Reverse DNS and single IP address space

Greg 'groggy' Lehey grog at
Thu Mar 27 16:55:52 PST 2003

On Thursday, 27 March 2003 at 11:31:54 -0700, James Earl wrote:
> I'm in the process of setting up primary and secondary name servers.
> This is my first time setting up named so I'm kinda a newbie in this
> area.
> My question is in regards to entries in named.conf and
> zone files.  In the FreeBSD Handbook and alot of other resources, I've
> noticed how the ip address is reversed, with part of the address left
> off.  For example:
> zone "" {
> 	type slave;
> 	file "s/";
> 	masters {
> 	};
> };

This is a slave entry.  It would be more interesting to see what the
master config looks like.  Anyway, this address is in an RFC 1918
non-routable address range.  That means it's not unique, and it's
completely meaningless on the global Internet.  In fact, I have that
address here :-)

 $ nslookup


> Now, what happens when I have only been allocated a single ip address
> from my ISP (well, actually two, one for the primary/master and one for
> the secondary/slave)? addresses come in blocks of 256.  You don't get the
choice, your ISP has to provide reverse DNS.  If he refuses, your only
options are to find a new ISP or a /24 address range for which you can
get reverse lookup.

On Thursday, 27 March 2003 at 11:52:04 -0700, 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?

No.  It depends on the name server you ask.  If you have set up your
name server as above, you'll get a response.  Nobody else will, for
the reasons mentioned above.

On Thursday, 27 March 2003 at 13:55:37 -0500, Jaime wrote:
>> 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?
> 	If you know enough about nslookup, then yes.  I'd suggest "host
> -v", though.  Its a bit easier.  :)

Is that easier than "nslookup"?

On Thursday, 27 March 2003 at 14:43:36 -0700, James Earl wrote:
> Thanks for the help everyone!  From your suggestions, it appears
> reverse DNS is setup properly.  Now if only my ISP could provide as
> good of support, as all of you provided me!  :)

Unfortunately, most of it was wrong :-(

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