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Versions: 00 01                                                         
dnsext                                                       A. Williams
Internet-Draft                                                  Motorola
Expires: December 30, 2002                                  July 1, 2002

                     A Locally Scoped DNS Namespace

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
   other groups may also distribute working documents as Internet-

   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 http://

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   This Internet-Draft will expire on December 30, 2002.

Copyright Notice

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


   This memo defines a locally scoped private DNS namespace.

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Table of Contents

   1.  Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1 The Existence of Scoped Addresses  . . . . . . . . . . . . . .  3
   1.2 Scoped Addresses and the DNS . . . . . . . . . . . . . . . . .  3
   2.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.1 The "private.arpa." namespace  . . . . . . . . . . . . . . . .  5
   3.2 Duplicate detection and resolution . . . . . . . . . . . . . .  6
   4.  Other issues . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.1 Merging of Networks  . . . . . . . . . . . . . . . . . . . . .  6
   4.2 Configuration Consistency in a Site  . . . . . . . . . . . . .  6
   4.3 Relationship to mDNS . . . . . . . . . . . . . . . . . . . . .  6
   4.4 Relationship to DDNS . . . . . . . . . . . . . . . . . . . . .  7
   4.5 Why not use a seperate QCLASS? . . . . . . . . . . . . . . . .  7
   4.6 Why not local.arpa or lcl.arpa?  . . . . . . . . . . . . . . .  7
       References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  9
   A.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 10

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1. Concepts

1.1 The Existence of Scoped Addresses

   Privately addressed networks are in widespread use today for a
   variety of reasons including address space shortage and a desire to
   have a separate addressing domain bordered by a security gateway.
   Well defined portions of the IPv4 address space have been reserved to
   support the desire to use private addresses in [RFC1918].

   In IPv6, the site-local address prefix [RFC2373] is reserved for use
   by those wanting to use private IPv6 addressing.  Private addresses
   are useful for people who wish to use IPv6 but are not connected to
   the global internet.

   IPv6 site local addresses can also be viewed as a way of providing
   stable addresses in the face of renumbering events.  A common case
   occurs when 6to4 [RFC3056] is used to provide global IPv6 addresses
   from a DHCP or dialup address subject to relatively frequent change.
   Site-local addresses allow un-interrupted operation of services
   within a site during periods when global addresses need to be changed
   or are unavailable.  It is expected that global addresses would be
   used in simultaneously with site local addresses.

   Private address ranges are in wide use today in administered networks
   (e.g.  corporates) and in un-administered networks (e.g.  the home).
   Private addresses are not unique in the global internet, and cannot
   be uniquely routed to, however they are typically allocated in a
   fashion that ensures their uniqueness and routeability within an
   administrative site.

1.2 Scoped Addresses and the DNS

   Generally it should be recognised that people wanting to use private
   addressing also wish to use the DNS to resolve names.  The current
   recommended approach is to set up two "views" of the DNS: one for the
   privately addressed hosts, and another for global hosts on the
   internet.  Various documents attempt to prohibit the placing non-
   globally scoped addresses into the global DNS since there are a
   variety of undesirable effects that come from doing so (e.g.  [I-
   D.ietf-dnsop-dontpublish-unreachable][RFC1918]).  The intention of
   this memo is to provide constructive guidance for people who will
   make use of locally scoped addresses and name spaces in spite of the
   admonishments against doing so.

   Rather than view these problems as arising "because non-globally
   scoped addresses are in the DNS", this document takes the view that
   problems arise because private (not globally useful) and global

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   addresses are returned together in a single response to a DNS query,
   and hosts do not or cannot distinguish between them.  Unfortunately,
   attempting to have the DNS server omit locally scoped addresses in
   responses "as appropriate" is understood to be infeasible.  The
   recommended split DNS approach results in different answers to the
   same DNS question depending on where you are in the network.

   This document proposes a locally scoped namespace to pair with
   locally scoped addresses: private.arpa.  The private.arpa.  domain
   suffix replaces the global domain suffix when a DNS record contains a
   private address, thus partitioning the private and global addresses
   into separate portions of the DNS namespace.  Clients can prefer
   local services using locally scoped addresses via a DNS suffix search

2. Rationale

   The private.arpa namespace provides a usable DNS domain name to use
   when a network does not have a globally allocated domain name.
   Typical examples are disconnected networks and also home networks
   which usually inherit the domain suffix of their ISP via DHCP, often
   in combination with a NAT.  An ISP is unlikely to be able to support
   DNS for each home for precisely the reasons listed above.  Further,
   most ISPs do not want to allow customers to add or remove DNS entries
   from their namespace, and getting a global domain name is
   complication a consumer can do without.

   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.

   A private DNS namespace allows standard DNS and dynamic updates to be
   used rather than a proprietary local name service such as NetBIOS
   naming or Appletalk NBP in the home.  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.  Devices can be configured with a name
   (rather than configuring the name server), and the devices can use
   dynamic update to populate the local DNS zone automatically.

   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.

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   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
   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[RFC1918] 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).

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

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[RFC2136] and draft-ietf-dhc-ddns-resolution-??.txt[ID-

   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 Merging of Networks

   Two organisations using privately addressed networks that merge run
   the risk of conflict in their address space.  In a similar way, two
   organisations using the private.arpa address space may also run the
   risk of conflicts during a subsequent merge of their networks.  One
   possible approach to minimising the risk is to create a sub-domain
   inside the private.arpa domain that is "reasonably unique".  One
   possibility might be to choose the company name (e.g.
   acme.private.arpa) as the domain suffix used, but still inside
   private.arpa.  Two organsiations that merge using different sub-
   domains inside private.arpa will not experience a conflict.

4.2 Configuration Consistency in a Site

   Since private.arpa (and the RFC1918 reverse maps for that matter) are
   not globally delegated, there is no chain of referrals that back-end
   resolvers may follow to locate a DNS server.  A site that makes use
   of back-end resolvers must ensure that they are configured to refer
   private.arpa requests (and RFC1918 backward maps if required) to the
   appropriate DNS server within the site.  It appears reasonable to
   require that all back-end resolvers be within the site.

4.3 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.

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4.4 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.

4.5 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"[ID-mini-dhcp]) with a
   DNS proxy may support the "private.arpa." namespace and existing
   clients can query it using their existing resolver code.

4.6 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
   "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"


   [I-D.ietf-dnsop-dontpublish-unreachable]  Hazel, P., "IP Addresses
                                             that should never appear in
                                             the public DNS", draft-
                                             unreachable-03 (work in
                                             progress), February 2002.

   [ID-mini-dhcp]                            Aboba, B., "The Mini-DHCP
                                             Server", ID draft-aboba-

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                                             dhc-mini-04.txt, September

   [ID-name-conflict]                        Stapp, M., "Resolution of
                                             DNS Name Conflicts Among
                                             DHCP Clients", ID draft-
                                             03.txt, November 2001.

   [RFC1034]                                 Mockapetris, P., "Domain
                                             names - concepts and
                                             facilities", STD 13, RFC
                                             1034, November 1987.

   [RFC1035]                                 Mockapetris, P., "Domain
                                             names - implementation and
                                             specification", STD 13, RFC
                                             1035, November 1987.

   [RFC1918]                                 Rekhter, Y., Moskowitz, R.,
                                             Karrenberg, D., Groot, G.
                                             and E. Lear, "Address
                                             Allocation for Private
                                             Internets", BCP 5, RFC
                                             1918, February 1996.

   [RFC2136]                                 Rekhter, Y., Thomson, S.,
                                             Bound, J. and P. Vixie,
                                             "Dynamic Updates in the
                                             Domain Name System (DNS
                                             UPDATE)", RFC 2136, April

   [RFC2373]                                 Hinden, R. and S. Deering,
                                             "IP Version 6 Addressing
                                             Architecture", RFC 2373,
                                             July 1998.

   [RFC3056]                                 Carpenter, B. and K. Moore,
                                             "Connection of IPv6 Domains
                                             via IPv4 Clouds", RFC 3056,
                                             February 2001.

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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/

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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   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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