"no data known" vrs "host not found"

[admjcd]

So your saying the mail server does some weird query to the dns server and the query fails. And this combined with something called "negative cacheing" takes place and the query sticks. 
Or does "negative cacheing" in itself explain why it keeps failing the way it does? 
Does this also explain why it does not fail on other servers? Is negative cacheing not enabled on them?
 Sorry if I sound like a begginer, but I hope I am at least asking the correct questions! 

-----Original Message-----
From: Barry Margolin [mailto:barmar at genuity.net] 
Sent: Monday, March 25, 2002 5:10 PM
To: comp-protocols-dns-bind at isc.org
Subject: Re: "no data known" vrs "host not found"


In article <a7o6oo$m35 at pub3.rc.vix.com>, admjcd  <admjcd at VOLPE.DOT.GOV> wrote:
>Yes, but it fails when I do an nslookup on my DNS server when it fails 
>with the mail.  So if we are getting the NDR I do an nslookup with the 
>type set to MX and get the "no data found" "authoritative answer" 
>response.  But when I do the same lookup at other dns servers on the 
>internet and the dns servers for the domain in question I get a correct 
>answer back while mine is failing.

It sounds like negative caching.  The mailer's query caused your server to cache the fact that the MX record doesn't exist, and your nslookup is then seeing this.

Since the other servers don't have the negative cache entry (because none of their client machines had recently performed an ANY query for the domain), when you query them they will forward the query to the authoritative server, and return the MX record properly.

-- 
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.