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Question

Difficulty: HardDNS Infrastructure and Record Types

A network administrator is troubleshooting an authentication service failure where an internal enterprise application requires reverse DNS verification before granting access to incoming clients. The administrator runs the command `dig -x 192.168.50.25` and receives the following output:

text
;; QUESTION SECTION:
;25.50.168.192.in-addr.arpa. IN PTR

;; AUTHORITY SECTION:
50.168.192.in-addr.arpa. 3600 IN SOA ns1.corp.internal. hostmaster.corp.internal. 2026072701 7200 3600 1209600 3600

Which resource record configuration must be added to the authoritative zone to resolve the hostname verification failure?

  1. A PTR record mapping 25.50.168.192.in-addr.arpa. to the client host's fully qualified domain nameAnswer
  2. B
    An A record mapping 25.50.168.192.in-addr.arpa. directly to 192.168.50.25 within the reverse lookup zone
  3. C
    A CNAME record alias configured to redirect reverse lookup queries over TCP port 53
  4. D
    An SRV record in the forward zone specifying UDP port 53 for reverse resolution services

Answer

A PTR record mapping 25.50.168.192.in-addr.arpa. to the host's fully qualified domain name must be created in the reverse lookup zone.
The output of `dig -x` demonstrates a query for a PTR record (`25.50.168.192.in-addr.arpa. IN PTR`). The absence of an ANSWER section confirms that no PTR record currently exists for IP 192.168.50.25. Adding a valid PTR record mapping the reverse address to the host's FQDN in the authoritative zone resolves the reverse lookup request.

Step-by-Step Solution

1
Analyze the command and output
The `dig -x` command issues a reverse lookup query for `25.50.168.192.in-addr.arpa.` expecting a PTR record. The output lacks an ANSWER section.
An missing ANSWER section with an AUTHORITY section indicates the reverse lookup zone exists, but no specific pointer record is configured for IP address 192.168.50.25.
2
Identify the required record type for reverse name resolution
Pointer (PTR) records map reverse IP domain paths (in-addr.arpa) to fully qualified domain names (FQDNs).
Applications performing reverse DNS security checks query PTR records to verify that an IP address maps back to an expected hostname.
3
Determine the correct administrative remediation
Add a PTR record for host 25 in the `50.168.192.in-addr.arpa.` zone pointing to the client's FQDN.
Creating this record populated the ANSWER section in subsequent queries, completing reverse hostname verification.

Key Concept

DNS Reverse Lookup and PTR Record Configuration
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Topics

DNS Infrastructure and Record Types

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