Reverse DNS and single IP address space

Jan Grant Jan.Grant at
Thu Mar 27 11:17:03 PST 2003

On Thu, 27 Mar 2003, James Earl wrote:

> On 2003.03.27 11:38 Victor Bondarenko wrote:
> > On Thu, Mar 27, 2003 at 11:31:54AM -0700, James Earl wrote:
> > [...]
> > > Or, do I even need to worry about reverse DNS entries since my ISP
> > > already has them setup?
> >
> > If your ISP has reverse DNS for your IP(s), there's really no point in
> > you mapping them on your own.  Your network might see whatever you've
> > mapped, but the rest of the world will see what your ISP maps.
> I'm assuming if I can use nslookup [ip-address] to get my hostname,
> that reverse DNS on the ISP is setup properly.  Is this an okay
> assumption?

Not necessarily.

Firstly, you want to know what nameserver you're getting that response

Secondly, if the nameserver you're getting the response from belongs to
the ISP, they might _think_ the reverse map is set up correctly, but
upstream nameservers may disagree.

You might want to pick a publicly-available nameserver and query it;
alternatively, use nslookup or dig to follow the chain "by hand" from a
root nameserver.

The odds are you're ok, but checking by hand involves a little more

You can do something like:

> dig ptr

and you'll get a bit of the prefix and the next nameserver down to
query (probably a list of them). Pick one and repeat the request until
you get your ptr record back. That's what the rest of the world would
do, effectively.


jan grant, ILRT, University of Bristol.
Tel +44(0)117 9287088 Fax +44 (0)117 9287112
I shave with Occam's Razor.

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