strange resolver behavour

[Chuck Swiger]

On Oct 12, 2010, at 8:30 AM, Tom Evans wrote:
>> Taking the '5.3. Master file example' in RFC1035, what is the A response
>> for 'ISI.EDU.' where the domain itself has no specific A RR?  Would it
>> be that of VENERA.ISI.EDU, or that of the first A listed, ie A.ISI.EDU?
> 
> That domain has an MX record, so it wouldn't do either. When I do a
> dig isi.edu, I just get a single A record, so I would assume an SMTP
> server would attempt to deliver mail there.

No, it would use the published MX records for the domain, and would first try delivering to vapor.isi.edu as the lowest precedence MX host listed:

% dig -t mx isi.edu
; <<>> DiG 9.6.0-APPLE-P2 <<>> -t mx isi.edu
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 9344
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;isi.edu.			IN	MX

;; ANSWER SECTION:
isi.edu.		706	IN	MX	30 boreas.isi.edu.
isi.edu.		706	IN	MX	50 yin.isi.edu.
isi.edu.		706	IN	MX	10 vapor.isi.edu.
isi.edu.		706	IN	MX	20 nitro.isi.edu.

Only if no MX records were found, would an MTA fall back to using the A record here:

;; QUESTION SECTION:
;isi.edu.			IN	A

;; ANSWER SECTION:
isi.edu.		86123	IN	A	128.9.176.20

Regards,
-- 
-Chuck