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Difficulty: MediumTroubleshooting DNS and Name Resolution Services

A network administrator is troubleshooting host reachability issues on an enterprise dual-stack network. Workstations cannot connect to an internal server using its hostname `app.internal.example.com`, though direct IPv6 ping tests succeed. The administrator executes the following troubleshooting command from a client system:

C:\> nslookup app.internal.example.com
Server: dns01.internal.example.com
Address: 10.0.0.53

*** dns01.internal.example.com can't find app.internal.example.com: Non-existent domain

Next, the administrator runs `nslookup -type=AAAA app.internal.example.com`, which successfully returns the IPv6 address `2001:db8:85a3::8a2e:370:7334`.

Which of the following is the root cause of the initial resolution failure?

  1. The DNS server lacks an IPv4 address (A) record for the target host, and the default query requested an A record.Answer
  2. B
    The local DNS client is configured to send DNS queries over TCP port 53 instead of UDP port 53.
  3. C
    The command-line lookup utility misinterpreted the non-authoritative answer flag returned by the DNS server.
  4. D
    The DNS server service is configured to listen on port 5353 instead of standard port 53.

Answer

The DNS server lacks an IPv4 address (A) record for the target host, and the default query requested an A record.
The initial nslookup command without specified parameters queries for IPv4 address (A) records by default. Because the DNS server only contains an IPv6 address (AAAA) record for app.internal.example.com, the initial lookup fails with a 'Non-existent domain' or record missing error. Explicitly setting -type=AAAA successfully retrieves the IPv6 mapping.

Step-by-Step Solution

1
Analyze the default nslookup output.
The tool queries the primary DNS server (10.0.0.53) for app.internal.example.com without explicit parameters, defaulting to an IPv4 host (A) record request.
Standard DNS resolution tools query for A records unless specified otherwise.
2
Compare the default command with the explicit -type=AAAA query.
The explicit query returns the IPv6 address 2001:db8:85a3::8a2e:370:7334.
This confirms that a valid AAAA record exists in DNS, but no A record has been created for IPv4.
3
Determine the root cause.
The failure occurs because the DNS zone lacks an A record while the client default lookup relies on A record resolution.
Creating an A record (or ensuring dual-stack applications query AAAA records) resolves the name resolution issue.

Key Concept

DNS Record Types and Default Command-Line Query Behavior
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Topics

Troubleshooting DNS and Name Resolution Services

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