Network Working Group D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Best Current July 8, 2007
Practice
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draft-crocker-dns-attrleaf-03
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Copyright (C) The IETF Trust (2007).
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. References -- Normative . . . . . . . . . . . . . . . . . . 7
5.2. References -- Informative . . . . . . . . . . . . . . . . . 7
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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1. Introduction
Historically, any DNS RR may occur for any domain name. The DNS
technical specifications assign 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 reserved DNS node names, beginning with
an underscore. The underscore construct is used to define a scope
for the occurrence of particular resource records, notably particular
uses of those RRs. 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 terms "resolution context" and "scoping rules" have
been suggested, in place of "semantic scope". In order to avoid
concern for matters of semantics, this specification uses the term
"scoping rules", to create a focus on the mechanics being defined,
rather than nuances of interpretation for the mechanism.
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.
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
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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
Some resource records are have a generic form, with additional rules
of use, internal syntax, or naming node naming conventions to
distinguish among particular types. The TXT and SRV records are the
notable concern for this. Some of these approaches scale poorly,
particularly when the same RR can be present in the same node, but
with different uses. An approach with good scaling properties uses
underscore-based names can be used to define sections with particular
uses for particular RRs.
In the case of TXT records, use for different scoping rules has
developed organically and largely without coordination. Underscore-
based names therefore provide an administrative way of separating TXT
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records that might have different uses, 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 a set of DNS Underscore Name
Registries, for DNS node names that begin with the underscore
character and have been specified in any published RFC.
The "DNS Underscore SCOPE Registry" creates the top-level of a
potentially multi-field sequence of underscore names. Additional
registries are defined by the specification that creates a particular
underscore name, if it provides for subordinate underscore
components.
The purpose of these tables is to define portions of the DNS for
which there is a scope of use, with specific meanings for specific
resource records, when they occur under the domain name having the
underscore name(s). They do not constrain the usage of other
resource records that are not specified. The purpose of the
registries is to avoid collisions resulting from the use of the same
underscore name, for different applications.
A request to register an entry in a DNS Underscore Name Registry MUST
contain:
Name: Specifies a textual name for this scoped portion of the
DNS. The name will usually be taken from the specification
cited in the "Defined" column and is intended for use in
discussions about the entry.
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Label: Specifies the underscore name that is being reserved.
The name may be specified directly or by citing a table of
names, with the implication that a name from the table will be
prefaced with an underscore. Referencing a table of names
incorporates those names into the table, so as to create a set
of additional entries.
Subordinate: Refers to a registry of underscore names that
defines the next level of domain name field, below the current
one. If this cell contains a reference, the RR cell must be
empty.
RR(s): Specifies the Resource Records that are explicitly
defined for the scope of this registration. The specification
is either by directly listing the RR(s) or by citing a table of
RRs. This cell in the table is to be empty, if the Subordinate
cell contains a reference.
Defined: Specifies the particular use for specific RR(s),
defined for use within the scope of the registered underscore
name.
+------+-------+------------------------+-------+-------------------+
| NAME | LABEL | SUBORDINATE | RR(s) | DEFINED |
+------+-------+------------------------+-------+-------------------+
| SRV | _tcp | DNS SRV Underscore | | [RFC3263] |
| TCP | | Name Registry | | |
| | | (Table 2) | | |
| SRV | _udp | DNS Underscore SRV | | [RFC3263] |
| UDP | | Name Registry | | |
| | | (Table 2) | | |
| SPF | _spf | | TXT | [RFC4408] |
| URI | _e2u | ENUM (Table 3) | | (new enumuri |
| | | | | draft pending. |
| | | | | /d) |
+------+-------+------------------------+-------+-------------------+
Table 1: DNS Underscore SCOPE Name Registry (with initial values)
Initial entries in the registry comprise:
+------+-------+-------------+-------+---------+
| NAME | LABEL | SUBORDINATE | RR(s) | DEFINED |
+------+-------+-------------+-------+---------+
Table 2: DNS Underscore SRV Name Registry (with initial values)
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+-------+-------+-------------+-------------------------+-----------+
| NAME | LABEL | SUBORDINATE | RR(s) | DEFINED |
+-------+-------+-------------+-------------------------+-----------+
| ENUM | _e2u | | IANA Service Table | (new |
| (or | | | enum-services; or RFC | enumuri |
| E2U?) | | | 3968, Section 6.5, | draft |
| | | | <experimental-service> | pending. |
| | | | | /d) |
+-------+-------+-------------+-------------------------+-----------+
Table 3: DNS Underscore ENUM Name Registry (with 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-04 (work in progress), October 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, and
Andrew Sullivan 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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