Questions about DNAME records

[Chris Buxton]

On Jun 15, 2009, at 2:37 AM, Braebaum, Neil wrote:
> I just have a couple of questions about DNAME records:-
>
> Say I have:-
>
> example.com.	IN	DNAME		example2.com.
>
> If I catered for the names in example2.com. that I want to be able to
> use from example.com., would subdomains and resource records be  
> allowed?
>
> And secondly, if the name enquired on, didn't exist in example2.com.,
> but example2.com. normally forwards elsewhere, would records not found
> in example2.com. be sent to where example2.com. forwards to, or would
> the question end at example2.com.?

It's not clear exactly what you're asking, but let's take a look at  
some examples.

The following is a perfectly valid zone:

$ORIGIN example.com.
$TTL 1d
	SOA	ns1.example2.com. hostmaster.example2.com. 1 1d 5m 1w 1h
	NS	ns1.example2.com.
	NS	ns2.example2.com.
	MX	10  mx1.example2.com.
	A	192.0.2.1
	DNAME	example2.com.

Now consider what happens if you add the following:

www	A	192.0.2.1

The zone is now invalid. Basically, a DNAME record marks the bottom of  
the namespace - the end of the branch. A name that owns a DNAME record  
may not have children except as implied by the DNAME record.

Now, ignoring that invalid "www" record, the zone above has an apex  
(example.com itself) and then essentially infinite ghostly children.  
Any valid query that lands in that domain (i.e. the qname ends in  
example.com) but is not for example.com itself will be answered by a  
synthetic CNAME record, like this:

qname.example.com.	CNAME	qname.example2.com.

If that alias points to a valid name in example2.com, then the query  
is answered positively. If it points to a CNAME record in the  
example2.com domain, then you have a CNAME chain (an alias of an alias  
of a third, referenced name), which then causes resolution to continue  
with the referenced name. (Is this what you meant by "forwarding"?)

Chris Buxton
Professional Services
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