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Difficulty: HardTroubleshooting DNS and Name Resolution Services

A network administrator is investigating reports that internal workstations are unable to connect to a web application at `payroll.internal.net`. To diagnose the issue, the administrator executes a hostname lookup using `nslookup` on a client machine, which returns the following output:

text
$ nslookup payroll.internal.net
Server: 10.1.1.5
Address: 10.1.1.5#53

Name: payroll.internal.net
Alias: app-server-04.internal.net

*** 10.1.1.5 can't find app-server-04.internal.net: Non-existent domain (NXDOMAIN)

Which of the following represents the primary root cause of this name resolution failure?

  1. The CNAME record for `payroll.internal.net` points to a canonical target (`app-server-04.internal.net`) that lacks a corresponding A or AAAA record in DNS.Answer
  2. B
    The client system failed to establish a TCP port 53 handshake, which is mandatory for resolving CNAME alias records.
  3. C
    The DNS server returned an authoritative error because `payroll.internal.net` requires an MX record rather than a CNAME alias.
  4. D
    The `nslookup` utility failed because it only inspects the local HOSTS file when resolving canonical alias addresses.

Answer

The CNAME record for `payroll.internal.net` points to a canonical target (`app-server-04.internal.net`) that lacks a corresponding A or AAAA record in DNS.
A CNAME (Canonical Name) record acts as an alias pointing to another domain name rather than directly to an IP address. When a client requests `payroll.internal.net`, the DNS server resolves `payroll.internal.net` to `app-server-04.internal.net` and then attempts to resolve `app-server-04.internal.net` to an IP address. Because `app-server-04.internal.net` does not have a valid A/AAAA host record in the zone, the DNS server returns an NXDOMAIN error for the target, causing the entire resolution attempt to fail.

Step-by-Step Solution

1
Analyze the `nslookup` command output.
The lookup successfully resolved `payroll.internal.net` to its alias target `app-server-04.internal.net`.
This confirms that the CNAME entry for `payroll.internal.net` exists and is functional.
2
Examine the second stage of the DNS resolution chain.
The DNS server returned `Non-existent domain (NXDOMAIN)` when attempting to resolve `app-server-04.internal.net`.
Resolving a CNAME record requires a valid A (IPv4) or AAAA (IPv6) record for the destination canonical hostname. Without an A/AAAA record for `app-server-04.internal.net`, final IP address resolution cannot complete.
3
Identify the corrective action required to fix the issue.
Create an A record for `app-server-04.internal.net` mapping to the server's correct IP address.
Adding the missing host record will allow future CNAME chain resolution to complete successfully.

Key Concept

DNS CNAME Record Resolution and Chaining Dependencies
Estimated Time:2m 0s
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Topics

Troubleshooting DNS and Name Resolution Services

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