From: Thomas Bridge (tbridge at domain vianetworks.ie)
Date: Thu 11 Apr 2002 - 21:06:18 IST
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One situation where you have to use CNAMES.
Suppose you get a block of say, 128 IP addresses from an ISP (for the sake
of argument, I'll use 192.168.71.0/25).
ISP is probably going to want to assign the rest of the Class C (ie
192.168.74.128/25) to another customer.
Who controls the reverse DNS? Well, currently it's the ISP. They aren't
going to delegate 71.168.192.in-addr.arpa to either customer, as that means
that the customer who doesn't get the delegation has to deal with the other
customer, rather than the ISP. This is clearly ludicrous - after all, would
you want to talk to the likes of Paul to get your reverse DNS sorted out :)
The current hack is documented in RFC 2317. The way around is as follows
(I'm doing this from memory, check the RFC for the recommended practice).
Subdeletgate the zone 0/188.8.131.52.in-addr.arpa to the first customer.
Add records for all the IP address as follows:
n IN CNAME n.0/184.108.40.206.inaddr.arpa.
Customer then adds the right hand side to the zone on his nameservers as
n IN PTR server.customer.ie.
There is currently no way to delegated reverse DNS for /25 and smaller
networks without using CNAMES.
Therefore, CNAMEs are required :)
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