Learning to Use SPF Macros for Reduced Maintenance, Scalability, and Flexibility

Flexible and dynamic SPF records are easier to manage and need less frequent updates. Such records are even more significant for organizations with an extensive email ecosystem as they allow scalable SPF configurations.

SPF macros serve this purpose. They are a feature that is part of the SPF record syntax and defines character sequences that are replaced by metadata from individual emails requiring SPF validation. 

Here’s a detailed explanation of how SPF macros work, their key components, benefits, etc. 

SPF Macros Definition

SPF macros are placeholders used within SPF records that dynamically expand to specific values based on the characteristics of the email being processed. These macros allow SPF policies to be more flexible and adaptive by incorporating variables such as the sender’s email address, IP address, and domain name. This enables administrators to create precise and dynamic rules for validating email senders, enhancing email security, and simplifying SPF record management.

As of now, not many domain owners use SPF macros as people are uncertain about them and also because using them is a bit complicated.

An SMTP Primer

SMTP, which is short for Simple Mail Transfer Protocol, is the standard that describes the language using which two computer hosts communicate to exchange an email message over the internet. 

This is how a standard SMTP transaction looks like-

• The client tries connecting to the server
• The server accepts the connection invitation
• The client greets with EHLO or HELO and tells its name
• The server reverts to the greeting
• The client describes the email sender using the command ‘MAIL FROM’
• The server accepts the sender
• The client describes the email’s intended recipients using the ‘RCPT TO’ command
• The server accepts or rejects the intended recipients
• The client uses the ‘DATA’ command to pass the full body of the email
• The exchange ends with the email being accepted or rejected by the server

SPF validates a domain’s usage at the beginning of an SMTP transaction.

Key Components

SPF macros are represented by single letters or characters enclosed in curly braces { } with the percentage sign (%) as the prefix. Here are the most commonly used macros-

• %{s}: The ‘s’ macro represents the sender’s email address, for example, user@domain.com.
• %{l}: It represents the sender’s local part, it’s ‘user’ in the above example
• %{o}: This indicates the sender’s domain, which is ‘domain.com’ in the above example
• %{d}: Much like ‘o,’ the ‘d’ macro represents the authoritative sending domain, which is the same as the sender’s domain in most cases, except for a few.
• %{i}: This helps extract the IP address of the message sender, e.g., 303.0.113.8.
• %{h}: It tells the HELP/EHLO domain.

How do SPF Macros Work?

SPF macros help domain owners indicate some mechanisms within an SPF record, thereby replacing these mechanisms. These references are used during a DNS query by receiving MTA to extract the mechanisms and for scaling and simplifying the SPF record.

Here’s an example of how Macros are used in an SPF record-

“v=spf1 include:%{i}_.%{d}._spf.duocircle.com ~all”

• In the above example, the ‘include:’ mechanism contains the SPF macros.
• The above example has two SPF macros. %{i} denotes the sender’s IP address, and %{d} represents the sender domain from the ‘MAIL FROM’ command.
• Let 192.168.3.400 be the sender’s IP address. So, when an email is dispatched from this IP, the receiving server queries the DNS to check the SPF record corresponding to the sender’s domain.
• Once the receiver looks up the sending domain’s SPF record, it encounters SPF macros, which are then replaced with their corresponding values.
• This scaled SPF record is further evaluated to ensure it’s valid and has no scope for errors.

Who Should Use SPF Macros?

Depending on the email structure of your organization and your tolerance for false positives and negatives, SPF macros can be used in various conditions.

Here are a few cases-

Companies With Several Domains

SPF macros, though particularly beneficial for enterprise-level organizations managing multiple domains, are valuable for organizations of any size. They offer greater flexibility and efficiency in SPF records compared to conventional SPF flattening techniques. This ensures smooth SPF functioning in complex multi-domain environments and eliminates the need for multiple SPF records.

Intricate and Extensive Email Infrastructures

Organizations with complex email systems may need to use several SPF mechanisms, and the most efficient approach is to utilize SPF macros. These macros allow referencing mechanisms, preventing the SPF record from becoming excessively lengthy and keeping it within the RFC-prescribed limit.

External Service Providers or Vendors

Organizations using multiple third-party email vendors can be assured that their SPF won’t fail, thanks to SPF macros. These macros enable easy optimization of third-party ‘includes’ while ensuring the SPF record stays within the permitted limits for DNS and void lookups.

As aforementioned, integrating SPF macros in an SPF record is complicated and should be done by an expert only. So, if you need a helping hand, contact us. We will be happy to help.