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Difficulty: MediumDNS Infrastructure and Record Types

A network systems engineer is deploying an IPv6-enabled mail gateway. Remote mail servers are failing reverse DNS checks when accepting outbound messages from the gateway's IP address (2001:db8:85a3::8a2e:370:73342001:db8:85a3::8a2e:370:7334). The engineer runs the following diagnostic command from a terminal:

text
$ host 2001:db8:85a3::8a2e:370:7334
Host 4.3.3.7.0.7.3.0.e.2.a.8.0.0.0.0.0.0.0.0.3.a.5.8.8.b.d.0.1.0.0.2.ip6.arpa not found: 3(NXDOMAIN)

Which of the following resource records must be configured in the authoritative DNS zone to resolve the NXDOMAIN error?

  1. A PTR record within the ip6.arpa domain that maps the reverse nibble format of the IPv6 address to the mail gateway's FQDNAnswer
  2. B
    An AAAA record mapping the mail gateway's FQDN to the 2001:db8:85a3::8a2e:370:7334 IPv6 address
  3. C
    A PTR record configured to deliver queries across TCP port 25 to match the mail transport protocol
  4. D
    An AAAA record configured to enforce TCP port 53 transport because IPv6 reverse strings exceed standard packet boundaries

Answer

A PTR record within the ip6.arpa domain that maps the reverse nibble format of the IPv6 address to the mail gateway's fully qualified domain name (FQDN).
Reverse DNS lookups resolve IP addresses into hostnames using Pointer (PTR) records. For IPv6, the address is expanded into 32 hexadecimal nibbles, reversed, and appended with .ip6.arpa. Creating a PTR record in this zone allows receiving servers to successfully resolve the mail gateway's IP address.

Step-by-Step Solution

1
Analyze the diagnostic command and output.
The query 'host 2001:db8:85a3::8a2e:370:7334' performs a reverse DNS lookup against the reverse IPv6 domain (ip6.arpa) and returns NXDOMAIN, indicating the record is missing.
Reverse DNS resolution queries IP addresses to retrieve associated domain names.
2
Identify the required DNS record type for reverse lookup.
A Pointer (PTR) record is specifically designed to map IP addresses to FQDNs in reverse lookup zones (.in-addr.arpa for IPv4 and .ip6.arpa for IPv6).
Forward lookup records (A or AAAA) map hostnames to IP addresses, which will not satisfy reverse DNS lookup requests.
3
Select the option specifying the correct record type and zone structure.
Creating a PTR record in the ip6.arpa zone resolves the reverse lookup failure.
This establishes the valid mapping required by receiving mail servers verifying origin IP addresses.

Key Concept

IPv6 Reverse DNS Resolution (PTR Records in ip6.arpa)
Estimated Time:1m 30s
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Topics

DNS Infrastructure and Record Types

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