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Detection of registered domains by reg-dom libs

Registered domains are top-level-domains (TLDs) and subdomains that have their own registrar, making them 'effective TLDs' that cannot be used arbitrarily, so while you can register any .com domain, the same is not true of .uk and many other domains. They are useful to know since there are a limited number of them (about 6,000), and that may be used to partially validate domain names in various circumstances without incurring the overhead of DNS lookups. For example, the co.uk domain has its own registrar, so the registered domain of abc.xyz.co.uk is co.uk, and the name is probably valid (a DNS lookup will confirm it). In the other direction, there is no such TLD as zz, so a domain name of abc.xyz.zz will be spotted as invalid without needing a DNS lookup. The list of known registered domains is maintained by Mozilla and is updated approximately monthly. You need a database of names to do this because otherwise it's not possible to tell which part of a domain name is registered.

The reg-dom libs are available in PHP, C and Perl. This library includes procedural PHP code. There are similar implementations available for python and ruby.

Example code:

$ingoingDomain = 'abc.xyz.example.com';
require 'effectiveTLDs.inc.php';
$tldTree = require 'regDomain.inc.php';
$registeredDomain = getRegisteredDomain($ingoingDomain, $tldTree);

Return values:

  1. NULL if ingoingDomain is a TLD (or effective TLD)
  2. The registered domain name if TLD is known
  3. just <domain>.<tld> if <tld> is unknown This case was added to support new TLDs in outdated reg-dom libs by a certain likelihood. This fallback method is implemented in the last conversion step and can be simply commented out.

You can regenerate the effective TLD tree structure with the script generateEffectiveTLDs.php with the following parameters:

./generateEffectiveTLDs.php > effectiveTLDs.inc.php
./generateEffectiveTLDs.php perl > effectiveTLDs.pm
./generateEffectiveTLDs.php c    > tld-canon.h

The code compiles to a large array; loading it is quite an expensive operation, so it's best used for batch processing large lists of domains.

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Reg-dom was written by Florian Sager, 2009-02-05, sager@agitos.de

Original code is available at http://www.dkim-reputation.org/regdom-lib-downloads/

This version cleaned up for E_STRICT and PHP 5.4 by Marcus Bointon http://github.com/Synchro