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DevOps / Cloud Networking Dns Loadbalancing Interview Questions

75 verified DevOps / Cloud Networking Dns Loadbalancing interview questions — solve with answers, learn from explanations, test yourself in a real simulation.

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Sample questions

Networking Dns LoadbalancingDifficulty 1
What is the primary purpose of a DNS A record?
  • aIt stores an alias so one hostname can point to another hostname.
  • bIt maps a hostname to an IPv4 address so clients can reach a server.✓
  • cIt defines which mail servers are responsible for a domain.
  • dIt sets the maximum time a resolver may cache a DNS answer.
Explanation:An A record maps a hostname directly to an IPv4 address. Aliasing one hostname to another is what a CNAME record does; mail server assignment is the job of an MX record; caching duration is controlled by TTL, not by the record type itself.
Networking Dns LoadbalancingDifficulty 1
Given the following zone records:
blog.example.com.    300  IN  CNAME  example.com.
example.com.          300  IN  A      203.0.113.10

A client queries blog.example.com. What IP address will it ultimately receive?
  • aNo IP — resolvers cannot automatically follow a CNAME to another record.
  • bThe resolver returns an error, because a name cannot have both a CNAME and an A record chain.
  • cIt depends entirely on which mail server is configured for example.com.
  • d203.0.113.10, the IP found by following the CNAME to example.com's A record.✓
Explanation:A CNAME is an alias: the resolver follows it to the target name and continues resolution from there, ultimately returning the A record's IP. Resolvers follow CNAME chains automatically as part of normal resolution; this has nothing to do with mail servers.
Networking Dns LoadbalancingDifficulty 2
You update your domain's A record to point to a new server's IP address, but for about an hour afterward some visitors still land on the old server. What is the most likely cause?
  • aDNS resolvers cached the previous record and won't re-query until the TTL expires.✓
  • bThe new server is misconfigured and rejecting all incoming traffic.
  • cThe domain registrar has not yet approved the record change.
  • dA load balancer between the client and server is still routing some requests to a stale backend server.
Explanation:This staggered, time-limited symptom is the classic signature of DNS caching: different resolvers cached the old answer at different times, so they expire and re-query at different times based on the TTL. Registrar approval and server misconfiguration would typically produce a different, non-time-bound symptom.
Networking Dns LoadbalancingDifficulty 1
In the following zone record:
www.example.com.   60   IN   A   203.0.113.5

What does the value 60 represent?
  • aThe number of retries a resolver performs before failing the query.
  • bThe priority value used for ordering when multiple A records exist for the same hostname.
  • cThe maximum number of seconds a resolver may cache this record (TTL).✓
  • dThe port number the server listens on for this record.
Explanation:In a zone file, the numeric value next to the record name is the TTL, in seconds — the maximum time resolvers may cache the answer. DNS A records don't carry priority (that's an MX concept) or a port number (that's an SRV concept).
Networking Dns LoadbalancingDifficulty 3
Your team lowers a record's TTL from 3600 to 60 seconds ahead of a planned migration, waits 10 minutes, then flips the record to the new server. During the migration, a small fraction of users still hit the old server for up to an hour afterward. What best explains this?
  • aThe zone file wasn't saved correctly, so the change never actually propagated.
  • bSome resolvers had already cached the answer under the old TTL, and that cache doesn't expire early.✓
  • cTTL only applies to CNAME records, not to A records, so lowering it had no effect here.
  • dDNS propagation always takes exactly 24 to 48 hours, regardless of any TTL setting.
Explanation:Lowering the TTL only affects new lookups made after the change — it can't retroactively shrink the caching window of answers a resolver already cached under the old, larger TTL. That's why waiting 10 minutes wasn't enough for every resolver to have picked up the new short TTL. TTL applies to any record type, and there's no fixed universal propagation time.
Networking Dns LoadbalancingDifficulty 1
What is the main purpose of a load balancer?
  • aTo distribute incoming traffic across multiple backend servers.✓
  • bTo encrypt all traffic exchanged between the client and the backend servers.
  • cTo cache static content on servers geographically closer to end users.
  • dTo translate human-readable domain names into IP addresses for clients.
Explanation:A load balancer's core job is distributing traffic across backends for capacity and resilience. Encrypting traffic (TLS termination) can be an optional LB feature, caching content closer to users is what a CDN does, and translating names to IPs is DNS's job.

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