Define a domains addresses sole in terms of another

[Barry Margolin]

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  204.1.5.153
a767.ms.akamai.net.  8  IN A  204.1.5.155
a767.ms.akamai.net.  8  IN A  204.1.5.170
a767.ms.akamai.net.  8  IN A  204.1.5.161
a767.ms.akamai.net.  8  IN A  204.1.5.152
a767.ms.akamai.net.  8  IN A  204.1.5.179
a767.ms.akamai.net.  8  IN A  204.1.5.185
a767.ms.akamai.net.  8  IN A  204.1.5.160

-- 
Barry Margolin, barmar at alum.mit.edu
Arlington, MA
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