Message-Id: <200005230246.WAA03750@hrothgar.gw.com>
To: ...
Subject: Notes on RFC-2317
Date: Mon, 22 May 2000 22:46:55 -0400
From: Kimmo Suominen <kim@tac.nyc.ny.us>

Hi!

I wrote down some notes on RFC-2317.  I've had discussions with all of
you regarding classless IN-ADDR.ARPA delegations, and I would very much
appreciate any comments you may have.  Please feel free to forward this
to other parties as you see necessary or appropriate.

The goal of these notes is to try and clarify the reasoning behind the
recommendations I've been making on implementing RFC-2317 delegations.
In particular the following issues keep coming up with again and again
with each vendor:

    - why use "-" instead of "/"
    - why use particular NS records
    - why delegate within IN-ADDR.ARPA

I am hoping that the these notes could eventually be used to convince
ISPs to provide an efficient and smooth implementation of RFC-2317 with
the least amount of headache for the end-user.

Regards,
+ Kim



NOTES ON IMPLEMENTING CLASSLESS IN-ADDR.ARPA DELEGATION PER RFC-2317

1.  Selecting the CNAME target zone

    RFC-2317 shows an example case where the target zone is a delegated
    sub-zone of the IN-ADDR.ARPA zone for the natural class C network.
    This will allow for the NS records for the zone can be independently
    selected (see benefits described below).  An example of such a zone
    would be 0-28.150.80.204.IN-ADDR.ARPA.

    Now pay careful attention to the last paragraph of RFC-2317.  There
    are broken resolver implementations that apply the "valid host name"
    restrictions on the CNAME target (it should only be applied to the
    PTR target name).  To avoid problems with such implementations it
    is best to use a character that is allowed in a hostname.  I prefer
    using a hyphen, as I did in the example above.

    Some ISPs may at first refuse to delegate these zones (without any
    explanation).  Approach such ISPs with the reasoning in here first,
    but if that fails consider using your "forward" zone as a fallback.

    There is nothing magic about the IN-ADDR.ARPA zone for RFC-2317
    delegations.  You will have to sacrifice the optimization provided
    by a correct IN-ADDR.ARPA delegation, but you will still retain
    the ease of local administration for all name changes.

    I recommend using a dedicated subdomain for the PTR records, e.g. if
    your "forward" domain is "HOME.GW.COM" use "REV.HOME.GW.COM" for the
    PTR records.

2.  Selecting the NS records

    The NS records for the delegated zone should include all the NS
    records of the parent zone, in addition to any NS records pointing
    to the public name servers the delegate may want to use.  Having the
    name servers of the parent zone secondary the delegated zone allows
    them to have the necessary authoritative data to return the CNAME
    target in the additional records of a response to a PTR record query
    (minimizing the number of queries needed to resolve an address).

    This can be achieved using any zone (i.e. even a subdomain of your
    "forward" domain), of course.  However, having the ISP delegate an
    IN-ADDR.ARPA zone for your PTR records rather than you delegating a
    zone to your ISP maintains the logical "owner" and "delegate" roles.

    If the primary server for the delegated zone is not permanently on
    the Internet (e.g. a dial-on-demand connection) then you would not
    want to advertise it in the NS records.  It would just be a stealth
    server which the advertised secondaries poll for updates.

3.  Example delegation

    To delegate our example zone 0-28.150.80.204.IN-ADDR.ARPA first look
    at the NS records of the parent zone 150.80.204.IN-ADDR.ARPA.  Let's
    say they are the following:

        $ORIGIN 150.80.204.IN-ADDR.ARPA.
        @    IN NS GRENDEL.GW.COM.
             IN NS PYRY.GW.COM.

    To delegate 204.80.150.0/28 to SRV.HOME.GW.COM you would then insert
    these records in the parent zone data:

        $ORIGIN 150.80.204.IN-ADDR.ARPA.
        0-28 IN NS SRV.HOME.GW.COM.
             IN NS GRENDEL.GW.COM.
             IN NS PYRY.GW.COM.
        $GENERATE 0-15 $ IN CNAME $.0-28.150.80.204.IN-ADDR.ARPA.

    The necessary modifications to /etc/named.conf will be left as an
    exercise to the reader.

Kimmo Suominen
Global Wire Oy