how DNS client perform reverse lookup

[Kevin Darcy]

Keith Ng wrote:

>Barry Margolin wrote:
>  
>
>>In article <ct3ioj$1qll$1 at sf1.isc.org>, Keith Ng <ngkeith at triumf.ca> 
>>wrote:
>>
>>
>>    
>>
>>>Hi,
>>>
>>>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.
>>
>>    
>>
>Thanks guys,
>
>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).

- Kevin