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Difficulty: MediumDNS Infrastructure and Record Types

A network administrator is troubleshooting an issue where external email servers are unable to deliver messages to `example.com`. The administrator executes a DNS query tool to check the domain's mail configuration and receives the following output:

$ dig MX example.com +noall +answer
example.com. 3600 IN MX 10 mail.example.com.

When sending test emails, remote mail servers report a host resolution failure when attempting to reach `mail.example.com`.

Which of the following DNS resource records is missing from the authoritative zone file for `example.com`?

  1. An A record mapping mail.example.com to its corresponding IPv4 addressAnswer
  2. B
    A CNAME record mapping example.com directly to mail.example.com
  3. C
    A PTR record mapping mail.example.com to the mail server IPv4 address
  4. D
    An SRV record mapping port 25 on TCP to mail.example.com

Answer

An A record mapping mail.example.com to its corresponding IPv4 address
The MX (Mail Exchanger) record returns the hostname mail.example.com as the designated mail server for the domain. However, MX records do not contain IP addresses. For mail servers to establish a TCP connection, the DNS zone must contain a forward lookup host record (an A record for IPv4 or an AAAA record for IPv6) that maps mail.example.com to its IP address.

Step-by-Step Solution

1
Analyze the dig output
The MX record correctly points traffic for example.com to the host mail.example.com with priority 10.
An MX (Mail Exchanger) record maps a domain name to a fully qualified domain name (FQDN) of a mail server, not directly to an IP address.
2
Identify the cause of host resolution failure
Connecting servers cannot resolve the hostname mail.example.com into a Layer 3 IP address.
Without an authoritative host address record (A record for IPv4 or AAAA for IPv6) for mail.example.com, client queries for the mail server's IP address fail.
3
Determine the required record type to fix the issue
Add an A record mapping mail.example.com to its public IPv4 address in the zone file.
An A record provides the IPv4 address mapping required for clients to open a TCP connection to the mail server.

Key Concept

DNS MX Record and Host A Record Dependency
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Topics

DNS Infrastructure and Record Types

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