A mail delivery puzzle

[Derek Atkins]

"Bill Horne" <bill at horne.net> writes:

> I'm still puzzled: this was a new entry, with no previous information. Since
> any previous lookups would have failed, what was there to cache?

DNS caches NXDOMAIN (negative results), too.

The point of caching is to reduce the load on the authoritative
server.  DNS information is assumed to be quazi-static (and when it is
not, you can lower the TTL -- time to live -- to inform the caches how
long they should keep the data).  If negative information were not
cached, then you would boink a server every time you requested the
(same) non-existent domain name.  That would sort of reduce the
effectiveness of having a cache, wouldn't it?

> Bill

-derek

-- 
       Derek Atkins
       Computer and Internet Security Consultant
       derek at ihtfp.com             www.ihtfp.com