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

[Daniel Kalchev]

> On 28.04.2017 г., at 9:23, Robert Martin-Legene <rlegene at gmail.com> wrote:
> 
> Just to make matters worse, you can't expect a domain name which do exist in DNS to not be available for registration.
> 
> Most registries publish their zones triggered by a cron job. Some maybe even less frequent than daily. Even if there is an hourly update it still leaves a window of opportunity where someone else could have time to register the domain name before you.
> 
> Some registries even only spool a new version when it is "really needed". It is a manual process and deletions may not qualify as important enough to warrant a few extra key presses.
> 

It is even worse, because DNS is (heavily) cached. You will have to make sure to ask the authoritative servers, and better all of them then filter out those with lower serials.

Daniel