Network Working Group D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Best Current October 23, 2006
Practice
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DNS Scoped Data Through Attribute Leaves
draft-crocker-dns-attrleaf-02
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Historically, any DNS RR may occur for any domain name. Recent
additions have defined DNS leaf nodes that contain a reserved node
name, beginning with an underscore. The underscore construct is used
to define a semantic scope for the associated, parent domain name,
within which the use of some RRs is constrained. Hence the
underscore construct defines a basic paradigm modification to the
DNS. This note explores the nature of this DNS usage and defines the
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procedures for registering "underscore names" with IANA.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Procedural Model . . . . . . . . . . . . . . . . . . . . . 4
1.3. Discussion Venue . . . . . . . . . . . . . . . . . . . . . 4
2. Scaling Benefits and TXT and SRV Resource Records . . . . . . . 4
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. References -- Normative . . . . . . . . . . . . . . . . . . 6
5.2. References -- Informative . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Introduction
Historically, any DNS RR may occur for any domain name. The DNS
technical specification assigns no semantics to domain names and no
constraints upon which resource records may be associated with a
particular name. Over time, some leaf node names, such as "www" and
"ftp" have come to imply support for particular services, but this is
a matter of operational convention, rather than defined semantics.
This freedom in the basic technology has permitted a wide range of
administrative and semantic policies to be used -- in parallel --
with the DNS. In the DNS, data semantics have been limited to the
specifications of particular resource records, on the expectation
that new ones would be added as needed. Although there remains the
view that this method of enhancement is preferred, alternative
approaches have been explored and gained widespread deployment.
Recent additions have defined DNS leaves that contain a reserved leaf
node name, beginning with an underscore. The underscore construct is
used to define a semantic scope for for the associated, parent domain
name, within which the use of some RRs is constrained. Hence the
underscore construct defines a basic paradigm modification to the
DNS. Within the scope of a defined underscore leaf, the uses of
specific resource records can be formally defined and constrained.
An established example is the SRV record [RFC2782] which generalizes
concepts long-used for email routing in the MX
record.[RFC0974][RFC2821] The use of special DNS names has
significant benefits and detriments. Some of these are explored in
[I-D.iab-dns-choices].
[Comment]: The term "resolution context" has been suggested, in
place of "semantic scope". It is not yet clear what resolution is
being given particular context by this proposal, whereas the
intent behind the phrase "semantic scope" is to note that the
interpretation -- ie, semantics -- of particular RRs is
constrained. [/Dave]
One use that has perhaps not been noticed is that the underscore
construct substantially changes possible concerns for scaling
effects. For example, different uses for the same RR, such as the
free-form TXT record, become manageable when those are defined to be
within different, scoped leaf nodes.
This note discusses this enhancement, provides an explicit definition
of it, and establishes an IANA registry for the reserved names
beginning with underscore.
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1.1. Disclaimer
This document does not seek to recommend or debate the merits of
using sub-domain names that begin with underscore. The practise
already exists, for multiple services. The sole goal for this
document is to specify a registry for the underscore-based names that
get used.
1.2. Procedural Model
NOTE: This procedure is modeled after that specified in [RFC2489].
"The author of a new DHCP option will follow these steps to obtain
approval for the option and publication of the specification of the
option as an RFC:
1. The author devises the new option.
2. The author documents the new option as an Internet Draft,
choosing a node name that has not yet been registered.
3. The author submits the Internet Draft for publication as an RFC,
either as an independent submission or as an IETF-approved
document.
4. The specification of the new option is reviewed for publication
by the appropriate bodies.
5. At the time of publication as an RFC, IANA formally lists the
node name."
1.3. Discussion Venue
Discussion about this draft is directed to the
dnsop@lists.uoregon.edu [1]mailing list of the IETF DNSOP Working
Group [2].
2. Scaling Benefits and TXT and SRV Resource Records
It is intended that additional semantics, associated with a domain
name, be provided by the definition and deployment of new resource
records, the use of underscore-based naming is sometimes used to
distinguish among different semantics for the same RR. The primary
examples of this are TXT and SRV records.
In the case of TXT records, use for different semantics has developed
organically and largely without coordination. Underscore-based names
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therefore provide an administrative way of separating TXT records
that might have different semantics, but otherwise would have no
syntactic markers for distinguishing among them.
In the case of the SRV RR this method of distinguishing among uses
was part of the design. [RFC2782] In reality, the SRV specification
defines an RR that may only be used for specific applications when
there is an additional specification. So the SRV specification is
best thought of as a template for future specifications. The
template definition includes reference to tables of names from which
underscore-names should be drawn. So, the set of <service> names is
defined in terms of other IANA tables, namely any table with symbolic
names. The other SRV naming field is <proto>, although its pool of
names is not explicitly defined.
3. IANA Considerations
IANA is requested to establish the DNS Underscore Name Registry, for
DNS node names that begin with the underscore character and have been
specified in any published RFC.
These documents provide specific meanings for specific resource
records. They do not constrain the usage of resource records that
are not specified. The purpose of this registry tis to avoid
collisions resulting from the use of the same underscore name, for
different applications.
A request to register an entry in the DNS Underscore Name Registry
MUST contain:
Name: Specifies the underscore name that is being reserved. The
name may be multi-part. That is, it may cover more than one
domain name field, such as "_sip._tcp".
RR(s) Specifies the Resource Records that are explicitly defined
for the scope of this registration.
RFC(s) These specify the semantics for the RRs defined for use
within the scope of the registered underscore name.
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Initial entries in the registry comprise:
+-----------+-------+-----------+
| NAME | RR(s) | RFC |
+-----------+-------+-----------+
| _sip._tcp | SRV | [RFC3263] |
| _sip._udp | SRV | [RFC3263] |
| _spf | TXT | [RFC4408] |
+-----------+-------+-----------+
Table 1: DNS Underscore Name Registry Initial Values
4. Security Considerations
This memo raises no security issues
5. References
5.1. References -- Normative
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006.
5.2. References -- Informative
[I-D.iab-dns-choices]
Faltstrom, P., "Design Choices When Expanding DNS",
draft-iab-dns-choices-03 (work in progress), April 2006.
[RFC0974] Partridge, C., "Mail routing and the domain system",
RFC 974, January 1986.
[RFC2489] Droms, R., "Procedure for Defining New DHCP Options",
BCP 29, RFC 2489, January 1999.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
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URIs
[1] <mailto:dnsop@lists.uoregon.edu>
[2] <http://ietf.org/html.charters/dnsop-charter.html>
Appendix A. Acknowledgements
Thanks go to Bill Fenner, Tony Hansen, Peter Koch, Olaf Kolkman for
diligent review.
Author's Address
Dave Crocker
Brandenburg InternetWorking
675 Spruce Dr.
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
Email: dcrocker@bbiw.net
URI: http://bbiw.net/
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