Hardening Linux Security Audits: Implementing Enhanced Custom Lynis Scripts for Password Policy and DNS Configuration Auditing

Enhancing Lynis with Custom Linux Security Tests

Cybersecurity / Linux Security • Auditing password aging policies and DNS resolver configurations through custom Linux security checks.
Written by Pushkar Regode

Introduction

Lynis is an open-source security auditing tool designed to evaluate Linux system configurations and identify potential vulnerabilities. In this project, I explored the existing Lynis framework and developed custom security tests to analyze password aging policies and DNS resolver configurations. These enhancements aim to provide deeper visibility into authentication hardening gaps and network resolver security risks.

Objective

  • To analyze the default Lynis custom test structure to understand its auditing mechanism and identify limitations in baseline password and DNS checks.
  • To develop and integrate two custom Lynis tests targeting password aging policy misconfigurations and DNS resolver security weaknesses.

Reference Script

Script Name: tests_custom

Description

The base script contains two custom tests for password policy and DNS configuration auditing. Test CUST-0001 reads /etc/login.defs and extracts PASS_MAX_DAYS, PASS_MIN_DAYS, and PASS_WARN_AGE using grep and awk, displaying all values with an unconditional [ OK ] status before performing threshold comparisons. Test CUST-0002 checks if /etc/resolv.conf exists and echoes configured nameserver entries. This script served as the structural template I used to build my own enhanced security checks without replacing the original tests.

Command

Output

Enhancement Options

Building upon the foundational logic of the reference script, the enhancements focus on identifying specific authentication and network configuration risks. The two enhancement options involve auditing password aging parameters with strict threshold validation and inspecting /etc/shadow for blank password accounts, and performing an in-depth DNS resolver security audit covering systemd stub detection, nameserver redundancy, EDNS0 verification, and search domain hijacking risk assessment.

Enhanced Script 1

Script Name: CUST-0001 (Password Aging & Shadow Account Audit)
GitHub Link: CUST-0001 →

Description

I created this custom test to inspect password aging policies for security misconfigurations. It validates PASS_MAX_DAYS against a 90-day threshold, enforces PASS_MIN_DAYS to prevent immediate password reuse, and verifies PASS_WARN_AGE ensures adequate user notification before expiry. Unlike the baseline script which displayed misleading [ OK ] statuses for insecure values like PASS_MAX_DAYS = 99999, this enhanced version only reports [ OK ] when values genuinely pass validation. It also inspects /etc/shadow for accounts with empty password hash fields, successfully detecting unauthenticated login vulnerabilities.

Command

Output

Enhanced Script 2

Script Name: CUST-0002 (DNS Resolver Security & Redundancy Audit)
GitHub Link: CUST-0002 →

Description

This second custom test specifically audits DNS resolver configuration in /etc/resolv.conf. It detects whether the file is a managed symlink or a static file vulnerable to overwriting, identifies the systemd-resolved local stub address 127.0.0.53 to clarify the actual DNS architecture, evaluates nameserver redundancy to prevent single-point-of-failure outages, verifies the edns0 option required for DNSSEC and large DNS payloads, and flags excessive search domain entries that risk DNS query leakage and suffix hijacking attacks.

Command

Output

Outcomes

  • Successfully developed and integrated custom Lynis security tests to accurately audit password aging policies and DNS resolver configurations, eliminating false-positive status reporting present in the baseline script.
  • Gained practical experience in Linux shell scripting, authentication configuration analysis (/etc/login.defs, /etc/shadow), and DNS resolver security evaluation including systemd stub detection and EDNS0 verification.

AI Disclaimer

I utilized AI as a supporting tool while preparing the custom Lynis tests and formatting the structure of this blog. Specifically, I used AI to refine my shell-script logic, understand password aging parameter validation, and establish the layout for publication. The custom tests were developed using established Linux commands and Lynis templates rather than relying on AI-generated vulnerabilities. Furthermore, all audit results and terminal outputs displayed in this report are derived directly from actual execution on my system. Ultimately, AI assisted primarily in clarifying technical explanations and enhancing the overall visual presentation of my original work.

Conclusion

This project successfully demonstrated how to extend Lynis by developing custom security checks for password aging policies and DNS resolver configurations. The integration of these scripts improved the auditing scope by eliminating misleading false-positive reports and adding multi-layered validation checks. These enhancements ultimately provide a more robust and accurate evaluation of Linux system security.

Acknowledgements

I would like to thank the School of Computer Science and Engineering (SCOPE), Vellore Institute of Technology (VIT), Chennai, for offering the Cyber Security course during the Fall Semester 2026–2027 with an industry-standard syllabus.

I would also like to express my gratitude to Dr. T. Subbulakshmi, Professor, SCOPE, VIT Chennai, for her guidance and for providing the opportunity to explore practical cybersecurity concepts through this project.

I acknowledge the Lynis founder and the Lynis development community for creating and maintaining this open-source security auditing tool. The existing Lynis tests, custom-test template, and documentation were valuable references while developing my custom security checks.

References

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