From: "Perry E. Metzger" <perry@imsi.com>
Date: Sat, 11 Feb 95 22:16:29 PST
To: Hal <hfinney@shell.portal.com>
Subject: Re: Does PGP scale well?
In-Reply-To: <199502120105.RAA20992@jobe.shell.portal.com>
Message-ID: <9502120616.AA18409@snark.imsi.com>
MIME-Version: 1.0
Content-Type: text/plain



Hal says:
> I was just reading RFC1034 about DNS, and one thing I noted was that
> there is a "reverse lookup" feature.  This allows you to go from,
> say, 156.151.1.101 to portal.com.  This problem seems similar in
> some ways to the key lookup problem since you have a relatively
> unstructured number and you want to use it as a lookup key.
> 
> According to the RFC, if you want to know what host machine is at
> address 156.151.1.101, you do a lookup of 156.151.1.101.IN-ADDR.ARPA.

No, you got that wrong. You do a lookup on 101.1.151.156.IN-ADDR.ARPA
-- note the component reversal. The reversal is very key.

> The RFC did not make it very clear how this is done.  Does this use a
> "flat" database?

No. Its fully distributed. The fact that networks are assigned in
heirarchical chunks should explain how its done, and why the bytes get
reversed for the lookup. As an example, MIT owns network 18, which is
to say that all MIT addresses are 18.XXX.XXX.XXX, and 18.IN-ADDR.ARPA
is a server at MIT. MIT may have sub-servers beyond that level, but
DNS makes us oblivious to this.

For IPv6, the reverse lookup is going to be segregated at the nybble
boundaries rather than at the byte boundaries because of CIDR style
classless allocation. The domain will be IP6.INT rather than IN-ADDR.ARPA

Perry