"no data known" vrs "host not found"

[Barry Margolin]

In article <a7g931$kmf at pub3.rc.vix.com>, admjcd  <admjcd at VOLPE.DOT.GOV> wrote:
>When ever we send mail we get back these NDRs when ever the domain is not
>correct. Sometimes we get them when the domain is correct. Both types are
>"host unknown".  One says  "no data known" and the others say "host not
>found" .  What is the difference between these two and why should we get
>the "no data known" intermittenly message when the domain does exist?

"Host not found" means the name doesn't exist.  "No data known" means the
name exists, but it doesn't have any records of the requested type; in the
case of mail, it requires either an MX record or an A record.

Something very weird is going on with the army.mil entries that result in
"no data known" messages.  They have MX records, but if you perform an ANY
query (as sendmail does), the MX record appears in the Authority section
instead of the Answer section.  And its TTL decreases on each query,
implying that the MX record is in the cache, not the authoritative data for
the zone.  You should probably let the Army hostmaster know that there's a
problem with their server.

% dig hua.army.mil any @ns01.army.mil

; <<>> DiG 8.3 <<>> hua.army.mil any @ns01.army.mil 
; (1 server found)
;; res options: init recurs defnam dnsrch
;; got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 6, ADDITIONAL: 3
;; QUERY SECTION:
;;	hua.army.mil, type = ANY, class = IN

;; ANSWER SECTION:
hua.army.mil.		1H IN SOA	ns01.army.mil. DOMAIN-REQUEST.AIMS7.army.mil. (
					10000		; serial
					10M		; refresh
					1S		; retry
					10M		; expiry
					1H )		; minimum

hua.army.mil.		1H IN NS	ns01.army.mil.

;; AUTHORITY SECTION:
hua.army.mil.		1H IN NS	ns03.army.mil.
hua.army.mil.		1H IN NS	ns02.army.mil.
hua.army.mil.		6h46m16s IN MX	10 emh1.hua.army.mil.
hua.army.mil.		1H IN NS	ns01.army.mil.
hua.army.mil.		1H IN NS	ns03.army.mil.
hua.army.mil.		1H IN NS	ns02.army.mil.

;; ADDITIONAL SECTION:
ns01.army.mil.		40m39s IN A	140.153.43.44
ns03.army.mil.		40m39s IN A	130.114.200.6
ns02.army.mil.		40m39s IN A	192.82.113.7

;; Total query time: 183 msec
;; FROM: tools.genuity.com to SERVER: ns01.army.mil  140.153.43.44
;; WHEN: Fri Mar 22 17:20:36 2002
;; MSG SIZE  sent: 30  rcvd: 263

-- 
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.