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Chapter 9: Domain Names, DNS and Subdomains

Interview Questions

These DNS questions definitely come up in cloud and DevOps interviews. While answering, say one command like dig out loud – the interviewer sees your hands-on experience.

Q1. Explain how DNS resolution works when you type a URL.
Browser/OS cache and hosts file → recursive resolver → root server → TLD server (returns NS of the domain) → authoritative name server (returns the record) → resolver caches it for the TTL and returns the IP → browser connects.
Q2. What is the difference between an A record and a CNAME?
An A record maps a name to an IPv4 address. A CNAME maps a name to another name, which is then resolved. A CNAME can't be used at the zone apex or alongside other records for the same name.
Q3. What is TTL? How would you plan a server migration with minimum DNS delay?
Time To Live, the number of seconds a resolver may cache a record. Lower the TTL (e.g. 300) well before the change, switch the record (or better, move the Elastic IP), verify, then raise the TTL again.
Q4. What is the difference between a registrar and a DNS hosting provider?
The registrar registers and renews the domain and stores which name servers are authoritative. The DNS host runs those name servers and answers queries with your records. They can be the same company or different ones (GoDaddy registrar + Route 53 DNS).
Q5. How do you host blog.example.com on the same EC2 instance as example.com?
Add a DNS record blog (A to the same Elastic IP), create /var/www/blog, add a server block/virtual host with server_name blog.example.com, test, reload and optionally extend the certificate with certbot.
Q6. Why use an Elastic IP or load balancer for DNS records?
The auto-assigned public IP changes on stop/start. A DNS record pointing to it would break, or point to someone else's instance.
Q7. What is a Route 53 Alias record?
A Route 53 extension that maps a name, including the zone apex, to AWS resources like load balancers, CloudFront or S3 websites. It follows their IP changes automatically, and alias queries to AWS resources are not charged.
Q8. Which commands do you use to check DNS?
dig name +short, dig @8.8.8.8 name, dig NS domain, dig +trace name, nslookup name, host name.

Extended teaching notes

Why this lesson matters

Cloud security is mostly IAM, network exposure, logging and secure defaults. The shared responsibility model means the provider patches the metal; you patch what you configure.

  • No public databases or public admin panels.
  • Roles / OIDC instead of long-lived access keys.
  • CloudTrail / Activity Logs / Audit Logs on and retained.
  • S3/Blob/GCS: Block Public Access; encrypt; least privilege.

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.