All,

In the document draft-howard-isp-ip6rdns-05, a proposal to manage the 
ip6-addr.arpa zone is to generate a PTR record on demand, and cache or 
prepopulate the forward (AAAA) entry for the duration of the time-to-live of 
the PTR.

It is possible to generate PTR responses to reverse lookups on the fly without 
need to keep the forward entry into cache memory.


Kazunori Fujiwara has implemented a prototype "On the Fly" DNS server which 
does not require caching.



  http://member.wide.ad.jp/~fujiwara/v6rev.html


The algorithm is simple. The DNS server builds the reverse host name as the 
concatenation of the IPv6 address (using a 32-character hexadecimal 
representation without :) with a fixed domain name suffix.
The generated hostname can be easily resolved by forward lookup since the 
generated hostname contains raw IPv6 address. The response is in the question.

Here below an example of reverse lookup matching a forward lookup.

The reverse lookup on IPv6 "2a01:c012:3456:789a:bcde:f012:3456:789a" gives the 
following:

# dig -x 2a01:c012:3456:789a:bcde:f012:3456:789a +short
2a01c0123456789abcdef0123456789a.ip6.abo.orange.fr.

In the forward lookup response, we retrieve the original IPv6 address:

# dig 2a01c0123456789abcdef0123456789a.ip6.abo.orange.fr -t aaaa +short
2a01:c012:3456:789a:bcde:f012:3456:789a


Philippe Lemordant

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