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Difficulty: MediumDNS Infrastructure and Record Types

An enterprise systems engineer is resolving name lookup and mail transport issues for the internal domain `hr.enterprise.internal`. The engineer executes the following diagnostic command to inspect the domain's resource records:

$ dig +noall +answer hr.enterprise.internal MX
hr.enterprise.internal. 3600 IN MX 10 smtp1.enterprise.internal.

A subsequent lookup query directed to `smtp1.enterprise.internal` returns no address records, preventing mail transfer agents from connecting to the server.

Based on standard DNS infrastructure requirements and protocol operations, which TWO of the following configurations or administrative actions are required to restore proper functionality?

  1. Add an A or AAAA record for host `smtp1.enterprise.internal` mapping the hostname to the mail server's IP address.Answer
  2. Ensure that network firewall rules permit client and server traffic on UDP port 53 for standard DNS name resolution queries.Answer
  3. C
    Replace the MX record entry with a CNAME record pointing directly to the target IP address of the mail host.
  4. D
    Configure DNS client resolvers to send name resolution queries to the DNS server using TCP port 25.
  5. E
    Require all client DNS host resolution queries to use TCP port 53 exclusively because UDP is reserved strictly for zone transfers.

Answer

The resolution requires adding an A or AAAA record mapping `smtp1.enterprise.internal` to its IP address, and ensuring firewalls permit UDP port 53 traffic for standard DNS name queries.
Mail Exchanger (MX) records point to host domain names rather than IP addresses directly. Consequently, an A record (for IPv4) or an AAAA record (for IPv6) must exist for the specified target hostname so that clients can resolve the mail server's IP address. Additionally, DNS name resolution relies on UDP port 53 for standard client query and response traffic.

Step-by-Step Solution

1
Analyze the MX record output from the diagnostic lookup.
The MX record points to the hostname `smtp1.enterprise.internal` as the designated mail server.
An MX record only specifies the domain name of the mail gateway, requiring an underlying address record to determine the target IP.
2
Identify the missing host address record required to complete host resolution.
An A record (IPv4) or AAAA record (IPv6) must be configured for `smtp1.enterprise.internal`.
Without an A/AAAA record, mail transfer agents cannot translate the host domain name returned by the MX query into a network layer IP address.
3
Identify the proper protocol and port configuration for DNS service traffic.
Ensure UDP port 53 is open and accessible.
Standard iterative and recursive DNS queries rely on UDP port 53 for lightweight, low-overhead communication.

Key Concept

DNS MX Record Host Resolution Dependency and Core DNS Port/Protocol Operation
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Topics

DNS Infrastructure and Record Types

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