dnsop D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Standards Track November 16, 2018
Expires: May 20, 2019

DNS Scoped Data Through "Underscore" Naming of Attribute Leaves


Formally, any DNS resource record 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 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.

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This Internet-Draft will expire on May 20, 2019.

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Table of Contents

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 resource record types 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.

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, containing the specific resource record sets can be formally defined and constrained. [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.

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

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 DKIM , 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.

Discussion Venue:
Discussion about this draft should be directed to the dnsop@ietf.org mailing list.
Please remove "Discussion Venue" paragraph prior to publication.

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 BCP14 [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:

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.

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.

1.5. History

Originally different uses of underscore-based 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 [RFC2782], [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 underscore-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 underscore-based names that were reserved without coordination, which invites an eventual name-assignment collision. The remedy is this base document, which defines a registry for these names, and attempts to register all those already in use, with a companion document [attrleaf-fix] developed to direct changes to the pre-registry specifications that used underscore-based (global) node names.

2. DNS Underscore Scoped Entry Registries Function

A registry for "global" DNS node 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.

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.

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.

Examples of Underscored Names

Only global underscored names are registered in the IANA Underscore Global table. From the example, that would mean registering "_service3", "_service4", and "_authority" are registered in the IANA _service1, _service2, _service3, _service 4, and _authority.

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:

Underscore Global Registry Entry Template
{RRTYPE} _{DNS global node name} {citation for the document making the addition.}

Note to RFC Editor:
Please replace the above "{RFC Attrleaf}" text with a reference to this document's RFC number. /d

4. IANA Considerations

DNS Underscore Global Scoped Entry Registry

Per [RFC8126] IANA is requested to establish the:[IANA] is used.

This section describes actions requested of IANA. The guidance in

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.

4.2. DNS Underscore Global Scoped Entry Registry Definition

A registry entry contains:

Each RR type that is to be used with a _Node Name MUST have a separate registry entry.

4.3. Initial entries

Initial entries in the registry are:

Underscore Global Registry (initial entries)
* _example Section 4.5
NULL _ta-* {Section 4.4} [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 [ACME]
TXT _dmarc [RFC7489]
TXT _domainkey [RFC6376]
TXT _mta-sts [MTA-STS]
TXT _spf [RFC7208]
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]

Under the NULL RR, the entry _ta-* denotes all node names beginning with the string _ta-*. It does NOT refer to a DNS wildcard specification.

4.4. _ta

Under the NULL RR, the entry _ta-* denotes all node names beginning with the string _ta-*. It does NOT refer to a DNS wildcard specification.

4.5. _example

The node name _example is reserved across all RRsets

4.6. Enumservices Registrations Registry

Please add a note to the Enumservice Registrations registry with the following -- or similar -- language:

5. Guidance for Expert Review

This section provides guidance for expert review of registration requests in the DNS Underscore Global Scoped Entry Registry.

The review is for the purposes of ensuring that:

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.

7. References

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.
[RFC3921] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence", RFC 3921, DOI 10.17487/RFC3921, October 2004.
[RFC4386] Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key Infrastructure Repository Locator Service", RFC 4386, DOI 10.17487/RFC4386, February 2006.
[RFC5026] Giaretta, G., Kempf, J. and V. Devarapalli, "Mobile IPv6 Bootstrapping in Split Scenario", RFC 5026, DOI 10.17487/RFC5026, October 2007.
[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.
[RFC5518] Hoffman, P., Levine, J. and A. Hathcock, "Vouch By Reference", RFC 5518, April 2009.
[RFC6118] Hoeneisen, B. and A. Mayrhofer, "Update of Legacy IANA Registrations of Enumservices", RFC 6118, DOI 10.17487/RFC6118, March 2011.
[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 .
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 7208, April 2014.
[RFC7489] Kucherawy, M. and E. Zwicky, "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.
[RFC7566] Goix, L. and K. Li, "Enumservice Registration for 'acct' URI", RFC 7566, DOI 10.17487/RFC7566, June 2015.
[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., RFC 7929, August 2016.
[RFC7929] Wouters, P.,
[RFC8126] Cotton, M., Leiba, B. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 8126, June 2017.
[RFC8145] Wessels, D., Kumari, W. and P. Hoffman, "Signaling Trust Anchor Knowledge in DNS Security Extensions (DNSSEC)", RFC 8145, April 2017.
[RFC8162] Hoffman, P. and J. Schlyter, "Using Secure DNS to Associate Certificates with Domain Names for S​/​MIME", RFC 8162, May 2017.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC952] Harrenstien, K., Stahl, M. and E. Feinler, "DOD Internet Host Table Specification", RFC 952, October 1985.

7.2. References - Informative

[attrleaf-fix] Crocker, D., "Changes to Rationalize Underscore DNS Node Names", I-D draft-crocker-attrleaf-simplification-00, 2017.

Appendix A. 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 drafts. For the later enhancements, thanks to: Stephane Bortzmeyer, Alissa Cooper, Bob Harold, Benjamin Kaduk, Mirja Kuehlewind, Warren Kumari, John Levine, Joel Jaeggli, Benno Overeinder, Eric Rescorla, Adam Roach, Petr Špaček, Ondřej 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.

The listed names Petr Špaček, Ondřej Sury -- Petr Špaček, Ondřej Sury -- render properly in xml-to-html conversion but the production xml2rfc engine does not render it properly to text. Making the xml version match the documented form for txt will mean that the names are not properly rendered for output formats that can support extended character sets. /d

Author's Address

Dave Crocker Brandenburg InternetWorking 675 Spruce Dr. Sunnyvale, CA 94086 USA Phone: +1.408.246.8253 EMail: dcrocker@bbiw.net URI: http://bbiw.net/