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DNS MX Record Misconfiguration

DNS MX Record Misconfiguration

Description

MX (Mail Exchanger) records in DNS are critical for email routing and delivery. Misconfigurations in MX records can lead to email delivery failures, increased vulnerability to email spoofing, and inefficient mail routing. The following areas are key concerns in MX record configuration:

  1. Record Format: MX records must adhere to the standard format of 'priority fully-qualified-hostname.domain.tld'. Incorrect formatting can lead to misinterpretation by email servers and DNS resolvers. Priority values should be valid integers, and hostnames must be properly formatted.

  2. Priority Values: MX record priorities should be within the valid range of 0-65535. The lowest value indicates the most preferred mail server. Improperly configured priorities can lead to inefficient email routing.

  3. Duplicate Records: Duplicate MX records should be avoided. They can cause confusion in DNS resolution, and may lead to unnecessary processing by mail servers. It's important to regularly audit MX records to ensure no duplicates exist.

  4. Hostname Validity: The hostnames specified in MX records must be valid and resolvable to IP addresses. Non-existent or unreachable hostnames can cause email delivery failures. It's crucial to ensure that these hostnames point to active mail servers through A or AAAA record lookups.

  5. Consistency with SPF Records: MX record hostnames should be included in the domain's SPF (Sender Policy Framework) record. Inconsistency between MX and SPF records can increase vulnerability to email spoofing and negatively impact email deliverability.

These misconfigurations can result in delayed or failed email delivery, increased susceptibility to email-based attacks, and overall degradation of an organization's email infrastructure reliability. The impact can range from minor inconveniences to severe disruptions in business communication.

Recommendation

To address MX record misconfigurations, implement the following:

  • Correct Record Format: Ensure all MX records follow the exact 'priority hostname.domain.tld' format.

    • Example:

      • 10 mail.example.com.

  • Validate Priority Values: Set integer priorities within the 0-65535 range, with lower values for preferred servers.

    • Example:

      • 10 primary-mail.example.com.
      • 6553 secondary-mail.example.com.

  • Manage Duplicate Records: Ensure there are no duplicate records.

    • Example:

      • Incorrect:
        • 10 mail1.example.com.
        • 10 mail1.example.com.
      • Correct:
        • 10 mail1.example.com.
        • 20 mail2.example.com.

  • Verify Hostname Resolvability: Confirm all MX hostnames resolve to valid IP addresses of active mail servers.

    • Example:

      • For 10 mail.example.com., ensure the domain is resolvable to a valid IP address, e.g.:
        • mail.example.com. IN A 203.0.113.1

  • Align with SPF Records: Include all MX hostnames in the domain's SPF record.

    • Example:

      • The MX record 10 mail.example.com. should be included in the SPF record, e.g.:
        • v=spf1 MX ip4:203.0.113.1 -all, where resolving the domain in the MX record would give us the ip 203.0.113.1.

Standards

  • PCI_STANDARDS:
    • REQ_1_3
    • REQ_4_1
    • REQ_12_2
  • GDPR:
    • ART_25
    • ART_32
  • SOC2_CONTROLS:
    • CC_4_1
    • CC_6_1
    • CC_6_6
    • CC_7_1
    • CC_7_2