ptr — SPF Mechanism

The SPF ptr mechanism authorises senders via reverse DNS lookup. RFC 7208 explicitly recommends against using it — learn why and what to use instead.

Type
Mechanism
Syntax
ptr[:<domain>]
DNS Lookup
Yes — counts toward 10-lookup limit
Example
ptr:example.com

💬 What This Mechanism Does

The ptr mechanism passes if a reverse DNS (PTR) lookup on the sending IP returns a hostname that resolves back to an IP in the sending domain. This is the Forward-Confirmed Reverse DNS (FCrDNS) check. While conceptually sound, it's slow, unreliable, and explicitly deprecated by RFC 7208.

When to Use This

  • Legacy records only — no new use cases recommended.

⚠️ Watch Out For

  • RFC 7208 §5.5 states: "use of this mechanism is discouraged."
  • PTR lookups are slow, can time out, and some nameservers don't support them.
  • Results are unreliable — many hosting providers don't set reverse DNS.
  • Replace with ip4/ip6 or a/mx for equivalent authorisation.

📚 RFC References

↗RFC 7208 §5.5 — The "ptr" Mechanism (discouraged)

🔗 Related SPF Elements

a
Mechanism
 ip4
Mechanism

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