Glue for exporting DHCP provisioning into Bind

Mark Andrews marka at isc.org
Thu Jun 14 05:48:11 UTC 2018


> On 14 Jun 2018, at 4:15 am, Philip Prindeville <philipp_subx at redfish-solutions.com> wrote:
> 
> My last message didn’t get any replies so let me try a more tangible question.
> 
> I’m looking at this article:
> 
> https://www.freeipa.org/page/Howto/DNS_classless_IN-ADDR.ARPA_delegation
> 
> in particular the example.
> 
> From the subnet 198.51.100.0/26 for a host they get the following RR’s:
> 
> 1.100.51.198.in-addr.arpa. 86400 IN	CNAME	1.0/26.100.51.198.in-addr.arpa.
> 1.0/26.100.51.198.in-addr.arpa.	86400 IN PTR	my.machine.example.
> 
> Dumb question… they end up with 5 octets before they add the suffix on and reverse the order:
> 
> 198.51.100.0.1
> 
> How are they deriving those last 2 digits?  The 198.51.100 is obvious… that’s 26 rounded down to the nearest multiple of 8 (/24) and taking that many net digits…  the /26 is of course the prefix.  0 would be the network start, I guess?  Since it’s a /26 prefix, subtracted from 32, that’s 6… 2^6 is 64, so there would be 0, 64, 128, and 256 as subnet starts…  And I’m guessing that 1 module 64 is 1… and that’s how they get the 1 digit?

The 1 comes from the last octet of the IP address.

If it was for 198.52.100.65/26 then the records would look like:

65.100.51.198.in-addr.arpa. 86400 IN CNAME 65.64/26.100.51.198.in-addr.arpa. (in 100.51.198.in-addr.arpa.)
65.64/26.100.51.198.in-addr.arpa. IN PTR my.machine.example. (in 64/26.100.51.198.in-addr.arpa.)


> What if the prefix were /22?

You delegate the 4 reverses for the /24’s that make up the /22 without applying the techniques in RFC 2317.
RFC 2317’s target prefixes lengths are /32-/25.  The intent is to not have to delegate a reverse zone for
each IP address in the range.

> 
> Thanks,
> 
> -Philip
> 
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Mark Andrews, ISC
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