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12. Domains and DNS: GoDaddy, Elastic IP, A/CNAME and Subdomains

12.6 Verifying DNS with dig, nslookup and curl

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Also in the AWS course: this lesson is the same as 9.9 Verify DNS: dig, nslookup and friends in the AWS course, which is the main version of this page.

dig example.com +short                  # A record → 203.0.113.10
dig www.example.com +short              # CNAME chain → example.com. → 203.0.113.10
dig example.com                         # full answer incl. TTL countdown
dig @8.8.8.8 example.com +short         # ask Google's public resolver
dig @1.1.1.1 example.com +short         # ask Cloudflare's resolver
dig NS example.com +short               # which name servers are authoritative?
dig MX example.com +short               # mail records
dig +trace example.com                  # follow root → TLD → authoritative
nslookup example.com                    # works on Windows, macOS and Linux
host example.com                        # short output (Linux)
curl -I http://example.com              # DNS + web server together

dig comes from bind-utils on Amazon Linux/CentOS (sudo yum install -y bind-utils) and dnsutils on Ubuntu (sudo apt install -y dnsutils).

Reading dig output:

;; ANSWER SECTION:
example.com.        587     IN      A       203.0.113.10
     name          TTL(s) class   type     value

Flush stale caches on your laptop if the server already shows the new IP but your browser still doesn't:

OS Command
Windows ipconfig /flushdns
macOS sudo dscacheutil -​flushcache; sudo killall -​HUP mDNSResponder
Linux (systemd-resolved) resolvectl flush-​caches
Chrome browser open chrome://​net-​internals/#dns → Clear host cache

Why this matters for security

The same dig commands are reconnaissance tools. dig +trace shows the delegation chain; dig TXT reveals SPF and verification records; testing curl -H "Host: ..." against an IP is how testers find virtual hosts that are not in public DNS.

Ravindra Bagale's Tip

When a domain "isn't working", split the problem in two: dig yourdomain.com +short tells you whether DNS is correct, and curl -I http://<EIP> -H "Host: yourdomain.com" tells you whether the web server is correct. If both are fine, it's just your laptop's cache – wait or flush it.

Practice task

Query your domain through 8.8.8.8 and 1.1.1.1, follow it with dig +trace, and flush your laptop's DNS cache.

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.