[dns-operations] Why are cnames called "canonical" names?

Mark Andrews marka at isc.org
Fri Apr 10 03:02:46 UTC 2015

In message <20150410023644.GA24751 at isc.org>, Evan Hunt writes:
> On Thu, Apr 09, 2015 at 09:28:57PM -0400, Mark E. Jeftovic wrote:
> > Thanks but that page doesn't go into why the term "canonical" was
> > adopted, as opposed to "alias" (which makes sense), or even "knickname",
> > "doppelganger" or something like "pointer".
> Read it as: (thing on the right) is the (type) of (thing on the left).
> "example.com. IN A" means " is the address of
> example.com".  Similarly, "www.example.com. IN CNAME xyzzy.example.com."
> means "xyzzy is the canonical name for www" (i.e., the true and correct
> name).
> If CNAME had been called ALIAS, then the line above would have been saying
> "xyzzy is an alias for www", but that's the opposite of what it's saying.
> www is the alias, and xyzzy is the real name of the server hosting it.
> I doubt anybody was anticipating CDNs in those days; everybody probably
> figured a CNAME RR would always point to the *one and only* true and
> correct hostname, instead of having multiple levels of indirection like
> you see if you look at "dig www.paypal.com" nowadays, so it made sense
> to refer to that as canonical. (Add it to the list of ways the DNS might
> have come out differently if we'd had a crystal ball.)

CNAME is not "IS-HOSTED-BY" but it is used like IS-HOSTED-BY for HTTP.

If you http://LHS/foo and http://RHS/foo do not get the same thing
for all possible foo then you are mis-using CNAME.  Yes, there is
a lot of mis-use of CNAME.

> -- 
> Evan Hunt -- each at isc.org
> Internet Systems Consortium, Inc.
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Mark Andrews, ISC
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PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org

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