spf -- expand and report on a given domain's SPF policy

The syntax for the Sender Policy Framework (SPF) definition via a DNS TXT record is complex and expansion and inspection of the final ruleset requires possibly multiple DNS lookups.

The spf utility can be used to easily expand a domain's policy and report on its definition from the command-line.

For a longer discussion of SPF, please see this blog post: https://www.netmeister.org/blog/spf.html

Sample output of the tool (trimmed for brevity):

$ spf github.com
github.com:
  policy:
    ip4:192.30.252.0/22 include:_netblocks.google.com include:_netblocks2.google.com include:_netblocks3.google.com include:spf.protection.outlook.com include:mail.zendesk.com include:_spf.salesforce.com include:servers.mcsv.net ip4:166.78.69.169 ip4:166.78.69.170 ip4:166.78.71.131 ip4:167.89.101.2 ip4:167.89.101.192/28 ip4:192.254.112.60 ip4:192.254.112.98/31 ip4:192.254.113.10 ip4:192.254.113.101 ip4:192.254.114.176 ip4:62.253.227.114 ~all

  valid

  pass:
    include (7 domains):
      _netblocks.google.com
      _netblocks2.google.com
      _netblocks3.google.com
      _spf.salesforce.com
      mail.zendesk.com
[...]

    ip4 (12 CIDRs / 1051 IPs):
      166.78.69.169/32
      166.78.69.170/32
      166.78.71.131/32
[...]

    _netblocks.google.com:
      policy:
        ip4:35.190.247.0/24 ip4:64.233.160.0/19 ip4:66.102.0.0/20 ip4:66.249.80.0/20 ip4:72.14.192.0/18 ip4:74.125.0.0/16 ip4:108.177.8.0/21 ip4:173.194.0.0/16 ip4:209.85.128.0/17 ip4:216.58.192.0/19 ip4:216.239.32.0/19 ~all

      valid

      pass:
        ip4 (11 CIDRs / 215296 IPs):
          108.177.8.0/21
          173.194.0.0/16
[...]
SPF record for domain 'github.com': valid

Total counts:
  Total # of DNS lookups            : 8

  pass:
    Total # of 'exists' directives  : 1
    Total # of 'include' directives : 7
    Total # of ip4 directives       : 45
    Total # of ip4 addresses        : 870,171
    Total # of ip6 directives       : 11
    Total # of ip6 addresses        : 29,712,752,120,897,178,112,958,136,320

All others: softfail

Requirements

spf(1) is written in Perl, and you will need the following modules installed:

• Net::DNS
• Net::Netmask
• Math::BigInt

Optional modules:

• Data::Dumper
• JSON

You may be able to install these dependencies via:

• NetBSD and other systems using pkgsrc: sudo pkg_add p5-JSON p5-Net-DNS p5-Net-Netmask p5-Math-BigInt or sudo pkgin install p5-JSON p5-Net-DNS p5-Net-Netmask p5-Math-BigInt
• Debian, Ubuntu, and related systems: sudo apt install libjson-perl libnet-dns-perl libnet-netmask-perl libmath-bigint-perl
• FreeBSD: sudo pkg install p5-JSON p5-Net-DNS p5-Net-Netmask p5-Math-BigInt

You can also find a Dockerfile in the 'misc' directory, if that's your jam.

Installation

To install the command and manual page somewhere convenient, run make install; the Makefile defaults to '/usr/local' but you can change the PREFIX:

$ make PREFIX=~ install

Documentation

Please see the manual page for all details:

NAME
     spf - report on the given domain's use of SPF

SYNOPSIS
     spf [-Vbhjv] [-f format] [-p policy] [-r address] domain

DESCRIPTION
     The spf tool allows you to inspect the Sender Policy Framework DNS records
     for the given domain.

OPTIONS
     The following options are supported by spf:

     -V		 Print version information and exit.

     -f format	 Output format (json, perl, text; default: text).

     -h		 Display help and exit.

     -p policy	 Expand and report on the given policy.	 Note: policy needs to
		 be quoted; see EXAMPLES.

     -r address	 Explicitly query this resolver.

     -v		 Be verbose.  Can be specified multiple times.

DETAILS
     The Sender Policy Framework (SPF) defined in RFC7208 specifies the format
     for the SPF DNS entries a domain may choose to apply.  These entries can
     range from the straight forward to the complex, harboring certain surprises
     or unintenionally obscuring important information, such as when one domain
     includes the SPF records of another domain.

     spf can be used to report on the comprehensive ruleset derived from the DNS
     record.  It does that by counting CIDRs, resolving e.g., MX records, and
     recursively looking up SPF records of any included domains.

EXAMPLES
     The following examples illustrate common usage of this tool.

     To report on the SPF records for the domain netmeister.org:

	   spf netmeister.org

     To query Quad9's public resolver for the same SPF records and report the
     results in json format:

	   spf -r 2620:fe::fe -f json netmeister.org

     To expand an arbitrary policy from the command-line:

	   spf -p "v=spf1 a:example.com include:example.net -all"

EXIT STATUS
     The spf utility exits 0 on success, and >0 if an error occurs.

SEE ALSO
     dig(1)

HISTORY
     spf was originally written by Jan Schaumann <jschauma@netmeister.org> in
     July 2022.

BUGS
     Please file bugs and feature requests by emailing the author.