Question about RFC-2317

[Barry Margolin]

In article <enk8l9$2nn6$1 at sf1.isc.org>,
 "Clenna Lumina" <savagebeaste at yahoo.com> wrote:

> Mark Andrews wrote:
> > Because the clients look up 1.2.0.192.IN-ADDR.ARPA not
> > 1.0/25.2.0.192.IN-ADDR.ARPA when that want to find the name
> > of the machine at 192.0.2.1.  Without the CNAME they would
> > get a NXDOMAIN response as there are no records at
> > 1.2.0.192.IN-ADDR.ARPA.
> 
> Then how does the request make it's way to "ns.A.domain", for example ? 

By following the alias in the CNAME record.

The point is that the original client doesn't know that the network is 
subnetted.  It just takes the address, reverses it, appends 
..in-addr.arpa and tries to look up the PTR record.  So it looks for 
1.2.0.192.in-addr.arpa.  "Normal" DNS hierarchy traversal gets it to the 
server that hosts 2.0.192.in-addr.arpa, which tells it that this is an 
alias for 1.0/25.2.0.192.in-addr.arpa.  This is what tells it that it 
should look in the 0/25 subdomain, rather than looking for the PTR 
record directly in this domain.

> If what you say is true, then wouldn't the "0/25" in "0/25            NS 
> ns.A.domain." be pointless. Also, how would it any different that "1 
> CNAME   1.0/25.2.0.192.in-addr.arpa.", which uses the same syntax?
> 
> The NS record is what opens the door to the PTR records on "ns.A.domain" 
> (in that example), doesn't it? I mean how else does it *GET* there if 
> not for that NS record? The CNAME doesn't provide such a route, it's 
> just an alias, is it not?

It's the other way around.  The CNAME record tells it to follow the NS 
record to find these PTR records.  Without the CNAME, how would the 
client know about the 0/25 subdomain?

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