[dns-operations] Curious use of cname
marka at isc.org
Mon Aug 11 01:48:33 UTC 2014
In message <53E6B826.3000204 at mail2000.us>, Yonghua Peng writes:
> > mail.china.com. IN MX 10 mx-mail-china-com.icoremail.net.
> Even if they have setup this, but the DNS clients never have the chance
> to get the correct MX but always get a CNAME answer. Am I right?
It would be a alternative to a CNAME.
You you are publishing email addresses as user at domain then there
really should be a MX record at domain. There really doesn't need
to be a double level of indirection.
Today you get:
;; ANSWER SECTION:
mail.china.com. 500 IN CNAME mail.china.com.cachecn.com.
mail.china.com.cachecn.com. 201 IN MX 10 mx-mail-china-com.icoremail.net.
A better cleaner setup would be:
;; ANSWER SECTION:
mail.china.com. 500 IN 10 mx-mail-china-com.icoremail.net.
I suspect that almost all of the presure to change the CNAME behaviour
for MX records came about because people abused CNAME for HTTP. It
has never been hard to add new record types to the DNS. People
just thought it was hard so they never started the process. As a
result SMTP behaviour was changed rather than the simple thing of
adding a record for HTTP to the DNS.
You end up with stupid sets of records like these because people
wern't willing to do the little bit of effort to actually get a
> $ dig mail.china.com mx @ns-bgp.china.com
> ; <<>> DiG 9.8.1-P1 <<>> mail.china.com mx @ns-bgp.china.com
> ;; global options: +cmd
> ;; Got answer:
> ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 23565
> ;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
> ;; WARNING: recursion requested but not available
> ;; QUESTION SECTION:
> ;mail.china.com. IN MX
> ;; ANSWER SECTION:
> mail.china.com. 500 IN CNAME mail.china.com.cachecn.com.
> ;; Query time: 75 msec
> ;; SERVER: 22.214.171.124#53(126.96.36.199)
> ;; WHEN: Mon Aug 11 09:24:44 2014
> ;; MSG SIZE rcvd: 69
> As you see, I queried for MX but got a CNAME. Thanks.
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: marka at isc.org
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