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32. Linux and Network Hardening

32.1 Hardening Principles

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Quick answer: the short, self-contained version of this lesson is the guide How to Harden a Linux Server (Checklist).

Principle Meaning Example
Least privilege Every user, process and service gets only the access it needs App user cannot use sudo; DB user has rights only on its own database
Reduce attack surface Fewer running services and open ports means fewer ways in Remove FTP/Telnet, close unused ports
Defence in depth Several layers, so one failure is not fatal Security group + host firewall + fail2ban + strong keys
Secure defaults Change default passwords, disable sample pages and debug No admin/admin, display_errors=Off
Patch and monitor Known bugs fixed quickly, activity logged sudo yum update, Wazuh (Chapter 31)

Industry checklists such as the CIS Benchmarks list hundreds of hardening settings per OS; tools like Lynis check a server against them.

sudo yum install -y lynis      # on Amazon Linux it may need the EPEL repo, or install from the CISOfy site
sudo lynis audit system        # prints warnings, suggestions and a hardening index

Ravindra Bagale's Tip

Hardening is not something you do once and forget. A new package, a new user, a new port – check again after every change. Run Lynis's "hardening index" once a month and see whether it goes up.

Practice task

Run sudo ss -tulnp on your lab server and list every listening port and the service behind it. Mark each one "needed" or "remove", then run Lynis and note its hardening index before you start this chapter.

Extended teaching notes

Why this lesson matters

Part 11 turns tool skills into professional defensive practice: web risks, cloud, SOC work, hardening, cryptography, Indian law and careers. Every example below stays inside systems you own or public teaching material.

Indian company (sample data)

Suppose we are a junior security engineer at an Infosys or TCS client project. You take this lesson's checklist, map it to three production services (for example a Spring Boot API, a Node.js portal and a Python job), and open tickets with evidence and owners.

Global practice

Follow public guidance from OWASP, CERT-In, CISA and cloud provider Well-Architected / security pillars. Cite them in reports.

Everyday analogy

A cricket team that only practises hitting but never fielding loses matches. Offence skills in a lab exist to improve fielding in real systems.

Apply across stacks

Wherever code appears, ask the same questions.

Java / Spring Boot

Parameterise queries, encode output, limit Actuator, patch dependencies.

.NET

EF Core parameters, HTTPS, Data Protection API, patch NuGet.

Node.js

helmet, parameterized SQL, npm audit, lockfiles.

Python

ORM, autoescape, pip-audit, Bandit.

Go / PHP / Ruby

Safe templates, prepared statements, govulncheck / composer audit / Brakeman.

nginx / Apache / Tomcat

Hide versions, TLS, least-open ports.

MySQL / PostgreSQL / MongoDB / Redis

No public ports, auth on, least-privilege users.

AWS / Azure / GCP

Roles not keys, private data stores, cloud logs on.

Defender worksheet

  1. Write a 5-line summary.
  2. Name one related public breach or advisory.
  3. Practise one safe command on a lab you own.
  4. Write the fix for a teammate.

Recap

  • Permission first.
  • Fix over spectacle.
  • Evidence in notes.

Defender's worksheet (do this today)

  1. Write the lesson title and date.
  2. List three owned systems where it applies.
  3. Run one safe lab check and paste evidence.
  4. Write a two-sentence fix for a teammate.
  5. Note one CERT-In or CISA advisory that relates (title + date).

Indian workplace (sample)

At a Wipro or LTIMindtree client SOC, turn this lesson into one playbook step with an owner and a due date.

Global practice

Compare your notes with OWASP, CIS Benchmarks or a cloud Well-Architected security question. Write the overlap in five lines.

Family test

Explain the risk and the fix without jargon. If a parent or sibling can repeat the warning, you are ready to teach juniors.

Common mistakes

  • Skipping permission.
  • Tool collecting without fixing.
  • Publishing lab screenshots that contain secrets.
  • Treating encoding as encryption.

Recap

  • Evidence, fix, owner, date.
  • Own lab only.

Key terms for this page

  • Control: a safeguard (patch, config, process).
  • Finding: a weakness with evidence.
  • Remediation: the fix and how you verified it.
  • Scope: systems you are allowed to touch.
  • Least privilege: only the access required for the task.
  • Defence in depth: several controls in layers.

Mini case study

Suppose we are a junior engineer at Zoho or Freshworks (sample data). A weekly review asks each person to present one control from their area. You pick a control from this lesson, show a before/after screenshot from a lab, and list the production ticket you would open. That habit builds both skill and credibility.