Using dig for checking domain registration

[Scott Haneda]

I have been using the below command to determine if a domain is  
registered.  I use this for an internal audit of what clients have  
come and gone, and what DNS records I need to clean up.

dig example.com NS +trace -4 @4.2.2.1 | grep -i ns1.example.com

I run it also with ns2.example.com in the grep to make certain my  
primary and secondary are listed.

My current trouble is that I am getting intermittent failures if the  
domain is not a tld of .com, .net, .org.

$dig customtruckgraphics.us NS +trace -4 @4.2.2.1 +short
NS F.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS M.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS G.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS E.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS D.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS L.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS C.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS K.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS I.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS J.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS A.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS B.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
NS H.ROOT-SERVERS.NET. from server 4.2.2.1 in 19 ms.
dig: couldn't get address for 'J.GTLD.BIZ': not found

Can someone explain to me what is happening here, and also, provide a  
suggestion on how to best test for the conditions I am after?

-- 
Scott * If you contact me off list replace talklists@ with scott@ *