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

If you plan to do bulk checks you will be soon disappointed by the built-in DNS resolver - it is synchronous. There is a nice cute library called ADNS (http://www.gnu.org/software/adns/) which offers asynchronous DNS queries. It is accompanied by a small command-line tool adnshost which I am using in combination with N-DJBDNS (http://pjp.dgplug.org/ndjbdns/) as a recursive resolver.

The exact arguments are adnshost -a -tns -f

The 4th column in the output will contain the result code:

• 0 if there is at least 1 name server assigned for the domain (most registrars assign a default one until the owner assigns his own)
• 300 or 301 if there are no name servers assigned

Checking 1.2M domain names takes about 4-6 hours but this will vary with network speed/congestion.