While I think it is a bad idea to limit which domains can use TLS in an <i>ad hoc</i> style, this issue with stuff like example.co.uk was solved in DNS a long time ago.<p>One of the main features of DNS is the ability to <i>delegate authority</i> for a zone. You use a NS record on a valid name that where <i>you</i> have authority to indicate that some other server is authoritative for a given subset of your zone.<p>If you bought example.com, you can create DNS records like<p><pre><code>    subdomin.example.com. 172800 IN NS ns1.example.com
</code></pre>
to indicate that ns1.example.com is the authority for the entire subdomain.example.com zone. This includes all <i>further</i> nesting of <i>.subdomain.example.com.<p>Instead of </i>guessing* who is responsible for a zone, you should simply ask the nearest upstream zone authority.<p>Also, relying on lists is almost always the wrong solution - if your security requires you to successfully enumerate something, you are doing it wrong. ( <a href="http:&#x2F;&#x2F;www.ranum.com&#x2F;security&#x2F;computer_security&#x2F;editorials&#x2F;dumb&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.ranum.com&#x2F;security&#x2F;computer_security&#x2F;editorials&#x2F;d...</a> )