SPF DNS Record Syntax Explained

DMARC Fundamentals: Everything You Need to Know About Email Authentication Protocols What is SPF? SPF DNS Record Syntax Explained

A typical SPF record in the DNS looks like the following:

v=spf1 ip4=192.0.2.0 ip4=192.0.2.1 include:examplesender.email -all

The SPF DNS method employs a list of 8 mechanisms that differentiate authorized email senders from unauthorized ones.

all: This mechanism is at the end of the SPF record and matches all the senders.
ip4: This mechanism allows IP addresses of the IPv4 network range of a pre-specified list to send emails using a given domain name.
ip6: This mechanism is similar to ip4 but works on the IPv6 network range.
a: When this mechanism is used, the IP address should strictly match the SPF DNS record unless a prefix length is provided. When the prefix length is provided, the system searches all the IP addresses for that prefix length.
mx: In the case of this mechanism, the entire list of records is tested in the order of specified priority.
ptr: The hostnames are validated using PTR queries. The invalid hostnames are rejected, while the valid ones are matched.
exists: This mechanism utilizes an A query based on which the existing IP addresses are validated and approved.
include: This mechanism searches the domain for a match. If a match is not found, it forwards the list for further processing.

Each of the mechanisms can use any one of the four qualifiers:

+ (Pass)

The Pass qualifiers list the domain-authorized email sender.

– (Fail)

The Fail qualifier lists the unauthorized senders.

～ (SoftFail)

The SoftFail qualifier gives the list of the in-transition unauthorized senders.

? (Neutral)

The Neutral qualifier is used to mark the questionable senders.

While the DNS processing is ongoing, a temporary error may be represented by the qualifier’ TempError.’ In contrast, a syntax or evaluation error is notified by ‘PermError.’ In the cases where the domain has not created the record yet, the qualifier ‘None‘ is observed.

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