From Computer Tyme Support Wiki

DNS Lookup to Separate the domain part of a hostname

This DNS lookup helps you find the main domain part of a hostname. Sometimes it is called two level TLDs and three level TLDs. Lookups are accomplished through DNS calls as follows:

dig perkel.com.rb.junkemailfilter.com   - returns 127.0.0.1
dig perkel.co.uk.rb.junkemailfilter.com - returns 127.0.0.2

This is a service of Junk Email Filter dot com. One of many technologies used in advanced email filtering.

Exim Configuration

If you are running Exim you can use this as follows:

1. This example calls rb_resolve as an ACL subroutine.

warn	set acl_c_rb_query = $sender_host_name

acl = rb_resolve set acl_c_sender_host_domain = $acl_c_rb_result

rb_resolve:

# Separates the domain part of a hostname - ftp.perkel.com returns perkel.com 
# DNS lookup returns 127.0.0.1 for single level domains
# DNS lookup returns 127.0.0.2 for two level domains
# DNS lookup returns 127.0.0.3 for three level domains

warn	set acl_c_rb_result =

set acl_c_rb_query = ${lc:$acl_c_rb_query}

accept	condition = ${if eq{$acl_c_rb_query}{}}

accept	!dnslists = rb.junkemailfilter.com/$acl_c_rb_query

warn	condition = ${if eq{$dnslist_value}{127.0.0.1}}

set acl_c_rb_result = ${sg{$acl_c_rb_query}{\N^(.*\.)?(.*\..*)$\N}{\$2}}

warn	condition = ${if eq{$dnslist_value}{127.0.0.2}}

set acl_c_rb_result = ${sg{$acl_c_rb_query}{\N^(.*\.)?(.*\..*\..*)$\N}{\$2}}

warn	condition = ${if eq{$dnslist_value}{127.0.0.3}}

set acl_c_rb_result = ${sg{$acl_c_rb_query}{\N^(.*\.)?(.*\..*\..*\..*)$\N}{\$2}}

accept