dnsext A. Williams
Internet-Draft Motorola
Expires: December 30, 2002 July 1, 2002
A Locally Scoped DNS Namespace
draft-williams-dnsext-private-namespace-01.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
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
list.
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
DNS.
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],
RFC1035[RFC1035]).
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-
name-conflict].
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"
namespace.
References
[I-D.ietf-dnsop-dontpublish-unreachable] Hazel, P., "IP Addresses
that should never appear in
the public DNS", draft-
ietf-dnsop-dontpublish-
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
2001.
[ID-name-conflict] Stapp, M., "Resolution of
DNS Name Conflicts Among
DHCP Clients", ID draft-
ietf-dhc-ddns-resolution-
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
1997.
[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
Australia
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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