Reverse DDNS on classless zones

Simon Hobson dhcp1 at
Wed Jul 18 07:10:12 UTC 2012

atif yameen wrote:

>and the following is my reverse zone in named.conf
>zone ""
>     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 
> to
>where as it should be :
>zone file shows the following: (creates a subzone 129 and then a 
>dotted name from the rest of the three octets)
>$TTL 302400     ; 3 days 12 hours
>250.25.119              PTR
>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 ""), 
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 zone of the form :

128  CNAME
129  CNAME

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 as the zone, it can literally be any 
reachable zone. So you could use, and then you'd have :

128  CNAME
And your client above would get an entry in the zone
250 PTR

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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