how DNS client perform reverse lookup
kcd at daimlerchrysler.com
Tue Jan 25 18:54:46 UTC 2005
Keith Ng wrote:
>Barry Margolin wrote:
>>In article <ct3ioj$1qll$1 at sf1.isc.org>, Keith Ng <ngkeith at triumf.ca>
>>>Would someone tell me how DNS client perform reverse lookup? Is there
>>>any root servers like forwards lookup for the in-addr.apra domain?
>>They start from the normal root servers. These servers have delegation
>>records for in-addr.arpa, just like they do for com, net, etc. The
>>client servers follow this reverse delegation in exactly the same way
>>that they follow forward delegations. As far as the nameservers are
>>concerned, there's no difference between forward and reverse DNS;
>>everything is just arbitrary labels to the servers and resolvers.
>I did not realize that the root servers for TLD are also responsible for
>the in-addr.arpa domain.
>Now it is making sense to me.
I think you're still not quite getting it. The root servers are servers
for the root zone. TLD servers are servers for a particular TLD zone
(i.e. the apex zone of the domain). in-addr.arpa is neither the root
zone nor the apex zone of a TLD. It's delegated *underneath* the .arpa
TLD. The fact that the same boxes that serve the root zone and the .arpa
TLD also serve the in-addr.arpa zone is only a coincidence, and may
change in the future. There's no hard requirement for a zone cut to
appear at every domain/subdomain boundary, but in this particular case,
there's two levels of delegation from root to in-addr.arpa. A recursive
resolver should, however, be able to find anything in the namespace by
following the delegations down (assuming of course that it can speak to
the authoritative nameservers at each delegation level).
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