Underscored DNS Names: Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves
How DNS names that start with an underscore scope a record to its parent domain, and the IANA registry that keeps them from colliding.
Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019, <https://www.rfc-editor.org/info/rfc8552>.
@misc{rfc8552,
series = {Request for Comments},
number = 8552,
howpublished = {RFC 8552},
publisher = {RFC Editor},
doi = {10.17487/RFC8552},
url = {https://www.rfc-editor.org/info/rfc8552},
author = {D. Crocker},
title = {{Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves}},
pagetotal = 15,
year = 2019,
month = mar,
} RFC text
Contents
Internet Engineering Task Force (IETF) D. Crocker
Request for Comments: 8552 Brandenburg InternetWorking
BCP: 222 March 2019
Category: Best Current Practice
ISSN: 2070-1721
Scoped Interpretation of DNS Resource Records through
"Underscored" Naming of Attribute Leaves
Abstract
Formally, any DNS Resource Record (RR) may occur under any domain
name. However, some services use an operational convention for
defining specific interpretations of an RRset by locating the records
in a DNS branch under the parent domain to which the RRset actually
applies. The top of this subordinate branch is defined by a naming
convention that uses a reserved node name, which begins with the
underscore character (e.g., "_name"). 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
"Underscored and Globally Scoped DNS Node Names" registry with IANA.
The purpose of this registry is to avoid collisions resulting from
the use of the same underscored name for different services.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8552.
Copyright Notice
Copyright (c) 2019 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
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-Based Scoping . . . . . . . . . . . . . . . . 3
1.2. Scaling Benefits . . . . . . . . . . . . . . . . . . . . 4
1.3. Global Underscored Node Names . . . . . . . . . . . . . . 4
1.4. Interaction with DNS Wildcards . . . . . . . . . . . . . 5
1.5. History . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. "Underscored and Globally Scoped DNS Node Names" Registry . . 6
3. Guidance for Registering RRset Use . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
4.1. "Underscored and Globally Scoped DNS Node Names" Registry 8
4.2. Enumservices Registrations Registry . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative References . . . . . . . . . . . . . . . . . . 12
6.2. Informative References . . . . . . . . . . . . . . . . . 15
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
The core Domain Name System (DNS) technical specifications ([RFC1035]
and [RFC2181]) 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 resource record
types would be added as needed. Unfortunately, the addition of new
resource record types has proven extremely challenging, with
significant adoption and use barriers occurring over the life of the
DNS.
1.1. Underscore-Based Scoping
As an alternative to defining a new RR TYPE, some DNS service
enhancements call for using an existing resource record type but
specifying 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 containing
the specific resource record sets that are formally defined and
constrained. Specifically:
The leaf occurs in a branch having a distinguished naming
convention: there is a parent domain name to which the scoped data
applies. The branch is under this name. The sub-branch is
indicated by a sequence of one or more reserved DNS node names; at
least the first (highest) of these names begins with an underscore
(e.g., "_name").
Because the DNS rules for a "host" (host name) do not allow use of
the underscore character, the underscored name is distinguishable
from all legal host names [RFC0952]. Effectively, this convention
for naming leaf nodes 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 are returned.
A simple example is DomainKeys Identified Mail (DKIM) [RFC6376],
which uses "_domainkey" to define a place to hold a TXT record
containing signing information for the parent domain.
This specification formally defines how underscored names are used as
"attribute" enhancements for their parent domain names. For example,
the domain name "_domainkey.example." acts as an attribute of the
parent domain name "example.". To avoid collisions resulting from
the use of the same underscored names for different applications
using the same resource record type, this document establishes the
"Underscored and Globally Scoped DNS Node Names" registry with IANA.
Use of such node names, which begin with an underscore character, is
reserved when they are the underscored name closest to the DNS root;
as in that case, they are considered "global". Underscored names
that are farther down the hierarchy are handled within the scope of
the global underscored node name.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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 the underscore character (e.g.,
"_name"). A global underscored node name is the one that is closest
to the root of the DNS hierarchy, also called the highest level or
topmost. In the presentation convention described in Section 3.1 of
[RFC1034], this is the rightmost 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.
1.4. Interaction with DNS Wildcards
DNS wildcards interact poorly with underscored names in two ways:
Since wildcards are only 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.
1.5. History
Originally, different uses of underscored node names developed
largely without coordination. For TXT records, there is no
consistent, internal syntax that permits distinguishing among the
different uses. In the case of the SRV RR and URI RR, distinguishing
among different types of use was part of the design (see [RFC2782]
and [RFC7553]). The SRV and URI specifications serve as templates,
defining RRs that might only be used for specific applications when
there is an additional specification. The template definition
included reference to two levels of tables of names from which
underscored names should be drawn. The lower-level (local scope) set
of "_service" names is defined in terms of other IANA tables, namely
any table with symbolic names. The upper-level (global scope) SRV
naming field is "_proto", although its pool of names is not
explicitly defined.
The aggregate effect of these independent efforts was a long list of
underscored names that were reserved without coordination, which
invites an eventual name-assignment collision. The remedy is this
base document and a companion document ([RFC8553]), which define a
registry for these names and attempt to register all those already in
use as well as to direct changes to the pre-registry specifications
that used global underscored node names.
2. "Underscored and Globally Scoped DNS Node Names" Registry
A registry for global DNS node names that begin with an underscore is
defined here. The purpose of the "Underscored and Globally Scoped
DNS Node Names" registry is to avoid collisions resulting from the
use of the same underscored name for different applications.
If a public specification calls for use of an underscored node
name, the global underscored node name -- the underscored name
that is closest to the DNS root -- MUST be entered into this
registry.
An underscored name defines the 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 a leaf that is underscore scoped, 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 reuse 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 node name.
Therefore, they are ignored by this "Underscored and Globally Scoped
DNS Node Names" registry. This registry is for the definition of
highest-level -- that is, global -- underscored node name used.
+----------------------------+
| NAME |
+----------------------------+
| _service1 |
| _protoB._service2 |
| _protoB._service3 |
| _protoC._service3 |
| _useX._protoD._service4 |
| _protoE._region._authority |
+----------------------------+
Table 1: Examples of Underscored Names
Only global underscored node names are registered in the "Underscored
and Globally Scoped DNS Node Names" registry. From the example
above, that would mean _service1, _service2, _service3, _service 4,
and _authority would be listed in the IANA registry.
o The use of underscored node names is specific to each RR TYPE 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 RR TYPEs. The sections below describe the way that
existing underscored names are used with the RR TYPEs that they
name.
o Definition and registration of subordinate underscored node names
are the responsibility of the specification that creates the
global underscored node name registry entry.
That is, if a scheme using a global underscored node name has one or
more subordinate levels of underscored node naming, the namespaces
from which names for those lower levels are chosen are controlled by
the parent underscored node name. Each registered global underscored
node name owns a distinct, subordinate namespace.
3. Guidance for Registering RRset Use
This section provides guidance for specification writers, with a
basic template they can use, to register new entries in the
"Underscored and Globally Scoped DNS Node Names" registry. The text
can be added to specifications using RR TYPE / _NODE NAME
combinations that have not already been registered:
Per RFC 8552, please add the following entry to the "Underscored
and Globally Scoped DNS Node Names" registry:
+---------+-------------------+-------------------------------------+
| RR Type | _NODE NAME | Reference |
+---------+-------------------+-------------------------------------+
| {RR | _{DNS global node | {citation for the document making |
| TYPE} | name} | the addition.} |
+---------+-------------------+-------------------------------------+
Table 2: Template for Entries in the
"Underscored and Globally Scoped DNS Node Names" Registry
4. IANA Considerations
IANA has established the "Underscored and Globally Scoped DNS Node
Names" registry. This section describes the registry, the
definitions, the initial entries, the use of_ta and _example, and the
use of [RFC8126] as guidance for expert review. IANA has also
updated the "Enumservices Registrations" registry with a pointer to
this document.
4.1. "Underscored and Globally Scoped DNS Node Names" Registry
The "Underscored and Globally Scoped DNS Node Names" registry
includes any DNS node name that begins 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 operates under the IANA rules for "Expert Review"
registration; see Section 4.1.5.
o The contents of each entry in the registry are defined in
Section 4.1.1.
o Each entry in the registry MUST contain values for all of the
fields specified in Section 4.1.1.
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.
4.1.1. Contents of an Entry in the "Underscored and Globally Scoped DNS
Node Names" Registry
A registry entry contains:
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. For characters in the name that have
an uppercase form and a lowercase form, the character
MUST be recorded as lowercase to simplify name
comparisons.
Reference: Lists the specification that defines a record type and
its use under this _Node Name. The organization
producing the specification retains control over the
registry entry for the _Node Name.
Each RR TYPE that is to be used with a _Node Name MUST have a
separate registry entry.
4.1.2. Initial Node Names
The initial entries in the registry are as follows:
+------------+-----------------------+---------------+
| RR Type | _NODE NAME | Reference |
+------------+-----------------------+---------------+
| * | _example | Section 4.1.4 |
| NULL | _ta-* {Section 4.1.3} | [RFC8145] |
| OPENPGPKEY | _openpgpkey | [RFC7929] |
| SMIMEA | _smimecert | [RFC8162] |
| SRV | _dccp | [RFC2782] |
| SRV | _http | [RFC4386] |
| SRV | _ipv6 | [RFC5026] |
| SRV | _ldap | [RFC4386] |
| SRV | _ocsp | [RFC4386] |
| SRV | _sctp | [RFC2782] |
| SRV | _sip | [RFC5509] |
| SRV | _tcp | [RFC2782] |
| SRV | _udp | [RFC2782] |
| SRV | _xmpp | [RFC3921] |
| TLSA | _dane | [RFC7671] |
| TLSA | _sctp | [RFC6698] |
| TLSA | _tcp | [RFC6698] |
| TLSA | _udp | [RFC6698] |
| TXT | _acme-challenge | [RFC8555] |
| TXT | _dmarc | [RFC7489] |
| TXT | _domainkey | [RFC6376] |
| TXT | _mta-sts | [RFC8461] |
| TXT | _spf | [RFC7208] |
| TXT | _sztp | [ZEROTOUCH] |
| TXT | _tcp | [RFC6763] |
| TXT | _udp | [RFC6763] |
| TXT | _vouch | [RFC5518] |
| URI | _acct | [RFC6118] |
| URI | _dccp | [RFC7566] |
| URI | _email | [RFC6118] |
| URI | _ems | [RFC6118] |
| URI | _fax | [RFC6118] |
| URI | _ft | [RFC6118] |
| URI | _h323 | [RFC6118] |
| URI | _iax | [RFC6118] |
| URI | _ical-access | [RFC6118] |
| URI | _ical-sched | [RFC6118] |
| URI | _ifax | [RFC6118] |
| URI | _im | [RFC6118] |
| URI | _mms | [RFC6118] |
| URI | _pres | [RFC6118] |
| URI | _pstn | [RFC6118] |
| URI | _sctp | [RFC6118] |
| URI | _sip | [RFC6118] |
| URI | _sms | [RFC6118] |
| URI | _tcp | [RFC6118] |
| URI | _udp | [RFC6118] |
| URI | _unifmsg | [RFC6118] |
| URI | _vcard | [RFC6118] |
| URI | _videomsg | [RFC6118] |
| URI | _voice | [RFC6118] |
| URI | _voicemsg | [RFC6118] |
| URI | _vpim | [RFC6118] |
| URI | _web | [RFC6118] |
| URI | _xmpp | [RFC6118] |
+------------+-----------------------+---------------+
Table 3: Initial Contents of the
"Underscored and Globally Scoped DNS Node Names" Registry
4.1.3. _ta
Under the NULL RR Type, the entry "_ta-*" denotes all node names
beginning with the string "_ta-*". It does NOT refer to a DNS
wildcard specification.
4.1.4. _example
The node name "_example" is reserved across all RRsets.
4.1.5. Guidance for Expert Review
This section provides guidance for expert review of registration
requests in the "Underscored and Globally Scoped DNS Node Names"
registry.
This review is solely to determine adequacy of a requested entry
in this registry, and it 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.
4.2. Enumservices Registrations Registry
The following note has been added to the "Enumservice Registrations"
registry:
When adding an entry to this registry, strong consideration should
be given to also adding an entry to the "Underscored and Globally
Scoped DNS Node Names" registry.
5. Security Considerations
This memo raises no security issues.
6. References
6.1. Normative References
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, DOI 10.17487/RFC0952,
October 1985, <https://www.rfc-editor.org/info/rfc952>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.
[RFC3921] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
RFC 3921, DOI 10.17487/RFC3921, October 2004,
<https://www.rfc-editor.org/info/rfc3921>.
[RFC4386] Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key
Infrastructure Repository Locator Service", RFC 4386,
DOI 10.17487/RFC4386, February 2006,
<https://www.rfc-editor.org/info/rfc4386>.
[RFC5026] Giaretta, G., Ed., Kempf, J., and V. Devarapalli, Ed.,
"Mobile IPv6 Bootstrapping in Split Scenario", RFC 5026,
DOI 10.17487/RFC5026, October 2007,
<https://www.rfc-editor.org/info/rfc5026>.
[RFC5509] Loreto, S., "Internet Assigned Numbers Authority (IANA)
Registration of Instant Messaging and Presence DNS SRV RRs
for the Session Initiation Protocol (SIP)", RFC 5509,
DOI 10.17487/RFC5509, April 2009,
<https://www.rfc-editor.org/info/rfc5509>.
[RFC5518] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009,
<https://www.rfc-editor.org/info/rfc5518>.
[RFC6118] Hoeneisen, B. and A. Mayrhofer, "Update of Legacy IANA
Registrations of Enumservices", RFC 6118,
DOI 10.17487/RFC6118, March 2011,
<https://www.rfc-editor.org/info/rfc6118>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[RFC7553] Faltstrom, P. and O. Kolkman, "The Uniform Resource
Identifier (URI) DNS Resource Record", RFC 7553,
DOI 10.17487/RFC7553, June 2015,
<https://www.rfc-editor.org/info/rfc7553>.
[RFC7566] Goix, L. and K. Li, "Enumservice Registration for 'acct'
URI", RFC 7566, DOI 10.17487/RFC7566, June 2015,
<https://www.rfc-editor.org/info/rfc7566>.
[RFC7671] Dukhovni, V. and W. Hardaker, "The DNS-Based
Authentication of Named Entities (DANE) Protocol: Updates
and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
October 2015, <https://www.rfc-editor.org/info/rfc7671>.
[RFC7929] Wouters, P., "DNS-Based Authentication of Named Entities
(DANE) Bindings for OpenPGP", RFC 7929,
DOI 10.17487/RFC7929, August 2016,
<https://www.rfc-editor.org/info/rfc7929>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8145] Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust
Anchor Knowledge in DNS Security Extensions (DNSSEC)",
RFC 8145, DOI 10.17487/RFC8145, April 2017,
<https://www.rfc-editor.org/info/rfc8145>.
[RFC8162] Hoffman, P. and J. Schlyter, "Using Secure DNS to
Associate Certificates with Domain Names for S/MIME",
RFC 8162, DOI 10.17487/RFC8162, May 2017,
<https://www.rfc-editor.org/info/rfc8162>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
<https://www.rfc-editor.org/info/rfc8461>.
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.
6.2. Informative References
[RFC8553] Crocker, D., "DNS Attrleaf Changes: Fixing Specifications
That Use Underscored Node Names", RFC 8553,
DOI 10.17487/RFC8553, March 2019,
<https://www.rfc-editor.org/info/rfc8553>.
[ZEROTOUCH]
Watsen, K., Abrahamsson, M., and I. Farrer, "Secure Zero
Touch Provisioning (SZTP)", Work in Progress, draft-ietf-
netconf-zerotouch-29, January 2019.
Acknowledgements
Thanks go to Bill Fenner, Dick Franks, Tony Hansen, Martin Hoffmann,
Paul Hoffman, Peter Koch, Olaf Kolkman, Murray Kucherawy, John
Levine, Benno Overeinder, and Andrew Sullivan for diligent review of
the (much) earlier draft versions. For the later enhancements,
thanks to Stephane Bortzmeyer, Alissa Cooper, Bob Harold, Joel
Jaeggli, Benjamin Kaduk, Mirja Kuehlewind, Warren Kumari, John
Levine, Benno Overeinder, Eric Rescorla, Adam Roach, Petr Spacek,
Ondrej Sury, 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
Dave Crocker
Brandenburg InternetWorking
675 Spruce Dr.
Sunnyvale, CA 94086
United States of America
Phone: +1.408.246.8253
Email: dcrocker@bbiw.net
URI: http://bbiw.net/
ISO Plain Language
On this page
§1Why Underscored Names Exist
The core DNS specifications assign no semantics to domain names or their parts, and set no constraints on which resource record types may be stored under which names. Over time some leaf names, such as www and ftp, have come to imply support for particular services, but that is operational convention rather than defined protocol semantics. This freedom has allowed a wide range of administrative and semantic policies to run side by side.
DNS data semantics have been limited to the specification of particular resource record types, on the expectation that new types would be added as needed. Adding a new type has proved extremely challenging, with significant adoption and use barriers over the life of the DNS.
The underscored naming convention is the alternative: instead of a new record type, a service uses an existing record type but places it in a restricted scope. The scoped data sits under a leaf node whose parent is the domain the data actually applies to. The branch under that parent is indicated by a sequence of one or more reserved node names, and at least the first, that is the highest, of those names begins with an underscore.
The DNS rules for a host name do not allow the underscore character, so an underscored name is distinguishable from every legal host name. The convention therefore creates a space for listing attributes, in the form of resource record types, that belong to the parent domain above the underscored branch.
The convention is particularly useful with generalised record types such as TXT, SRV and URI. It separates one use of them from others. Without that separation, an undifferentiated mass of records is returned to the client, which then has to parse through all of them hoping to find the relevant ones, and in some cases the results are ambiguous because a record type does not adequately identify its own purpose. With underscore-based scoping, only the relevant record sets come back.
The DNS deliberately says nothing about what names mean, so services cannot tell each other apart by name alone. Instead, a service hangs its records under a label that starts with an underscore, a character no host name can contain, and that makes the records easy to separate and to look up.
- The DNS gives names no meaningThe core specifications set no constraints on which record types may be stored under which names.
- New record types are hard to addAdding a type has proved extremely challenging, with significant adoption and use barriers.
- A character host names cannot useThe underscore cannot appear in a legal host name, so an underscored name is always distinguishable.
- Separation for the clientWithout scoping, an undifferentiated mass of records comes back and the client must parse all of it.
§1.1How Underscore-Based Scoping Works
Scope here means a static property of the DNS name, not something that depends on the nature of the query. The scope is a leaf node holding the resource record sets that are formally defined and constrained.
The example given is DomainKeys Identified Mail, which uses _domainkey to define a place to hold a TXT record containing signing information for the parent domain. The name _domainkey.example. acts as an attribute of the parent domain name example..
The convention is also about scale. Some record types are used in a way that creates scaling problems when the whole record set for a domain is gathered at the leaf node for that name. Instead of putting the records at the node that would otherwise hold them, an increasingly popular approach places them under a specially named branch beneath that node. The rules for naming that branch define the context for interpreting the record set. A direct lookup at the subordinate leaf node then produces only the desired record types, at no greater cost than a typical DNS lookup.
Instead of putting a service's records at example.com, they go at _domainkey.example.com. A client that asks for that name gets the service's records and nothing else, and it costs one ordinary lookup.
- example The parent domain the attribute belongs to.
- _domainkey The global underscored node name, registered with IANA.
- TXT record Signing information for the parent domain.
- _domainkey The global underscored node name, registered with IANA.
§1.3Global Underscored Node Names
The DNS uses names organised in a tree. A domain name might have several node names beginning with an underscore. The global underscored node name is the one closest to the root of the DNS hierarchy, the highest or topmost. In the presentation convention of Section 3.1 of RFC 1034 this is the rightmost name that begins with an underscore; in other presentation environments it may sit elsewhere, which is why the word global is used.
To avoid collisions between services using the same underscored name with the same record type, the document establishes the Underscored and Globally Scoped DNS Node Names registry with IANA. Use of node names beginning with an underscore is reserved when the underscored name is the one closest to the DNS root, in which case it is considered global. Underscored names farther down the hierarchy are handled within the scope of the global underscored node name.
A name may have several underscore labels in it. The registry only records the topmost one, because that is the part that has to be unique across the whole Internet. The lower parts are the business of whoever registered the top one.
| Arrangement | Where the RRset lives | What the client gets |
|---|---|---|
| Unscoped | At the domain name itself | Every record of that type at the domain |
| Underscore scoped THIS RFC | Below a branch named with an underscore | Only the records of the service that owns the name |
§1.4Wildcards and Underscored Names
DNS wildcards interact badly with underscored names in two ways. Since wildcards are only 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 and return a record of the type that the underscored name controls but that is not intended for the underscored context and does not conform to its rules.
The document also records the history. Different uses of underscored node names developed largely without coordination. For TXT records there is no consistent internal syntax that lets one tell the different uses apart. For the SRV and URI records, telling the different uses apart was part of the design, and those specifications serve as templates defining records that might only be used for specific applications when there is an additional specification. The SRV template referred to two levels of tables of names, a local scope set of _service names and a global scope field of _proto. The aggregate effect of these independent efforts was a long list of underscored names reserved without coordination, which invites an eventual name-assignment collision. The remedy is this document and a companion one, RFC 8553, which define a registry for the names and try to register those already in use.
Wildcards and underscored names do not mix well. A wildcard matches any name, so it can hand back a record that belongs to the underscored branch without following the rules of that branch. The document also explains how the collection of underscored names grew up without coordination, which is why a registry is needed.
§2The Registry and Its Rules
If a public specification calls for the use of an underscored node name, the global underscored node name, the one closest to the DNS root, MUST be entered into the registry.
An underscored name defines the scope of use for specific record types, which are associated with the domain name that is the parent of the branch the underscored name defines. A given name defines a specific, constrained context for one or more record types, and use of those types inside it conforms to the defined constraints. Within a leaf that is underscore scoped, other record sets that are not part of the scope MAY be used.
Structurally the registry is a single, flat table of record types under node names beginning with an underscore. In some cases the full scoping name is multi-part, a sequence of underscored names, and semantically that sequence represents a hierarchical model. It is reasonable to reuse a subordinate underscored name in a different global context, because a subordinate name is meaningful only within the scope of the global underscored node name, so subordinate names are ignored by this registry. The registry is for the definition of the highest-level, global, underscored node name.
The use of underscored node names is specific to each record type being scoped. Each name defines a place, but does not define the rules for what appears underneath that place, either as further underscored naming or as a leaf node with resource records. Those rules come from the specifications for individual record types. Definition and registration of subordinate underscored node names are the responsibility of the specification that creates the global registry entry, so each registered global underscored node name owns a distinct subordinate namespace.
The registry includes any DNS node name that begins with the underscore character, ASCII 0x5F, and is the underscored node name closest to the root. It operates under the IANA rules for Expert Review. Every entry MUST contain values for all the fields, and the combination of record type and node name MUST be unique. The table is maintained sorted by record type and then by node name. The required reference for an entry must have a stable resolution to the organisation controlling that entry.
An entry holds three fields. The record type lists a type defined for use within the scope. The node name specifies a single underscored name that defines a reserved name, the global entry name for the scoped record types associated with it, and characters that have an upper and a lower case form MUST be recorded in lower case to simplify name comparisons. The reference lists the specification that defines the record type and its use under the node name, and the organisation producing that specification retains control of the registry entry.
Each record type used with a node name MUST have a separate registry entry. The document also gives specification writers a template for requesting a new entry, with the record type, the node name and the citation.
The registry has one row for each pair of record type and underscored name, and the pair must be unique. The record is lower case. The reference field points at the document that defines the use, and whoever wrote that document keeps control of the entry.
- _domainkeyTXT — Signing information for the parent domain, from RFC 6376.
- _dmarcTXT — Email domain policy, from RFC 7489.
- _mta-stsTXT — Mail transport security policy, from RFC 8461.
- _tcpSRV — A TCP service, from RFC 2782.
- _sipSRV — A SIP service, from RFC 5509.
- _tcpTLSA — A TLS certificate association, from RFC 6698.
- _daneTLSA — DNSSEC-based authentication of named entities, from RFC 7671.
- _emailURI — An email contact, from RFC 6118.
- _ta-*NULL — All node names beginning with that string, from RFC 8145.
- _exampleAny — Reserved across all RRsets.
§4IANA Considerations
IANA has established the Underscored and Globally Scoped DNS Node Names registry. The section describes the registry, the definitions, the initial entries, the use of _ta and _example, and the use of RFC 8126 as guidance for expert review. IANA has also updated the Enumservices Registrations registry with a pointer to this document.
Two special entries are explained. Under the NULL record type, the entry _ta-* denotes all node names beginning with the string _ta-*; it does not refer to a DNS wildcard specification. The node name _example is reserved across all record sets.
The initial contents include _example for all record types, _ta-* for NULL, _openpgpkey for OPENPGPKEY, _smimecert for SMIMEA, and a set of SRV names such as _dccp, _http, _ipv6, _ldap, _ocsp, _sctp, _sip, _tcp, _udp and _xmpp. For TLSA there are _dane, _sctp and _udp. For TXT there are _acme-challenge, _dmarc, _domainkey, _mta-sts, _spf, _sztp, _tcp, _udp and _vouch. For URI there is a long list of names covering the many telephone and messaging services, such as _acct, _email, _fax, _h323, _sip, _sms, _web and _xmpp.
The registry starts with the names already in use, so that existing services keep working and new ones can check whether a name is taken.
§5Security Considerations
This memo raises no security issues. It defines a naming convention and a registry for it, and does not change how records under those names are published.
The document raises no security questions of its own.