[dns-operations] check if a domain has been registered via DNS

[Viktor Dukhovni]

> On Apr 27, 2017, at 1:33 PM, Doug Barton <dougb at dougbarton.us> wrote:
> 
> What would work for your situation is to test for the presence of NS records at the registry level, and if they are absent to fall back to whois to confirm.

Some subdomains of public suffix parent domains are CNAMEs, and only
the target of the CNAME has NS records.

;; QUESTION SECTION:
;vwtelecom.eu.                  IN      NS

;; ANSWER SECTION:
vwtelecom.eu.           86400   IN      CNAME   www.vwtelecom.com.
www.vwtelecom.com.      86400   IN      CNAME   vwtelecom.com.
vwtelecom.com.          86400   IN      NS      ns1.openprovider.nl.
vwtelecom.com.          86400   IN      NS      ns3.openprovider.eu.
vwtelecom.com.          86400   IN      NS      ns2.openprovider.be.

-- 
	Viktor.