SPF

SPF: Sender Policy Framework

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About

SPF (Sender Policy Framework) is an anti-spam protocol in which a DNS record specifies what IP addresses are allowed to send messages with a "from" address coming from that domain. The IP addresses may be specified literally (numerically, using CIDR address-range syntax) or as domain-names. SPF is defined in RFC 7208 (status).

DNS Configuration

Discussion of the DNS record format is in Section 3. SPF uses TXT records; there must be only one SPF record per domain or subdomain. A record's contents must start with "v=spf1" in order to be recognized as an SPF record.

The remainder of the contents consists of a set of terms, formatted according to these nearly-indecipherable rules:

  terms            = *( 1*SP ( directive / modifier ) )

  directive        = [ qualifier ] mechanism
  qualifier        = "+" / "-" / "?" / "~"
  mechanism        = ( all / include / a / mx / ptr / ip4 / ip6 / exists )
  modifier         = redirect / explanation / unknown-modifier
  unknown-modifier = name "=" macro-string
                     ; where name is not any known modifier

  name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )

This seems to translate to "one or more terms, where each term starts with either {a qualifier followed by a directive (which is a qualifier plus a mechanism)} or a modifier, and multiple terms are separated by spaces".

Tentatively, it's best to think of "modifiers" as a small set of additional directives. It's noted in that they're intended to allow for future expansion

Examples

This config seems to be at least partly functional:

TXT v=spf1 mx mx:mail.vbz.net a a:mail.vbz.net   ~all

This config does not get through to Gmail:

TXT v=spf1 ip4:165.227.176.23 ip6:2604:a880:800:a1::126e:6001 -all

Links

• Wikipedia (Sender Policy Framework)
• RFC 7208 (status)