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Versions: (draft-crocker-dns-attrleaf) 00 01
02 03 04 05 06 07 08 09 10 11 12 13
14 15 16 RFC 8552
dnsop D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Best Current Practice July 21, 2018
Expires: January 22, 2019
DNS Scoped Data Through "Underscore" Naming of Attribute Leaves
draft-ietf-dnsop-attrleaf-12
Abstract
Formally, any DNS resource record may occur under any domain name.
However some services have defined an operational convention, which
applies to DNS leaf nodes that are under a DNS branch having one or
more reserved node names, each beginning with an _underscore. The
underscored naming construct defines a semantic scope for DNS record
types that are associated with the parent domain, above the
underscored branch. This specification explores the nature of this
DNS usage and defines the "DNS Global Underscore Scoped Entry
Registry" with IANA. The purpose of the Underscore registry is to
avoid collisions resulting from the use of the same underscore-based
name, for different services.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on January 22, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Underscore Scoping . . . . . . . . . . . . . . . . . . . 3
1.2. Scaling Benefits . . . . . . . . . . . . . . . . . . . . 4
1.3. "Global" Underscored Node Names . . . . . . . . . . . . . 4
1.4. Interaction with DNS wildcards . . . . . . . . . . . . . 5
2. DNS Underscore Scoped Entry Registries Function . . . . . . . 5
3. RRset Use Registration Template . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. DNS Underscore Global Scoped Entry Registry . . . . . . . 7
4.2. DNS Underscore Global Scoped Entry Registry Definition . 7
4.3. Initial entries . . . . . . . . . . . . . . . . . . . . . 8
5. Guidance for Expert Review . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6.1. Interaction with DNS wildcards . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The core Domain Name System (DNS) technical specifications assign no
semantics to domain names or their parts, and no constraints upon
which resource record (RR) types are permitted to be stored under
particular names [RFC1035], [RFC2181]. 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. DNS data semantics have been
limited to the specification of particular resource record types, on
the expectation that new ones would be added as needed.
Unfortunately, the addition of new resource record types has proven
extremely challenging, over the life of the DNS, with significant
adoption and use barriers.
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1.1. Underscore Scoping
As an alternative to defining a new RR type, some DNS service
enhancements call for using an existing resource record type, but
specify a restricted scope for its occurrence. Scope is meant as a
static property, not one dependent on the nature of the query. It is
an artifact of the DNS name. That scope is a leaf node, within which
the uses of specific resource record sets can be formally defined and
constrained. The leaf occurs in a branch having a distinguished
naming convention: At the top of the branch -- beneath the parent
domain name to which the scope applies -- one or more reserved DNS
node names begin with an underscore ("_"). Because the DNS rules for
a "host" (host name) do not allow use of the underscore character,
this distinguishes the underscored name from all legal host names
[RFC952]. Effectively, this convention for leaf node naming creates
a space for the listing of "attributes" -- in the form of resource
record types -- that are associated with the parent domain, above the
underscored sub-branch.
The scoping feature is particularly useful when generalized resource
record types are used -- notably "TXT", "SRV", and "URI" [RFC1035],
[RFC2782], [RFC6335], [RFC7553]. It provides efficient separation of
one use of them from others. Absent this separation, an
undifferentiated mass of these "RRsets" is returned to the DNS
client, which then must parse through the internals of the records in
the hope of finding ones that are relevant. Worse, in some cases the
results are ambiguous because a record type might not adequately
self-identify its specific purpose. With underscore-based scoping,
only the relevant "RRsets"s are returned.
A simple example is DKIM [RFC6376] , which uses "_domainkey" for
defining a place to hold a "TXT" record containing signing
information for the parent domain.
This specification formally defines how underscored labels are used
as "attribute" enhancements for their parent domain names. For
example, domain name "_domainkey.example." acts as an attribute of
the parent domain name "example." To avoid collisions resulting from
the use of the same underscore-based labels for different
applications using the same resource record type, this document
establishes the DNS Underscore Global Scoped Entry IANA Registry.
Use of such node names, which begin with underscore, are reserved
when they are the underscored name closest to the DNS root; they are
considered "global". Underscore-based names that are farther down
the hierarchy are handled within the scope of the global underscore
name.
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Discussion Venue: Discussion about this draft should be directed
to the dnsop@ietf.org [1] mailing list.
NOTE TO RFC EDITOR: Please remove "Discussion Venue" paragraph
prior to publication.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. Scaling Benefits
Some resource record types are used in a fashion that can create
scaling problems, if an entire RRset associated with a domain name is
aggregated in the leaf node for that name. An increasingly-popular
approach, with excellent scaling properties, places the RRset under a
specially named branch, which is in turn under the node name that
would otherwise contain the RRset. The rules for naming that branch
define the context for interpreting the RRset. That is, rather than:
domain-name.example
/
RRset
the arrangement is:
_branch.domain-name.example
/
RRset
A direct lookup to the subordinate leaf node produces only the
desired record types, at no greater cost than a typical DNS lookup.
1.3. "Global" Underscored Node Names
As defined in [RFC1034] the DNS uses names organized in a tree-
structured, or hierarchical fashion. A domain name might have
multiple node names that begin with an _underscore. A "global"
underscored node name is the one that is closest to the root of the
DNS hierarchy, also called the highest-level or top-most. In the
presentation convention described in Section 3.1 of [RFC1034] this is
the right-most name beginning with an underscore. In other
presentation environments it might be positioned differently. To
avoid concern for the presentation variations, the qualifier "global"
is used here.
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1.4. Interaction with DNS wildcards
DNS wildcards interact poorly with underscored names in two ways.
Since wildcards only are interpreted as leaf names, one cannot create
the equivalent of a wildcard name for prefixed names. A name such as
label.*.example.com is not a wildcard.
Conversely, a wildcard such as *.example.com can match any name
including an underscored name. So, a wildcard might match an
underscored name, returning a record that is the type controlled by
the underscored name but is not intended to be used in the
underscored context and does not conform to its rules.
2. DNS Underscore Scoped Entry Registries Function
A registry for "global" DNS nodes names that begin with an underscore
is defined here. The purpose of the Underscore Global Registry is to
avoid collisions resulting from the use of the same underscore-based
name, for different applications.
o If a public specification calls for use of an underscore-prefixed
domain node name, the "global" underscored name -- the underscored
name that is closest to the DNS root -- MUST be entered into this
registry.
An underscored name defines scope of use for specific resource record
types, which are associated with the domain name that is the "parent"
to the branch defined by the underscored name. A given name defines
a specific, constrained context for one or more RR types, where use
of such record types conforms to the defined constraints.
o Within an underscore scoped leaf, other RRsets that are not
specified as part of the scope MAY be used.
Structurally, the registry is defined as a single, flat table of RR
types, under node names beginning with underscore. In some cases,
such as for use of an "SRV" record, the full scoping name might be
multi-part, as a sequence of underscored names. Semantically, that
sequence represents a hierarchical model and it is theoretically
reasonable to allow re-use of a subordinate underscored name in a
different, global underscored context; that is, a subordinate name is
meaningful only within the scope of the global underscored name.
Therefore they are ignored by this DNS Underscore Global Scoped Entry
Registry. This registry is for the definition of highest-level --
ie, global -- underscored node name used.
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+----------------------------+
| NAME |
+----------------------------+
| _service1 |
| ._protoB._service2 |
| _protoB._service3 |
| _protoC._service3 |
| _useX._protoD._service4 |
| _protoE._region._authority |
+----------------------------+
Examples of Underscored Names
Only global underscored names are registered in the IANA Underscore
Global table.
o The use of underscored node names is specific to each RRTYPE that
is being scoped. Each name defines a place, but does not define
the rules for what appears underneath that place, either as
additional underscored naming or as a leaf node with resource
records. Details for those rules are provided by specifications
for individual RRTYPEs. The sections below describe the way that
existing underscore labels are used with the RRTYPEs that they
name.
o Definition and registration of subordinate, underscore node names
is the responsibility of the specification that creates the global
registry entry.
That is, if a scheme using a global underscore node name has one or
more subordinate levels of underscore node naming, the namespaces
from which names for those lower levels are chosen are controlled by
the parent underscore node name. Each globally-registered underscore
name owns a distinct, subordinate name space.
3. RRset Use Registration Template
This section provides a basic template that can be used to register
new entries in the IANA DNS Underscore Global Scoped Entry Registry,
if the global underscored name above the RRTYPE is not already
registered. The text can be added to specifications using
RRTYPE/_Node-name combinations that have not already been registered.
"Per {RFC Attrleaf} please add the following entry to the DNS
Underscore Global Scoped Entry Registry:"
Note to RFC Editor: Please replace the above "{RFC Attrleaf}" text
with a reference to this document's RFC number. /d
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+----------+-------------------+------------------------------------+
| RR Type | _NODE NAME | REFERENCE |
+----------+-------------------+------------------------------------+
| {RRTYPE} | _{DNS global node | {citation for the document making |
| | name} | the addition.} |
+----------+-------------------+------------------------------------+
Table 1: Underscore Global Registry Entry
4. IANA Considerations
Per [RFC8126], IANA is requested to establish the:
DNS Underscore Global Scoped Entry Registry
This section describes actions requested of IANA. The guidance in
[IANA] is used.
4.1. DNS Underscore Global Scoped Entry Registry
The DNS Global Underscore Scoped Entry Registry is any DNS node name
that begin with the underscore character ("_", ASCII 0x5F) and is the
underscored node name closest to the root; that is it defines the
highest-level of a DNS branch, under a "parent" domain name.
o This registry is to operate under the IANA rules for "Expert
Review" registration; see Section 5.
o The contents of each entry in the Global registry are defined in
Section 4.2.
o Each entry in the registry MUST contain values for all of the
fields specified in Section 4.2.
o Within the registry, the combination of RR Type and _Node Name
MUST be unique.
o The table is to be maintained with entries sorted by the first
column (RR Type) and, within that, the second column (_Node Name).
o The required Reference for an entry MUST have a stable resolution
to the organization controlling that registry entry.
4.2. DNS Underscore Global Scoped Entry Registry Definition
A registry entry contains:
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RR Type: Lists an RR type that is defined for use within this
scope
_Node Name: Specifies a single, underscored name that defines a
reserved name; this name is the "global" entry name for the
scoped resource record types that are associated with that
name
References: Lists specification that defines a record type and its
use under this Name. The organization producing the
specification retains control over the registry entry for
the _Node Name
Each RR type that is to be used MUST have a separate registry entry.
4.3. Initial entries
Initial entries in the registry are:
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+------------+-----------------+------------+
| RR Type | _NODE NAME | REFERENCE |
+------------+-----------------+------------+
| OPENPGPKEY | _openpgpkey | [RFC7929] |
| SMIMEA | _smimecert | [RFC8162] |
| SRV | _dccp | [RFC2782] |
| SRV | _sctp | [RFC2782] |
| SRV | _tcp | [RFC2782] |
| SRV | _udp | [RFC2782] |
| TLSA | _sctp | [RFC6698] |
| TLSA | _tcp | [RFC6698] |
| TLSA | _udp | [RFC6698] |
| TXT | _mta-sts | [MTA-STS] |
| TXT | _acme-challenge | [ACME] |
| TXT | _dmarc | [RFC7489] |
| TXT | _domainkey | [RFC6376] |
| TXT | _spf | [RFC7208] |
| TXT | _vouch | [RFC5518] |
| URI | _iax | [RFC7553] |
| URI | _acct | [RFC7553] |
| URI | _dccp | [RFC7553] |
| URI | _email | [RFC7553] |
| URI | _ems | [RFC7553] |
| URI | _fax | [RFC7553] |
| URI | _ft | [RFC7553] |
| URI | _h323 | [RFC7553] |
| URI | _ical-sched | [RFC7553] |
| URI | _ical-access | [RFC7553] |
| URI | _ifax | [RFC7553] |
| URI | _im | [RFC7553] |
| URI | _mms | [RFC7553] |
| URI | _pres | [RFC7553] |
| URI | _pstn | [RFC7553] |
| URI | _sctp | [RFC7553] |
| URI | _sip | [RFC7553] |
| URI | _sms | [RFC7553] |
| URI | _tcp | [RFC7553] |
| URI | _udp | [RFC7553] |
| URI | _unifmsg | [RFC7553] |
| URI | _vcard | [RFC7553] |
| URI | _videomsg | [RFC7553] |
| URI | _voice | [RFC7553] |
| URI | _voicemsg | [RFC7553] |
| URI | _vpim | [RFC7553] |
| URI | _xmp | [RFC7553] |
+------------+-----------------+------------+
Table 2: Underscore Global Registry (initial entries)
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5. Guidance for Expert Review
This section provides guidance for expert review of registration
requests in the of DNS Underscore Global Scoped Entry Registry.
This review is solely to determine adequacy of a requested entry
in this Registry, and does not include review of other aspects of
the document specifying that entry. For example such a document
might also contain a definition of the resource record type that
is referenced by the requested entry. Any required review of that
definition is separate from the expert review required here.
The review is for the purposes of ensuring that:
o The details for creating the registry entry are sufficiently
clear, precise and complete
o The combination of the underscored name, under which the listed
resource record type is used, and the resource record type, is
unique in the table
For the purposes of this Expert Review, other matters of the
specification's technical quality, adequacy or the like are outside
of scope.
6. Security Considerations
This memo raises no security issues.
6.1. Interaction with DNS wildcards
DNS wildcards interact poorly with underscored names in two ways.
Since wildcards only are interpreted as leaf names, one cannot create
the equivalent of a wildcard name for prefixed names. A name such as
label.*.example.com is not a wildcard.
Conversely, a wildcard such as *.example.com can match any name
including an underscored name. So, a wildcard might match an
underscored name, returning a record that is the type controlled by
the underscored name but is not intended to be used in the
underscored context and does not conform to its rules.
7. References
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7.1. Normative References
[ACME] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", I-D draft-ietf-acme-acme-11, March 2018.
[IANA] M. Cotton, B. Leiba, and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", RFC 8126,
June 2017.
[MTA-STS] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", I-D draft-ietf-uta-mta-sts.
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC5518] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
Reference", RFC 5518, April 2009.
[RFC6335] Cotton, M., Eggert, L., Tpuch, J., Westerlund, M., and S.
Cheshire, "nternet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", RFC 6335, Aug
2011.
[RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", RFC 6376, Sept 2011.
[RFC6698] Hoffman, J. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August .
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[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in E-Mail, Version 1",
RFC 7208, April 2014.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, March 2015.
[RFC7553] Falstrom, P. and O. Kolkman, "The Uniform Resource
Identifier (URI) DNS Resource Record", RFC 7553,
ISSN 2070-1721, June 2015.
[RFC7929] Wouters, P., , RFC 7929, August 2016.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", RFC 8126,
June 2017.
[RFC8162] Hoffman, P. and J. Schlyter, "Using Secure DNS to
Associate Certificates with Domain Names for S/MIME",
RFC 8162, May 2017.
[RFC952] Harrenstien, K., Stahl, M., and E. Feinler, "DOD Internet
Host Table Specification", RFC 952, October 1985.
7.2. URIs
[1] mailto:dnsop@ietf.org
Appendix A. Acknowledgements
Thanks go to Bill Fenner, Dick Franks, Tony Hansen, Martin Hoffmann,
Peter Koch, Olaf Kolkman, and Andrew Sullivan for diligent review of
the (much) earlier drafts. For the later enhancements, thanks to:
Stephane Bortzmeyer, Bob Harold, Warren Kumari, John Levine, Joel
Jaeggli, Petr Špaček, Ondřej Surř, Paul Vixie,
Tim Wicinski, and Paul Wouters.
Special thanks to Ray Bellis for his persistent encouragement to
continue this effort, as well as the suggestion for an essential
simplification to the registration model.
Author's Address
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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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