DNS Connection Question

[trlists at clayst.com (trlists at clayst.com)]

Back with another question ... really just networking, not Linux, hope 
that's OK here ...

I am looking at a domain that uses servers ns1.host1.net and 
ns2.host2.net as its authoritative nameservers.  The domain behaves 
fine, web site can be accessed via HTTP, etc.

However I cannot open a connection to the same nameservers on port 53.  
If I try it times out.  I've tried from several different platforms in 
different locations on the net.

I'm using the ability to open that connection as a vaguely reasonable 
indication that the nameserver name is valid, and the server exists.  
This is to prevent mistaken entries in a form where I ask for a 
nameserver name.  In this case my code thinks the server is not valid, 
but apparently it is.  (My code works for other nameservers, and gives 
the expected reports as to whether they exist or not.)

What might I be missing here?  What could be different about my 
connection from the connection opened by my ISP's DNS to get the IP 
when requested?  I'm opening the connection with a PHP fsockopen() call 
but I get the same results attempting to telnet to port 53 on the named 
machines -- it just times out.

The only thing I can think of is that the ISP's DNS has the info cached 
and meanwhile the real nameserver is down.  If that's the case, how 
long is it likely to take before the domain stops behaving because its 
authoritative nameserver is down?

I am not a DNS expert and if any of the above reflects a 
misunderstanding of how the system works, feel free to correct me -- 
and thanks for any input!

--
Tom