Inverse DNS with Multiple DNS servers

Kevin Darcy kcd at
Tue Oct 23 03:28:48 UTC 2001

Mark A. Bialik wrote:

> So, just so I understand... a netblock (Class C for example) can only
> have one server provide reverse-mapping (if the whole block is
> contiguous).

The namespace is delegated on octet boundaries. So the reverse
zone corresponding to a class-C netblock can only be served by one *set* of
nameservers. Is that what you meant to say?

> But, if you subnet that baby four times (.192), you could
> ask the upstream ISP to delegate four different zones, each to a
> different DNS server on each of the four subnets?

It doesn't matter whether the DNS server(s) are on the subnets
corresponding to the reverse zones or not. They can be anywhere. The only
thing that matters is which nameservers control the PTR records
corresponding to the IP addresses in the relevant subnet ranges. In the
case of "traditional", octet-boundary address ranges, the controller of the
PTRs is determined by straight subzone delegation, whereas with RFC 2317,
the ISP "delegates" control of those PTRs indirectly via CNAMEs.

- Kevin

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