Many people may not realize it, but the Sender Policy Framework (SPF) specification has a limit on the number of DNS lookups (10) required to fully resolve an SPF record. One typically quickly exceeds this limit through the reckless use of the include modifier.
Section 10.1, "Processing Limits" of the SPF RFC specifies the following in regards to DNS lookups:
SPF implementations MUST limit the number of mechanisms and modifiers that do DNS lookups to at most 10 per SPF check, including any lookups caused by the use of the "include" mechanism or the "redirect" modifier. If this number is exceeded during a check, a PermError MUST be returned. The "include", "a", "mx", "ptr", and "exists" mechanisms as well as the "redirect" modifier do count against this limit. The "all", "ip4", and "ip6" mechanisms do not require DNS lookups and therefore do not count against this limit. The "exp" modifier does not count against this limit because the DNS lookup to fetch the explanation string occurs after the SPF record has been evaluated.
This limit is in place to prevent SPF lookups from being a useful avenue for Denial of Service attacks.
Is your SPF record validating? You can find out here.
Sender Policy Framework (SPF) is a crucial email authentication protocol that helps prevent email spoofing and enhances email deliverability. However, it's essential to be aware of certain limitations, particularly concerning DNS lookups within SPF records. In this section, we'll delve into SPF record lookups, their significance, and how to ensure your SPF record remains compliant.
SPF records are designed to verify the authenticity of sender domains by checking their DNS records. However, there's a limitation in place to prevent abuse and potential Denial of Service (DoS) attacks. According to Section 10.1, "Processing Limits" of the SPF RFC, SPF implementations must limit the number of DNS lookups to a maximum of 10 per SPF check. This includes any lookups triggered by the "include" mechanism or the "redirect" modifier.
In practical terms, this means that if your SPF record includes too many "include" mechanisms, "a", "mx", "ptr", or "exists" mechanisms that lead to DNS lookups, you risk exceeding the 10-lookup limit. When this limit is surpassed during an SPF check, a PermError is returned, potentially causing SPF validation failures.
To avoid SPF validation issues related to DNS lookup limitations, it's crucial to optimize your SPF record. Consider the following best practices:
- Minimize "include" Mechanisms: Reduce the use of the "include" mechanism to only include domains that are essential for your email delivery.
- Use IP Mechanisms: Whenever possible, use "ip4" and "ip6" mechanisms instead of "include" to specify IP addresses directly, as these mechanisms don't require DNS lookups.
- Monitor SPF Records: Regularly review and update your SPF record to ensure it remains efficient and compliant with SPF specifications.
- SPF Record Checker: Utilize SPF record checkers or validation tools to verify the correctness of your SPF record, such as here.
By understanding SPF record lookup limitations and adhering to best practices, you can maintain a reliable and effective SPF record that enhances your email authentication and delivery while staying within the specified DNS lookup limits.