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Difficulty: MediumDNS Infrastructure and Record Types

A network administrator is troubleshooting an issue where external mail gateways reject outbound emails sent from `example.com` due to failed Sender Policy Framework (SPF) validation checks. The administrator executes the following `dig` commands to inspect the DNS domain records:

text
$ dig MX example.com +short
10 mail01.example.com.

$ dig A mail01.example.com +short
192.0.2.25

Which of the following DNS resource records must be created in the `example.com` DNS zone to resolve the SPF validation failures?

  1. A TXT record containing a policy string that explicitly designates authorized sending mail servers and IP addresses for the domainAnswer
  2. B
    A CNAME record aliasing `mail01.example.com` to `spf.example.com` to delegate authentication handling
  3. C
    An MX record with a lower preference value specifically designated for SPF verification traffic
  4. D
    A PTR record mapping TCP port 25 directly to `mail01.example.com` for reverse transport protocol authentication

Answer

The network administrator must create a TXT record that specifies the SPF policy string containing authorized IP addresses and mail servers for the domain.
The option specifying a TXT record containing a policy string is correct because SPF relies on machine-readable text records in DNS to list authorized sending hosts and IP addresses for a domain.

Step-by-Step Solution

1
Identify the purpose of Sender Policy Framework (SPF).
SPF is an email authentication protocol designed to detect email spoofing by verifying sender IP addresses.
Receiving email systems validate incoming messages against sender rules published in the domain's public DNS.
2
Evaluate which DNS record type stores SPF policy statements.
SPF mechanisms and policies are defined using standard DNS TXT resource records.
TXT records store arbitrary human-readable and machine-readable text strings, including SPF, DKIM, and DMARC authentication records.

Key Concept

DNS Record Types and Mail Security (TXT for SPF/DKIM/DMARC)
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Topics

DNS Infrastructure and Record Types

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