"no data known" vrs "host not found"

Barry Margolin barmar at genuity.net
Tue Mar 26 00:03:50 UTC 2002


In article <a7o8n2$mdh at pub3.rc.vix.com>, admjcd  <admjcd at VOLPE.DOT.GOV> wrote:
>So your saying the mail server does some weird query to the dns server
>and the query fails. 

The query isn't particularly weird, but the army.mil servers seem to
respond incorrectly to it.

>		      And this combined with something called "negative
>cacheing" takes place and the query sticks. 

Right.  That's the whole point of caching -- you remember an answer that
you previously received, so you don't have to fetch it again.  "Negative"
caching just means remembering that a query failed (so you don't bother
trying it again); normal caching refers to remembering the value returned
by a successful query.

>Or does "negative cacheing" in itself explain why it keeps failing the
>way it does? 

It should clear up after an hour, as that's the negative cache TTL on the
hua.army.mil domain (but the next time your mail server tries to send mail
there it will perform the same query and cache the result).

>Does this also explain why it does not fail on other servers? Is negative
>cacheing not enabled on them?

No, my guess is that their mail servers aren't making the query that causes
the problem in the first place.  All modern BIND servers perform negative
caching, and I don't think it's even a configuration option.

-- 
Barry Margolin, barmar at genuity.net
Genuity, Woburn, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
Please DON'T copy followups to me -- I'll assume it wasn't posted to the group.


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