Reverse DNS and single IP address space

[Jan Grant]

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
from.

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
work.

You can do something like:

> dig @a.root-servers.net 1.0.168.192.in-addr.arpa. 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.

Cheers,

-- 
jan grant, ILRT, University of Bristol. http://www.ilrt.bris.ac.uk/
Tel +44(0)117 9287088 Fax +44 (0)117 9287112 http://ioctl.org/jan/
I shave with Occam's Razor.