nslookup can't but browser can !

[Stanley Liu]

Prashant Ranade wrote:

> now on this nt box in its DNS tab in network protocol we have a IP address of
> our ISPs nameserver. if I run nslookup on this box and point it to itself by
> saying "server ntbox" (assuming ntbox the name of this nt machine which is
> running DNS) and try to resolve www.yahoo.com I get an answer which I know is
> from its cache.

That's correct (ie. your DNS gets the information from its cache about
www.yahoo.com when you are using nslookup)

> so I then pointed the nslookup to the ISPs nameserver for the same query
> expecting that it will give an non-authoritative answer but it returned
> saying "unknown host"

When you are using nslookup, effectively it is looking into the dns's cache.
Therefore, when it returned "unknown host", it means www.yahoo.com does not exist
in your ISP's cache (or already expired?)

> if the ISPs dns dosn't know where www.yahoo.com is then where is my dns
> getting the resolution from(other than cache)...

Your DNS does not get www.yahoo.com from your ISP's DNS.  It grabs www.yahoo.com
from the root down recursively.  When someone in your company try to resolve
www.yahoo.com and your DNS does not have it in its cache, it will go to root (.)
to find the DNS for "com."  It will then go to the DNS which hosts "com." to find
"yahoo.com." and so on until "www.yahoo.com." is resolved.

> this confused me totally... all my theries about DNS and browsing have beed
> proved wrong...

Should get Cricket's book on DNS & BIND.  It is a good book.  It is one of the
technical books that I owned that I read more than once and bought a new edition
when it came out;-)  Disclaimer: I have no relation to Cricket and no financial
interest in O'reilly the publisher.  Just a satified customer.

--
Stanley Liu
stanley.liu at tmca.com.au