Checking domain name availability with DNS records

Question

How accurately can a domain name\'s availibility be determined by checking for the existence of NS or SOA (Start of Authority)

Answer 1

In practice, the DNS lookup works 99% of the time. If you writing something like Ajaxwhois, I'd suggest going this route. If you have a million names to check, I would do a first pass with a DNS lookup, and then maybe do a second pass on the ones which you may consider purchasing.