RFC 8552 BEST CURRENT PRACTICE DNS · NAMING · REGISTRIES
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.
RFC EDITOR
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>.
BIBTEX
@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/
AI ELI5
On this page
PROBLEMThe DNS lets any record type sit under any name, so services that reuse generic types such as TXT, SRV and URI have no clean way to separate one use from another, and independent uses of the same underscored name could collide.
MECHANISMRecords are placed under a reserved node name that begins with an underscore, beneath the domain they apply to. The underscored name closest to the root is recorded in a new IANA registry, paired with the record type.
WHY IT MATTERSA lookup of the underscored name returns only the relevant records, and the registry avoids name collisions. The RFC is a Best Current Practice and has not been obsoleted or updated.
§1Why DNS Needs Scoped Names
The core DNS specifications, RFC 1035 and RFC 2181, assign no meaning to domain names or their parts. They also put no limits on which resource record (RR) types can be stored under which names. A name such as www or ftp may suggest a service, but that is an operational habit, not protocol meaning.
Adding a new RR type has proven extremely challenging, with significant adoption and use barriers. So some service designers reuse an existing type and restrict where it may appear. That restriction is the idea this RFC describes and formalizes.
The DNS lets you put any kind of note under any name, so notes for different jobs get mixed up. Making a brand new kind of note is slow and hard, so people look for a tidier trick.
§1.1Underscore-Based Scoping
Instead of defining a new RR type, a service can use an existing type but place it in a scoped leaf. Scope here is a static property of the DNS name, not something that depends on the query. The leaf sits in a branch under the parent domain that the data applies to, and the branch is marked by one or more reserved node names. At least the first (highest) of them begins with an underscore, such as _name.
Host names cannot contain an underscore (RFC 952), so an underscored name can never be confused with a legal host name. The result is a space for "attributes", in the form of RR types, that belong to the parent domain. For example, _domainkey.example. is an attribute of example.. DKIM (RFC 6376) uses _domainkey to hold a TXT record with signing information for the parent domain.
This helps most with general-purpose types: TXT, SRV and URI. Without scoping, a client gets back an undifferentiated mass of these records and must parse them to find the relevant ones, and sometimes the result is ambiguous. With scoping, only the relevant RRsets are returned.
Think of putting a labeled folder inside a bigger folder. A folder named with an underscore can only be a special folder, never a normal name. When you open it, you find only the papers meant for that one job.
§1.2Scaling and Global Names
Some RR types cause scaling problems if the whole RRset for a name is collected in one leaf. The alternative described here puts the RRset under a specially named branch, so it lives at _branch.domain-name.example rather than at domain-name.example. A direct lookup of that subordinate leaf returns only the desired record types, at no greater cost than a typical DNS lookup.
A domain name can have several node names that begin with an underscore. The one closest to the root, the topmost, is the global underscored node name. In the usual presentation of a name, this is the rightmost underscored label. Names lower down are handled within the scope of the global one.
A lookup can go straight to the one small folder you need instead of dumping out the whole cabinet. The folder name closest to the top of the cabinet is the one that must be unique across everyone.
- example. parent domain the data applies to
- _domainkey.example. global underscored node name
- TXT RRset signing information for the parent domain
- _domainkey.example. global underscored node name
§1.5History and Wildcard Problems
Different uses of underscored names grew up without coordination. TXT records have no consistent internal syntax for telling uses apart. The SRV (RFC 2782) and URI (RFC 7553) records were designed to distinguish uses, and act as templates that need a further specification for each application. SRV names come from two levels: a lower-level _service and an upper-level _proto, whose pool of names was not explicitly defined.
The combined effect was a long list of uncoordinated reserved names, which invites an eventual name-assignment collision. The remedy is this document and a companion, RFC 8553, which define the registry and try to register names already in use.
DNS wildcards also interact poorly with underscored names. Wildcards apply only to leaf names, so there is no equivalent of a wildcard for prefixed names; label.*.example.com is not a wildcard. Conversely, *.example.com can match an underscored name and return a record of the controlled type that was never meant for that context.
For a long time, many groups made up their own special folder names without checking with each other, so two could have picked the same one. The fix is a shared list. Also, the catch-all trick in DNS can accidentally hand out the wrong thing from a special folder.
§2The Underscored Node Names Registry
The "Underscored and Globally Scoped DNS Node Names" registry exists to avoid collisions when the same underscored name is used for different applications. The rule is: if a public specification calls for an underscored node name, the global one, closest to the DNS root, MUST be entered in the registry.
Only global names are listed. As the RFC's example, from names like _service1, _protoB._service2 and _protoE._region._authority, the registry would list _service1, _service2, _service3, _service4 and _authority. Each registered global name owns a distinct subordinate namespace, and the specification that creates the entry is responsible for defining and registering the lower-level names. Other RRsets that are not part of the scope MAY still be used in an underscore-scoped leaf.
Each entry has three fields: the RR Type, the _NODE NAME (recorded in lowercase) and a Reference to the specification. The combination of RR Type and name MUST be unique, and every RR type used with a name needs its own entry. The organization producing the referenced specification keeps control over the entry. Section 3 gives specification writers a template: "Per RFC 8552, please add the following entry to the registry".
The shared list has three columns: what kind of note, which folder name, and who wrote the rules. The same pair can only appear once, and the folder name is always written in small letters so people compare names the same way.
| Field | What it holds |
|---|---|
| RR Type | An RR TYPE defined for use within this scope |
| _NODE NAME | A single underscored global name, recorded in lowercase |
| Reference | The specification defining the type and its use under the name |
§4.1Initial Entries and Expert Review
The registry starts with entries drawn from existing specifications. Examples in the RFC's table include TXT with _dmarc, _domainkey, _spf and _acme-challenge; TLSA with _tcp and _udp; SRV with _tcp, _udp and _sip; and many URI entries from RFC 6118. Two are special. The NULL entry _ta-* means every name that begins with the string _ta-, and does not refer to a DNS wildcard. The name _example is reserved across all RRsets.
New entries use Expert Review. The review only checks that the details for the entry are sufficiently clear, precise and complete, and that the combination of underscored name and RR type is unique in the table. Other questions of technical quality are out of scope.
The RFC also adds a note to the "Enumservice Registrations" registry: when adding an entry there, strong consideration should be given to also adding one to the underscored-names registry.
The list started with names people were already using. When someone asks to add a new one, a reviewer only checks that the request is clear and that nobody else has the same pair, not whether the idea is a good one.
§5Security Considerations
The RFC's security section is a single sentence: this memo raises no security issues. The related risk worth noting from the body of the text is the wildcard interaction above, where a wildcard record can match an underscored name and return data that does not follow that name's rules.
The authors say this list does not add new dangers. The only trap they point out is that a catch-all entry might answer for a special folder by mistake.
§8References
[RFC1034]RFC 1034Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.November 1987[RFC1035]RFC 1035Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.November 1987[RFC2181]RFC 2181Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997.July 1997[RFC2782]RFC 2782Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.February 2000[RFC6118]RFC 6118Hoeneisen, B. and A. Mayrhofer, "Update of Legacy IANA Registrations of Enumservices", RFC 6118, March 2011.March 2011[RFC6335]RFC 6335Cotton, M., et al., "IANA Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, August 2011.August 2011