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What is the purpose of the 'ptr' SPF mechanism?

The Sender Policy Framework (SPF) is a critical email authentication standard that helps protect your domain from being used for spam and phishing. It works by creating a DNS TXT record that lists all the authorized servers permitted to send email on behalf of your domain. When an email is received, the recipient's mail server checks this record to verify the sender is legitimate. SPF records are made up of several parts, including 'mechanisms', which define the rules for checking a sender's identity.

One of these is the ptr mechanism. In theory, its purpose is to perform a reverse DNS lookup to validate the sending server. The idea is to check if the sending IP address has a PTR record (a pointer record) that points back to a hostname within the specified domain. If it does, and that hostname then resolves back to the original IP address, the check passes.

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Easy365Manager says:
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The ptr mechanism checks if a reverse lookup on the sender's IP address matches the sender's domain name. The site explicitly states: "do not use".
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Why you should avoid the 'ptr' mechanism

While the concept behind the ptr mechanism might seem sound, it is highly discouraged and considered deprecated for several important reasons. Most modern email systems will treat an SPF record containing a ptr mechanism as invalid, potentially leading to a permanent error (PermError) during SPF evaluation. The primary issues with using ptr are:

  • It's inefficient and slow: The ptr mechanism requires multiple DNS lookups for a single check. First, a reverse lookup for the IP, then a forward lookup for the resulting hostname. This process is resource-intensive and can contribute to DNS lookup timeouts, which can cause legitimate emails to fail authentication.
  • It's unreliable: The effectiveness of this method depends entirely on senders maintaining accurate PTR records. Many legitimate mail servers do not have properly configured reverse DNS records, which would cause the check to fail even for valid emails.
  • It's deprecated by the RFC: The official specification for SPF, RFC 7208, explicitly advises against its use. It is considered an outdated part of the SPF standard.
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IETF Datatracker says:
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This mechanism is slow, it is not as reliable as other mechanisms in cases of DNS errors, and it places a large burden on the .arpa name servers. If used, proper PTR records must be in place for the sender's mail servers. Its use is NOT RECOMMENDED.

What to use instead of 'ptr'

Instead of relying on the obsolete ptr mechanism, you should always use more direct and reliable mechanisms to build your SPF record. These methods are efficient, clear, and fully supported.

Commonly used mechanisms include:

  • a: Authorizes servers by the domain's A or AAAA records.
  • mx: Authorizes servers listed in the domain's MX records.
  • ip4/ip6: Directly specifies authorized IPv4 or IPv6 addresses.
  • include: Includes the SPF record from another domain, commonly used for third-party sending services.
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DuoCircle says:
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DuoCircle notes that SPF recommendations caution against using the ptr mechanism. Avoiding it, along with other unused services, is a key step to preventing SPF errors and staying within the 10-lookup limit.

Conclusion

In short, the purpose of the ptr mechanism was to provide a way to verify a sending server through reverse DNS. However, due to its inefficiency, unreliability, and official deprecation, it should not be used in modern SPF records. Sticking to standard mechanisms like include and ip4 ensures your SPF record is effective, efficient, and compliant with current email authentication standards.

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