Reverse DDNS on classless zones

[Simon Hobson]

atif yameen wrote:

>and the following is my reverse zone in named.conf
>
>zone "128-25.25.119.129.in-addr.arpa"
>{
>     type master;
>     file "4981050.65ED25AA06C49D877204B11DDCB30C43.db";
>     allow-transfer { x.x.x.x; };
>     allow-update {  <IP-Address-of-DHCP-Server; };
>     also-notify { x.x.x.x; };
>};
>
>when a client in the said network obtains an ip address, the dhcp 
>server registers its reverse name as follows :
>
>Jul 17 20:25:37 new dhcpd: Added reverse map from 
>250.25.119.129.128-25.25.119.129.in-addr.arpa to a.b.com
>where as it should be : 250.128-25.25.119.129.in-addr.arpa
>
>zone file shows the following: (creates a subzone 129 and then a 
>dotted name from the rest of the three octets)
>
>$ORIGIN 129.128-25.25.119.129.in-addr.arpa.
>$TTL 302400     ; 3 days 12 hours
>250.25.119              PTR     a.b.com.
>
>Any Ideas why this is happening?

Yes. The DNS system does not support splitting of zones on anything 
but "dots" - and in particular it does not support what you are 
trying to do directly. "128-25" does not mean anything at all to it - 
it's just a string of characters.

When registering an entry, all that happens is the four octets are 
reversed, prepended to the base zone name (default "in-addr.arpa"), 
and the result is used.

So what you are seeing is correct behaviour, but there's a step 
missing. To make lookups work, you need 128 CNAME entries in the real 
25.119.129.in-addr.arpa zone of the form :

128  CNAME 128.25.119.129.128-25.25.119.129.in-addr.arpa.
129  CNAME 129.25.119.129.128-25.25.119.129.in-addr.arpa.

In BIND you can use the GENERATE statement to create these in one line.

You may also realise that suddenly you don't need to use 
128-25.25.119.129.in-addr.arpa as the zone, it can literally be any 
reachable zone. So you could use reverse.b.com, and then you'd have :

In 25.119.129.in-addr.arpa
128  CNAME 128.25.119.129.reverse.b.com.
...
And your client above would get an entry in the 25.119.129.reverse.b.com. zone
250 PTR a.b.com

Yes, classless DNS is a right pain !

FYI, in IPv6 the addresses use hex, and zones are split on 4 bit 
boundaries. Combine the larger assignments with this and there should 
never be any need for this sort of fudge.

-- 
Simon Hobson

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