Reverse mapping question

[Bill Larson]

"shh at tamu.edu" wrote:
> 
> Thanks for the reply, I think I understand now... but I have some more questions,
> if you don't mind.  In a forward lookup (ie, resolving name -> IP address), would
> it matter if they had delegated the reverse block to me?  My understanding is that
> a forward lookup would work like this:

This appears to be a confusing issue with DNS, but it is really quite simple.
A "reverse" lookup is simply a DNS query for some other information.  It occurs
in exactly the same manner as a "forward" lookup.

For example, if your system were to perform a reverse lookup of the IP address
"1.2.3.4", what would occur would be the system would reverse the IP address
and add ".in-addr.arpa" to this string, and finally do a simple DNS
lookup for
a pointer ("PTR") record for the complete string.  In this case, the system
would be looking for a PTR record for 4.3.2.1.in-addr.arpa.

Please note, and remember, this in-addr.arpa DNS domain functions identically
to a "xyz.com" or "tamu.edu" domain.  It is simply just another domain
and DNS
lookups are nothing special.

Now, there is always the question of who is responsible for providing DNS
information for any domain.  For example, who has been delegated the responsibilty
for the "tamu.edu" zone?  The same question can be asked about the in-addr.arpa
zones, who is responsible for providing DNS information for "4.3.2.1.in-addr.arpa"?
This responsibility is delegated in an identical manner as the forward zones.

So, to rephrase your questions, consider "can I use a fully qualified
domain name
on a system when I don't control the domain?"  The answer should be
obvious as
"yes".  The same question when asking about reverse pointers is also
yes.  It
doesn't matter.

To reiterate, in terms of DNS there is nothing special about reverse
pointers.  Queries
for pointer ("PTR") records follow the identical logic as normal
queries.  What can be
done with the delegation of the forward DNS information can also be done
with the
reverse information.

> Is reverse mapping really necessary?  Would it be feasible for me to host a domain,
> without having the IP address delegated to my name server's control?

Lots of organizations don't worry about the in-addr.arpa zone
information.  They
function, but then their users do run into problems occassionally where
this info
is required.  Now there is a problem, but their solution is "don't use
the site
that requires it".  Personally, this is NOT acceptable, but ...

You definitely CAN host a domain without having the responsibility for the
IP addresses.  Remember, "hosting" a domain is simply mapping the names into
IP addresses.  This says nothing about mapping the IP address back into
a host
name.  This works quite well.

Bill Larson (wllarso at swcp.com)