Network Working Group D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: BCP March 30, 2011
Expires: October 1, 2011
DNS Scoped Data Through Attribute Leaves
draft-crocker-dns-attrleaf-04
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 DNS records associated with the parent
domain. This note explores the nature of this DNS usage and defines
the "underscore names" registry with IANA.
Status of this Memo
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This Internet-Draft will expire on October 1, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Scaling Benefits and TXT and SRV Resource Records . . . . . . . 4
3. Underscore DNS Registry Function . . . . . . . . . . . . . . . 4
4. DNS Underscore Registry Definition . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. References -- Normative . . . . . . . . . . . . . . . . . . 7
7.2. References -- Informative . . . . . . . . . . . . . . . . . 7
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
The core DNS technical specifications assign no semantics to domain
names or their parts, and no constraints upon which resource records
(RRs) may be associated with particular names. 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 protocol 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. Data semantics
have been limited to the specification of particular resource
records, on the expectation that new ones would be added as needed.
Some recent service enhancements have defined define a restricted
scope for the occurrence of particular resource records. That scope
is a leaf node, within which the uses of specific resource records
can be formally defined and constrained. This leaf has a
distinguished naming convention: It uses a reserved DNS node name
that begins with an underscore. Because host names are not allowed
to use the underscore character, this distinguishes the name from all
legal host name. Effectively, this convention creates a space for
attributes that are associated with the parent domain, one level up.
An established example is the SRV record [RFC2782] which generalizes
concepts long-used for email routing by the MX record
[RFC0974][RFC2821]. The use of special DNS names has significant
benefits and detriments. Some of these are explored in [RFC5507].
[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.
The scoping feature is particularly useful when generalized resource
records are used -- notably TXT and SRV. It provides efficient
separation of one use of them from another. Absent this separation,
an undifferentiated mass of these RRs are returned to the client
which then must parse through the internals of the records in the
hope of finding ones that are relevant. With underscore-based
scoping, only the relevant RRs are returns.
This specification discusses this enhancement, provides an explicit
definition of it, and establishes an IANA registry for the reserved
names that begin with underscore.
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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 have a generic form, and support a variety of
uses. Each additional use defines its own rules and, possibly, its
own internal syntax and 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 leaf node, but with different
uses. An increasingly-popular approach, with excellent scaling
properties, uses an underscore-based name to a define place in the
DNS that is constrained to particular uses for particular RRs. This
means that a direct lookup produces only the desired records, at no
greater cost than a typical lookup.
In the case of TXT records, use for different scoping rules has
developed organically and largely without coordination. One side-
effect of this is no consistently distinguishable internal syntax for
the records; even internal inspection might not be a reliable means
of distinguishing among them. Underscore-based names therefore
provide an administrative way of separating TXT 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. Underscore DNS Registry Function
This specification defines a registry for DNS nodes names, used to
specify scope of use for specific resource records (RR). That is, a
given names defines a specific, constrained context for the use of
such records. This does not constrain the use 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,
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for different applications.
Structurally, the registry is defined as a single, flat table of
names that begin with underscore. In some cases, such as for SRV, an
underscore names might have further constraints, such as being valid
only "under" some other underscore name. Semantically, this is a
hierarchical model, thereby making a flat registry unexpected.
The registry requires such hierarchies to be registered as a
combinatorial case analysis set, with each entry being a full
sequence of underscore names. Given a naming scheme that is actually
structured, this flat design is inelegant. However it has the
benefit of being extremely simple, with the added advantage of being
easier for readers to understand, as long as these cases are small
and few.
+----------------------------+
| NAME |
+----------------------------+
| _protoA |
| _service1._protoB |
| _service2._protoC |
| _service2._protoC |
| _service3._protoD._useX |
| _protoE._region._authority |
+----------------------------+
Example of Underscore Names
The reasons for choosing a simplified registry design are:
o the belief that listing multi-level schemes as complete
combinations will be simpler than formulating sub-tables, simples,
and
o the view that requiring readers to parse through a possible
hierarchy of multiple registries -- one per level -- will
encourage errors.
4. DNS Underscore Registry Definition
A registry entry MUST contain:
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Name: Specifies a textual name for a scoped portion of the DNS.
The name will usually be taken from the specification cited in
the "Purpose" column and is intended for use in discussions
about the entry.
DNS Label(s): Specifies a sequence of one or more underscore
names that define a single name reservation.
Constraints: Specifies any restrictions on use of the name.
RR(s): Lists the RRs that are defined for use within this scope.
References Lists specifications that define the records and their
use under this Name.
Purpose: Specifies the particular purpose/use for specific
RR(s), defined for use within the scope of the registered
underscore name.
5. IANA Considerations
IANA is requested to establish a DNS Underscore Name Registry, for
DNS node names that begin with the underscore character (_) and have
been specified in any published RFC, or are documented by a
specification published by another standards organization. The
contents of each entry are defined in Section 3.
+-----+----------------+------------+-------+-----------+-----------+
| NAM | DNS LABEL | CONSTRAINT | RR(s) | REFERENCE | PURPOSE |
| E | | S | | S | |
+-----+----------------+------------+-------+-----------+-----------+
| SIP | _sip._tcp | | NAPTR | [RFC3263] | Locating |
| TCP | | | | | SIP |
| | | | | | Servers |
| SIP | _sips._tcp | | NAPTR | [RFC3263] | Locating |
| S | | | | | SIP |
| TC | | | | | Servers |
| P | | | | | |
| SIP | _sip._udp | | SRV | [RFC3263] | Locating |
| UDP | | | | | SIP |
| | | | | | servers. |
| SPF | _spf | | TXT | [RFC4408] | |
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| DKI | _domainkey | | TXT | [RFC4871] | Public |
| M | | | | | key for |
| | | | | | verifying |
| | | | | | DKIM |
| | | | | | signature |
| | | | | | . |
| ADS | _adsp._domaink | | TXT | [RFC5617] | |
| P | ey | | | | |
| PKI | _PKIXREP._ldap | | SRV | [RFC4386] | LDAP PKI |
| LDA | | | | | Repositor |
| P | | | | | y |
| PKI | _PKIXREP._http | | SRV | [RFC4386] | HTTP PKI |
| HTT | | | | | Repositor |
| P | | | | | y |
| PKI | _PKIXREP._ocsp | | SRV | [RFC4386] | OCSP PKI |
| OCS | | | | | Repositor |
| P | | | | | y |
+-----+----------------+------------+-------+-----------+-----------+
Table 1: DNS Underscore SCOPE Name Registry (with initial values)
6. Security Considerations
This memo raises no security issues.
7. References
7.1. 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.
7.2. References -- Informative
[RFC0974] Partridge, C., "Mail routing and the domain system",
RFC 974, January 1986.
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[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC4386] Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key
Infrastructure: Repository Locator Service",
February 2006.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[RFC5507] Faltstrom, P., Ed. and R. Austein, Ed., "", RFC 5507,
April 2009.
[RFC5617] Sendmail, Inc., Cisco Systems, Inc., Yahoo! Inc., and
Taughannock Networks, "DomainKeys Identified Mail (DKIM)
Author Domain Signing Practices (ADSP)", August 2009.
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 of the earlier drafts.
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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