Define a domains addresses sole in terms of another
barmar at alum.mit.edu
Tue Jul 10 00:48:00 UTC 2007
In article <f6u9f2$1uva$1 at sf1.isc.org>,
Stephane Bortzmeyer <bortzmeyer at nic.fr> wrote:
> > webmail IN CNAME www
> Forbidden, you cannot have a CNAME going to a CNAME.
Yes you can. The RFC recommends against it for performance reasons, but
doesn't prohibit it. It even mentions that resolvers must follow CNAME
chains, but may have limits on the number of CNAMEs that will be
followed in order to avoid loops.
All the web sites that use DNS-based load balancing like Akamai and
Savvis ITM would be in big trouble if CNAME chains weren't allowed.
$ dig download.microsoft.com
; <<>> DiG 9.3.4 <<>> download.microsoft.com
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 5269
;; flags: qr rd ra; QUERY: 1, ANSWER: 13, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;download.microsoft.com. IN A
;; ANSWER SECTION:
download.microsoft.com. 2566 IN CNAME main.dl.ms.akadns.net.
main.dl.ms.akadns.net. 52 IN CNAME dom.dl.ms.akadns.net.
dom.dl.ms.akadns.net. 52 IN CNAME dl.ms.d4p.net.
dl.ms.d4p.net. 3790 IN CNAME dl.ms.georedirector.akadns.net.
dl.ms.georedirector.akadns.net. 1189 IN CNAME a767.ms.akamai.net.
a767.ms.akamai.net. 8 IN A 18.104.22.168
a767.ms.akamai.net. 8 IN A 22.214.171.124
a767.ms.akamai.net. 8 IN A 126.96.36.199
a767.ms.akamai.net. 8 IN A 188.8.131.52
a767.ms.akamai.net. 8 IN A 184.108.40.206
a767.ms.akamai.net. 8 IN A 220.127.116.11
a767.ms.akamai.net. 8 IN A 18.104.22.168
a767.ms.akamai.net. 8 IN A 22.214.171.124
Barry Margolin, barmar at alum.mit.edu
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