DNS reverse lookup

Question

I'm doing the configuration of a DNS server with Ubuntu 12.04 and Bind9. I have a list of clients like this:

192.168.101.23
192.168.101.24
192.168.101.25
192.168.102.23
192.168.102.24
192.168.102.25

This is the zone defined for forward lookup, nothing to worry about here:

CC1 IN  A   192.168.101.22
CC2 IN  A   192.168.101.23
CC3 IN  A   192.168.101.24
CC4 IN  A   192.168.102.22
CC5 IN  A   192.168.102.23
CC6 IN  A   192.168.102.24

But it's not clear to me how I'am supposed to define the reverse lookup, with two or more subnets:

22     IN      PTR     CC1.dns.net.
22     IN      PTR     CC4.dns.net.  (I think, this is way too wrong. Should I write down 102.22 instead of 22?)

Any ideas?

Answer 1

It depends on the $ORIGIN of your zone. Usually the interesting part of a reverse DNS zone in bind looks like this:

$ORIGIN 101.168.192.in-addr.arpa.  
1    IN PTR cc1.dns.net.
2    IN PTR cc2.dns.net.

$ORIGIN 102.168.192.in-addr.arpa.  
1    IN PTR foo.dns.net.
2    IN PTR bar.dns.net.

Or in an aggregated section:

$ORIGIN 168.192.in-addr.arpa.  
1.101    IN PTR cc1.dns.net.
2.101    IN PTR cc2.dns.net.
1.102    IN PTR foo.dns.net.
2.102    IN PTR bar.dns.net.

Just consider the $ORIGIN a suffix to every record. So a

1.101 + 168.192.in-addr.arpa.

becomes

1.101.168.192.in-addr.arpa.

which is exactly 192.168.101.1 in forward notation.

You can leave off the $ORIGIN and write complete entries like this:

1.101.168.192.in-addr.arpa.   IN PTR   cc1.dns.net.

But nobody really likes to type that much. :)

Oh, and of course you want to put the reverse (PTR) records into another DNS zone configured in your /etc/bind/named.conf.local - they do not belong into the same file as the forward (A/CNAME) records.