Reasons Behind Discouraging the Use of PTR Mechanism in an SPF Record

Domain owners who care about email delivery and prevention from phishing attacks take no chances when it comes to the validation and correctness of their SPF records. One of the common elements causing issues in an SPF record is the use of the PTR mechanism due to its slow processing and unreliable nature. 

Per RFC 7208, SPF evaluation is capped at 10 DNS mechanism lookups and 2 void lookups per check — exceeding either limit produces a PermError that fails authentication for every message from the domain.

This guide explores more on why experts deprecate the use of the PTR mechanism in an SPF record.

Touching the Base- What is a PTR Mechanism and What Does it Actually Do?

The PTR mechanism is used for doing a reverse DNS lookup, which means retrieving the domain name corresponding to the queried IP address. It’s the opposite of the A record. Its procedure goes on as follows:

• The linked IP address is used to conduct reverse mapping in the “in-addr.arpa” format for IPv4 and in “ip6.arpa.” for IPv6 to find any domain names.

• Next, a forward lookup is performed for each domain name. 

• The match is successful if the connecting IP address and the returned IP addresses are the same. 

These steps make it an unreliable and slow mechanism; hence, experts disapprove of its use in a valid SPF record. 

3 Reasons Why Including the PTR Mechanism is a Wrong Move

In 2023, almost 333 billion emails were exchanged worldwide, and this number is anticipated to grow to 392.5 billion by 2026. With emails being such a critical medium of communication, you must understand the logic behind the discouragement of including a PTR mechanism in your SPF record.

Slow and Unreliable

The PTR mechanism can lead to delays and possible DNS errors because of the extra lookups it requires. In terms of ensuring dependable email authentication, it is less efficient compared to other mechanisms.

How does a DNS query work

Overload on Name Servers

Conducting PTR lookups puts a substantial strain on .arpa name servers, rendering it unfeasible for widespread implementation. This strain on name servers has the potential to amplify response times and result in service disruptions.

SPF Validation Failures

Major email recipients might opt to skip or disregard the PTR mechanism because of caching constraints, leading to potential SPF validation failures.

Alternative Mechanisms

Now that you can’t use the PTR mechanism, here are some of its alternatives-

‘A’ Mechanism

This mechanism enables the linking of a domain name to one or more IPv4 addresses, ensuring that the connecting IP address aligns with the IP address associated with the domain name.

‘MX’ Mechanism

The MX mechanism defines that the domain’s incoming messages are officially permitted to dispatch messages on behalf of the organization or domain owner. It validates that the sending server’s IP address corresponds to one of the authorized MX records for the domain. Essentially, the MX mechanism helps enhance email authentication by verifying the legitimacy of mail servers designated to handle incoming emails for a given domain.

‘IP4’ and ‘IP6’ Mechanisms

They verify that the linked IP address is the same as the IP4 or IP6 address specified.

‘Include’ Mechanism

The use of the ‘include’ mechanism allows you to add sending sources of third-party senders allowed to dispatch emails on your behalf. 

Final Words

The limitations of SPF can be overcome by combining it with DKIM and DMARC. Together, this trio disallows hackers from impersonating you or your employees and attempt phishing attacks. DMARC aggregate and forensic reports give you insights into email activities and inform you of suspicious messages sent from your domain. Carefully monitoring DMARC reports and adjusting DMARC policies accordingly reduces the instances of false positives and offers you protection from spammers.