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Network Working Group                                        A. Williams
Internet-Draft                                                  Motorola
Expires: August 21, 2002                               February 20, 2002

          A Private DNS Namespace for Automatic Configuration

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on August 21, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   This memo defines a locally scoped private DNS namespace.  Such a
   namespace supports self-configured authoritative nameservers in home
   or zeroconf environments where global names for devices are not
   required, yet local name resolution is beneficial.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.1 The "private.arpa." namespace  . . . . . . . . . . . . . . . .  4
   3.2 Duplicate detection and resolution . . . . . . . . . . . . . .  5
   4.  Other issues . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.1 Relationship to mDNS . . . . . . . . . . . . . . . . . . . . .  5
   4.2 Relationship to DDNS . . . . . . . . . . . . . . . . . . . . .  5
   4.3 Why not use a seperate QCLASS? . . . . . . . . . . . . . . . .  5
   4.4 Why not local.arpa or lcl.arpa?  . . . . . . . . . . . . . . .  5
       References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  6
   A.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
       Full Copyright Statement . . . . . . . . . . . . . . . . . . .  8

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1. Introduction

   An attraction of the multicast DNS proposals discussed on the dnsext
   mailing list recently is that they can be used by non-administrators
   in environments like the home.  This has been achieved by a
   combination of a transport change and a well defined namespace.  The
   transport change (i.e.  the use of multicast) is intended to avoid
   the need to configure information like DNS server addresses.  The
   namespace that has usually gone along with the proposals is intended
   to allow automatic population of a DNS zone.  Implicit in the
   discussion has been the notion that devices will be configured with a
   name, which is then used to populate the DNS zone.

   It has also been recognised that the namespace and the lookup
   mechanism are largely independent and should be defined seperately.
   The current dnsext working group multicast DNS proposal no longer
   specifies a private namespace, and so this document has been written
   to fill that gap.

   As yet, there does not appear to be consensus that the approach
   described here is a good idea.  This draft is an attempt to collect
   together the ideas presented on the mailing list and provide a focus
   for further discussion.  Participants may require a flame retardant

2. Rationale

   The primary motivation for proposing a well defined locally scoped
   private address space is to support automatic self-configuration of
   DNS servers.  Environments which stand to benefit are home networks
   and zeroconf networks.

   In home networks, users tend to name their devices and expect their
   device names to be automatically visible in the namespace.  This is
   in contrast to the usual method of populating DNS zones by listing
   device names and addresses in a master file.  Manual construction and
   maintenance of DNS zone files cannot be expected because many home
   networks are without administrators.

   Home and zeroconf networks for the most part do not have part of the
   global DNS namespace delegated to them.  A well defined private
   namespace (e.g.  "private.arpa.") allows devices to construct a fully
   qualified domain name for use locally, and corrals the automatically
   configured names in the global DNS namespace.

   A well defined namespace allows ISPs to provide authoritative
   negative responses to DNS requests that leak out of private networks.
   DNS response times are reduced for applications inside the private

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   network, and top level nameserver traffic is reduced.

   Private namespaces are already in use in environments like the home.
   Each vendor currently makes an arbitrary choice as to what domain
   suffix to use.  Suggesting an appropriate private domain name
   encourages interoperability and avoids some truly bad choices (e.g.
   a domain suffix of "." so that each device has a FQDN of "thing1.",
   "thing2.", etc.  This runs the risk of hiding a global TLD should a
   user happen to name their device "com").

3. Definitions

3.1 The "private.arpa." namespace

   The DNS domain "private.arpa." using the address class "IN" is
   defined to be a locally scoped private address space.  Local scoping
   implies that names registered inside this domain are available only
   within a physical or administrative network boundary.  As a private
   namespace, names in "private.arpa." are not visible across the global
   internet in much the same way as RFC1918[1] private addresses are not
   globally usable addresses.  The sets of names available in the
   "private.arpa." namespace of each site are disjoint.

   The "private.arpa." namespace co-exists with and is orthogonal to the
   global DNS namespace.  It is desirable that a network using
   "private.arpa."  for local names still be able to look up the global

   Any DNS server may be authoritative for the "private.arpa." domain.
   If a site contains more than one DNS server, coordination between
   them will be required.

   The "private.arpa." zone may be populated automatically using Dynamic
   DNS, zone file updates, from a co-located DHCP server, via hosts
   using multicast DNS, or some other technique.

   The "arpa" top-level DNS server is authoritative for "private.arpa.",
   which is an empty zone.  This will result in negative responses being
   sent for all lookups in the zone.

   DNS servers or backend resolvers run by network providers may also be
   authoritative for "private.arpa.".  This zone is expected to be
   empty, and serves to limit useless queries to the root nameservers.
   See RFC1912 for similar examples ("localhost", "0.0.127.in-
   addr.arpa", etc).

   Within a site, "private.arpa." may have additional structure
   according to the usual rules of the DNS namespace (RFC1034[2],

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3.2 Duplicate detection and resolution

   Hosts wanting to automatically update RRs in the "private.arpa."
   namespace must perform collision detection and resolution.  If DDNS
   is being used, collision resolution should be performed as described
   in RFC2136[4] and draft-ietf-dhc-ddns-resolution-??.txt[6].

   A DNS server updated by a co-located a DHCP server that does not use
   DDNS must also perform collision detection and resolution.

4. Other issues

4.1 Relationship to mDNS

   The "private.arpa." namespace is orthogonal to the use of multicast
   DNS.  Names in the "private.arpa." namespace may be queried via
   unicast or multicast DNS.

4.2 Relationship to DDNS

   DNS Dynamic Updates may be used in "private.arpa." namespace.  Other
   methods for automatically registering DNS names in the
   "private.arpa." namespace may also be used.

4.3 Why not use a seperate QCLASS?

   Another way to support self-configuring authoritative DNS servers is
   to use a different DNS query class.  This would have the effect of
   creating a new DNS namespace consisting only of automatically
   configured names and resource records.  It is assumed that the
   majority of the resource records already defined for the "IN" class
   would be used in this new class.

   The drawbacks of this approach are essentially related to backward
   compatibility and deployment.  Existing clients would need to be
   modified to query names using the new QCLASS.  In contrast, a home
   gateway (see for example "The Mini-DHCP Server"[5]) with a DNS proxy
   may support the "private.arpa." namespace and existing clients can
   query it using their existing resolver code.

4.4 Why not local.arpa or lcl.arpa?

   The particular name chosen is not particularly important.
   Historically the "local.arpa." and "lcl.arpa."  namespaces have been
   associated with various multicast DNS proposals.  Rather than reuse
   the name, a distinct name was chosen to highlight that the

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   "private.arpa." namespace has nothing to do with how it is looked up,
   and has no dependencies on multicast.

   Another factor is that code has already been written and deployed
   which uses the "local.arpa" namespace as a trigger to make multicast
   DNS queries.  If a name is in the "local.arpa" domain, then multicast
   will be used.  This behaviour is not desirable for the "private.arpa"


   [1]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E.
        Lear, "Address Allocation for Private Internets", RFC 1918,
        February 1996.

        1034, November 1987.

        SPECIFICATION", RFC 1035, November 1987.

   [4]  Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
        Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
        November 2001.

   [5]  Aboba, B., "The Mini-DHCP Server", ID draft-aboba-dhc-mini-
        04.txt, September 2001.

   [6]  Stapp, M., "Resolution of DNS Name Conflicts Among DHCP
        Clients", ID draft-ietf-dhc-ddns-resolution-03.txt, November

Author's Address

   Aidan Williams
   Motorola Australian Research Centre
   Locked Bag 5028
   Botany, NSW  1455

   Phone: +61 2 9666 0500
   EMail: Aidan.Williams@motorola.com
   URI:   http://www.motorola.com.au/marc/

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Appendix A. Acknowledgements

   Many people on the dnsext mailing list have contributed to the
   discussions on multicast DNS and the namespace issues it brought up.
   The discussion was helpful and at times most enlightening.

   Contributors to the discussion include: Bernard Aboba, Harald
   Alvestrand, Richard Barr Hibbs, Eric Brunner-Williams, Randy Bush,
   Stuart Cheshire, Matt Crawford, Alain Durand, Robert Elz, Levon
   Esibov, Patrick Falstrom, Olafur Gudmundsson, Erik Guttman, Eric A.
   Hall, Jun-ichiro itojun Hagino, Christian Huitema, Richard Johnson,
   Bill Manning, Tomohide Nagashima, Thomas Narten, Dan Nicolae, Erik
   Nordmark, Masataka Ohta, JINMEI Tatuya, David Terrell, Dave Thaler,
   Sander Van-Valkenburg, Paul A Vixie, Bill Woodcock, and Brian Zill.

   The author also wishes to thank Kwan-Wu Chin for a number of
   stimulating conversations.

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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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