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Difficulty: MediumUtilizing Command-Line Network Troubleshooting Utilities

A network technician executes `nslookup -type=TXT _dmarc.example.com` to verify an email authentication policy record. The command returns the following terminal output:

Server: UnKnown
Address: 192.168.1.2

Non-authoritative answer:
_dmarc.example.com text = "v=DMARC1; p=reject;"

Based on this command-line output, which of the following statements correctly interprets the diagnostic result?

  1. The record data was successfully retrieved from a local caching DNS resolver rather than the authoritative DNS server for example.com.Answer
  2. B
    The query failed completely because the local DNS server host name could not be resolved and is displayed as 'UnKnown'.
  3. C
    The lookup returned invalid data because DMARC policies must be published using A records rather than TXT records.
  4. D
    The query was rejected because the nslookup utility attempted to query the DNS server over TCP port 25 instead of UDP port 53.

Answer

The record data was successfully retrieved from a local caching DNS resolver rather than the authoritative DNS server for example.com.
The 'Non-authoritative answer' output header signifies that the server responding to the query (192.168.1.2) obtained the record via recursive resolution or local cache rather than directly holding the authoritative zone files for example.com. The TXT query successfully returned the expected DMARC record.

Step-by-Step Solution

1
Analyze the output header line 'Server: UnKnown / Address: 192.168.1.2'.
Identifies that the local computer queried the internal DNS server at 192.168.1.2, whose reverse DNS PTR lookup is unconfigured.
Understanding local DNS server settings prevents confusing unmapped server hostnames with query failures.
2
Examine the answer section designation 'Non-authoritative answer:'.
Confirms the response originated from a recursive or caching server rather than the authoritative name server hosting the primary zone.
Non-authoritative answers occur when a resolver holds cached records or queries third-party root/TLD servers on behalf of the client.
3
Evaluate the record payload '_dmarc.example.com text = "v=DMARC1; p=reject;"'.
Confirms valid TXT record contents were successfully fetched.
DMARC policies are properly published within TXT resource records.

Key Concept

Interpreting nslookup output headers and non-authoritative DNS lookup results
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Topics

Utilizing Command-Line Network Troubleshooting Utilities

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