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45. Practice Exercises with Hints

45.3 Linux on EC2 – Commands You Must Type Blind

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Amazon Linux habit: sudo yum install, sudo service <name> start|stop|restart|status. Permissions, users, logs, processes – Blue daily bread.

Exercise Task Hint
E1 Install and start Nginx on Amazon Linux; prove status sudo yum install nginx -​y then sudo service nginx start and sudo service nginx status
E2 Create user amir, add to wheel carefully, set chmod 400 on SSH key on client Never chmod 777 keys; ssh -​i key.​pem ec2-​user@... from your PC
E3 Find failed SSH logins in logs (path may vary) /var/log/secure on Amazon Linux; sudo grep -​i failed /​var/​log/​secure \| tail
E4 Explain why chmod 777 /​var/​www/​html is a student favourite mistake World-writable web root = defacement / webshell drop easy
E5 Write a one-line cron that runs a backup script at 02:00 (lab) 0 2 * * * /​home/​ec2-​user/​backup.​sh – test script path first
E6 ss -tulnp or netstat – list listening ports; mark unexpected Blue baseline: know what should listen
Red team (attacker) does Blue team (defender) detects / stops
Brutes SSH on open port 22 from internet Key-only SSH; fail2ban; SG allow your IP; no password auth
Looks for world-writable web dirs after foothold Correct ownership; no 777; file integrity / package verify

Ravindra Bagale's Tip

Students write systemctl start and get confused with the Amazon Linux notes. In this book, service control = sudo service. For enabling at boot, sudo systemctl enable is OK. Keep this in mind.

Lab

On OWN Free Tier Amazon Linux: install httpd, sudo service httpd start, open SG 80 from your IP, browse Elastic IP. Then sudo service httpd stop and confirm site down. Notes: 6 command lines + screenshot.

Extended teaching notes

Why this lesson matters

Use this lesson to strengthen systems you own: understand the risk, practise safely in a lab, then write and apply the fix.

Apply the fix across stacks

Same class of bug, same idea in every language.

Java / Spring Boot

Parameterise queries, encode output, CSRF tokens, limited Actuator, dependency scanning.

ASP.NET Core

EF Core / parameterised SQL, Razor encoding, HTTPS, Data Protection.

Node.js / Express

helmet, parameterised SQL, npm audit, avoid eval and shell concat.

Python / Django / Flask / FastAPI

ORM, autoescape, pip-audit, Bandit; never shell=True with user input.

PHP / Laravel

PDO/Eloquent, Blade escaping, composer audit.

Go / Ruby

database/sql placeholders / ActiveRecord; html/template / ERB escaping; govulncheck / Brakeman.

nginx / Apache / Tomcat

Hide versions, TLS, security headers, least-open ports.

MySQL / PostgreSQL / MongoDB / Redis

Private networks, auth on, least-privilege users, no 0.0.0.0/0.

AWS / Azure / GCP

Roles not long-lived keys, private data stores, cloud audit logs on.

Mini case study

Suppose we are on a purple-team exercise for an Indian mid-size company (sample data). Red notes a technique from this lesson against a lab clone. Blue must show the control that stops it, the log that would have fired, and the ticket that hardens production. That is the standard this course trains you for.

Defender's worksheet (do this today)

  1. Write the lesson title, date and the exact lab target you are allowed to use.
  2. List three owned systems (VMs, apps, or cloud accounts) where this lesson applies.
  3. Run one safe check from the lesson on a lab system only. Paste the command and output.
  4. Write a two-sentence fix you would give a teammate.
  5. Name one related public advisory (CERT-In, CISA, NVD, or a vendor bulletin) with its date.

Indian workplace (sample data)

Suppose we are a junior analyst at Infosys, TCS, Wipro or a fintech SOC in Bengaluru/Pune. Turn this lesson into one checklist item with an owner, a due date and a place to store evidence (ticket + screenshot).

Global practice

Compare your notes with OWASP, CIS Benchmarks, NIST CSF/IR guidance or a cloud Well-Architected security question. Write five lines on what overlaps.

Everyday analogy

Explain the risk and the fix to a family member without jargon. If they can repeat the warning, you understand it well enough to teach juniors.

Key terms

  • Control: a safeguard (patch, config, process, detection).
  • Finding: a weakness backed by evidence.
  • Remediation: the fix plus how you verified it.
  • Scope: systems you have written permission to touch.
  • Least privilege: only the access the task needs.
  • Defence in depth: several layers so one failure does not end the story.

Common mistakes

  • Skipping written permission "just for a quick scan".
  • Practising on hostel, office or public targets without authorisation.
  • Collecting tool output without writing the fix.
  • Copying attack steps onto production to "verify". Defenders verify with versions, configs and patches.
  • Publishing screenshots that contain secrets, cookies or internal hostnames.

Recap

  • Evidence, fix, owner, date.
  • Own lab only.
  • Fix > spectacle.