Parent is a CNAME

Sam Wilson Sam.Wilson at ed.ac.uk
Wed Dec 2 17:52:02 UTC 2009


In article <mailman.1165.1259775639.14796.bind-users at lists.isc.org>,
 Joseph S D Yao <jsdy at tux.org> wrote:

> On Wed, Dec 02, 2009 at 12:47:08PM +0000, Sam Wilson wrote:
> > In article <mailman.1153.1259725836.14796.bind-users at lists.isc.org>,
> >  Joseph S D Yao <jsdy at tux.org> wrote:
> [incorrectly]
> > > No.
> ...
> > Not true.  CNAME chains - CNAMEs pointing to other CNAMEs - are 
> > inefficient and discouraged but the DNS spec is built to ensure that 
> > they work.  Check out www.google.com sometime (or www.google.co.uk) and 
> > wonder at how many people would be annoyed if they didn't.
> 
> 
> CNAME chains have nothing to do with this.  THIS is perfectly legal:
> 
> a	CNAME	b
> b	CNAME	c
> c	CNAME	d
> d	CNAME	extra-ordinary
> 
> although, as mentioned, inefficient.

My bad - I read your initial statement as banning names with CNAME 
records from the RHS of other RRs, not from the LHS, and I was offering 
a counterexample.

> THIS is not legal:
> 
> a	CNAME	b
> a	CNAME	c
> a	A	1.1.1.1

To be pedantic, the first alone is legal, but once that exists neither 
of the second nor third is legal.

> ...
> > > Why not do this?
> > > 
> > > subdomain.b     A     7.8.9.10
> > > subdomain.b     NS    ns1.subdomain.b
> > > ns1.subdomain.b    A     7.9.11.13
> > 
> > If b was itself delegated the CNAME would be problematical again.
> ...
> 
> 
> And if all the name servers crashed, then the domain would be unserved.
> Why introduce unnecessary hypotheticals?  ;-)

Because you introduced a delegation for a and I wanted to head off the 
idea that you could delegate b and then point a to it as a CNAME.  You'd 
need to use DNAME in that situation.

> And, as pointed out in another post, the CNAME does not appear to be
> problematic in this case, even were it to exist.

Indeed.

Sam



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