Interesting subdomain question.

[Mark_Andrews at iengines.com]

> 
> 
> Hi,
> 
> 	I have run into an interesting dns issue with a server under my
> care.
> 
> 	This server is primary for domain z.com. Under z.com, there exists
> several subdomains for regional offices. such as:
> 
> 	a.z.com
> 	b.z.com
> 	c.z.com
> 
> So far, no problem! 
> 
> But, in the zone file for z.com, subdomains a, b, and c are not officially
> delegated. There are no records in z.com's zone file like this:
> 
> $origin z.com
> a		86400	IN	NS ns1.a.z.com
> 		86400   IN	NS ns2.a.z.com
> ns1.a.z.com	86400 	IN	A	1.2.3.4
> ns2.a.z.com	86400 	IN	A	5.6.7.8
> 
> 
> The primary name server for z.com is a secondary of all of the subdomains
> in question. So the zone files for the subdomains are being pulled in from
> the relevant subdomain name servers. 
> 
> But when I check the zone transfers on the secondaries for z.com, the
> delegation information for a.z.com, b.z.com, and c.z.com is included in
> z.com zone transfer. Nothing on the primary for z.com is telling it about
> the subdomain delegation.
> 
> From my point of view, it is merging the zone info it is receiving, as a
> subdomain secondary, into the parent z.com zone file and then sending that
> out to z.com's secondary name servers.
> 
> It all works, but, I can't figure out why it works. What is telling the
> primary for z.com to merge the z.com subdomains when z.com has not
> offically delegated them?
> 
> There is no reference to the subdomains in the zone file for z.com. At
> all. 
> 
> 
> Regards,
> 	Alan Kirchhoff
> 
> 
	Actually this is a bug but one that is impossible to correct
	in BIND 4/8 due to internal data structures.

	BIND 9 will correct this in that the zone transfer will
	contain the contents of z.com with nothing mixed in from the
	child zones.

	As to why it works.  The NS records from the parent zone are
	thrown away when the server serves both the parent and child
	zones.  Because of this we cannot see the difference between
	a parent zone that had NS records at bottom of zone and one
	that didn't.  Outgoing zone transfers just use the NS RRset
	from the child zone.

	Mark

--
Mark Andrews, Internet Engines Inc. / Internet Software Consortium
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at iengines.com