Not getting anticipated answer

Kevin Darcy kcd at daimlerchrysler.com
Fri Jun 14 23:07:57 UTC 2002


no mail wrote:

> Trying to get the MX records for a zone update.cio.com, I instead seem to
> get the SOA record for cio.com. I can just go there and look, and find some
> MX records, but the mail server is not that bright. Is it configured wrong
> or is the DNS for this zone (it's not my zone so I can't fix it) wrong?
> This stuff is all running on Windows 2000.
>
> Here's what I get from a dig of update.cio.com:
>
> Looking up [update.cio.com]
> ; <<>> DiG 2.2 <<>>
>
> res_mkquery(0, update.cio.com, 1, 15)
>
> Querying server (# 1) address = 10.11.1.253
>
> got answer:
>
> HEADER:
>
> opcode = QUERY, id = 2, rcode = NOERROR
>
> header flags: qr ra
>
> Ques: 1, Ans: 0, Auth: 3, Addit: 0
>
> QUESTIONS:
>
> update.cio.com, type = MX, class = IN
>
> NAME SERVERS:
>
> cio.com
>
> type = NS, class = IN, ttl = 20 hours 32 mins 1 sec, dlen = 14
>
> nameserver = ns1.wcom.net
>
> cio.com
>
> type = NS, class = IN, ttl = 20 hours 32 mins 1 sec, dlen = 6
>
> nameserver = ns2.wcom.net
>
> cio.com
>
> type = NS, class = IN, ttl = 20 hours 32 mins 1 sec, dlen = 6
>
> nameserver = ns3.wcom.net
>
> ;; Total query time: 0 sec
>
> ;; FROM: jeff to SERVER: sfsdhcp.sfs.accuride.com 10.11.1.253
>
> ;; WHEN: Fri Jun 14 15:04:07 2002
>
> ;; MSG SIZE sent: 32 rcvd: 94
>
> [End Query]

update.cio.com owns no MX records, but it does own an A record. So when you do
an MX query of update.cio.com, you get what's known as a "NODATA" response,
i.e. the name exists, but it owns no records of the type you asked for. NODATA
responses are distinct from NXDOMAIN responses, which denote that the queried
name doesn't exist at all. The SOA record is used for "negative caching", i.e.
to keep track of how long the nameserver should remember the NODATA response
to the query. See RFC 2308 for more details.


- Kevin




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