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Interview question · Network Security & Protocols

How does protective DNS filtering work, and how do DNS over HTTPS and DNS over TLS change what a network can see and enforce?

Blocking bad domains at the resolver is one of the cheapest controls there is, until the client stops asking your resolver. A strong answer covers both halves.

mediumNewPreparation tags: Cloudflare, Google, CiscoUpdated Nov 2026

Blocking bad domains at the resolver is one of the cheapest controls there is, until the client stops asking your resolver. A strong answer covers both halves.

Make your first attempt

What would you say before reading the answer?

  1. 01 State the assumptions your answer depends on.
  2. 02 Explain the mechanism and choose a control.
  3. 03 Name a condition that would change your decision.

The worked answer includes an approach, scoring criteria and follow-up questions. Compare the reasoning with your own.

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What if · the follow-ups in this answer

The answer works through 4 follow-ups like an interviewer would

A company relies on a protective DNS service to block phishing and malware domains. A phishing page still loaded on a managed laptop, and the DNS logs show no query for it.

  1. How could the page load with no query in the company's DNS logs?
  2. How do you bring the browser back under policy?
  3. What if the attacker's domain is brand new and on no list?
  4. What does a resilient DNS control look like?

Each rung has a nudge, a model answer, the principle that carries over and the weak answer to avoid. Premium opens every answer and ladder.

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