List Google's outbound SMTP server address blocks

google-spf

#!/bin/sh

set -eu

listblocks()
{
    local i

    for i in $(dig +short "${1}" txt)
    do
	r="$(echo "${i}" | tr -d ' "')"
	case "${r}" in
	v=spf1|[-?~]all|'')
	    ;;
	include:*)
	    listblocks "${r#include:}"
	    ;;
	ip[46]:*)
	    printf "%s\n" "${r#ip[46]:}"
	    ;;
	a:*)
	    dig +short "${r#a:}" a
	    dig +short "${r#a:}" aaaa
	    ;;
	#mx:*)
	#   ;;
	*)
	    printf "# %s\n" "${r}"
	    ;;
	esac
    done
}

if [ ${#} -lt 1 ]
then
    set -- _spf.google.com
fi

for block
do
    echo "# ${block}"
    listblocks "${block}"
done