systems.
Where is the File Located?
The nessusd.rules file resides on the Nessus server. It’s found in the configuration directory, and administrative access is required to modify it.
Understanding the Syntax
The file uses simple rules to either allow or deny specific actions. These rules can apply to:
- Hosts: IP addresses, subnets, or ranges.
- Plugins: Specific vulnerability checks by their unique plugin IDs.
- Ports: Specific ports or port ranges.
Rules typically define whether to include or exclude a target, plugin, or port from scans.
Common Use Cases
- Excluding Sensitive Systems
Prevent scans on critical systems, such as production databases, to avoid disruptions. - Disabling False Positive Plugins
If a particular plugin is known to generate inaccurate results in your environment, you can exclude it from running. - Avoiding Specific Ports
To ensure certain services or ports (e.g., administrative protocols) are not scanned. - Restricting Scans to Approved Hosts
Ensure that scans only run on authorized systems by defining allowed hosts.
How to Apply Changes
After modifying the file, save your changes and restart the Nessus service. This ensures the new rules are applied globally across all scans.
Testing Rules
To verify that your rules are working:
- Run a test scan on targets included or excluded by your rules.
- Check the scan results to ensure the rules were enforced correctly.
Best Practices
- Backup the File
Always create a backup of thenessusd.rulesfile before making changes. - Document Rules
Keep a record of why each rule was added to maintain clarity and ease of future updates. - Test Before Production
Validate changes in a test environment to avoid unintended scan behavior. - Avoid Conflicting Rules
Ensure that rules are clear and don’t overlap, which could cause unexpected behavior. - Regular Reviews
Periodically review and update the file to ensure it aligns with current needs and policies.
Limitations
- The rules apply globally to all scans on the Nessus server. For scan-specific configurations, it’s better to use policies within the Nessus interface.
- Managing this file requires manual intervention and familiarity with the Nessus backend, making it less user-friendly than UI-based configurations.
Why It’s Underutilized
The nessusd.rules file is not widely discussed because:
- It’s a backend feature that requires manual configuration.
- Most users rely on policies and settings within the Nessus interface, which are more intuitive.
- It’s often reserved for advanced use cases in large or sensitive environments.
The nessusd.rules file is a hidden gem that provides unparalleled control over Nessus scans. By defining clear and effective rules, you can improve scan precision, protect sensitive systems, and align Nessus behavior with your organization’s policies. When used correctly, it’s an indispensable tool for administrators managing complex or critical environments.
