dnsop D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Best Current Practice March 31, 2018
Expires: October 2, 2018

DNS Scoped Data Through '_Underscore' Naming of Attribute Leaves


Formally, any DNS resource record may occur for any domain name. However some services have defined an operational convention, which applies to DNS leaf nodes that are under a DNS branch having one or more reserved node names, each beginning with an underscore. The underscore 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

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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 ones would be added as needed. Unfortunately, the addition of new resource record types has proven extremely challenging, over the life of the DNS, with significant adoption and use barriers.

1.1. _Underscore Scoping

As an alternative to defining a new RR type, some DNS service enhancements call for using an existing resource record type, but specify a restricted scope for its occurrence. Scope is meant as a static property, not one dependent on the nature of the query. It is an artifact of the DNS name. That scope is a leaf node, within which the uses of specific resource record sets can be formally defined and constrained. The leaf occurs in a branch having a distinguished naming convention: At the top of the branch -- beneath the parent domain name to which the scope applies -- one or more reserved DNS node names begin with an underscore ("_"). Because the DNS rules for a "host" (host name) are not allowed to use the underscore character, this distinguishes the underscore name from all legal host names [RFC952]. Effectively, this convention for leaf node naming creates a space for the listing of 'attributes' -- in the form of resource record types -- that are associated with the parent domain, above the underscored sub-branch.

The scoping feature is particularly useful when generalized resource record types are used -- notably TXT, SRV, and URI [RFC1035], [RFC2782], [RFC6335], [RFC7553]. It provides efficient separation of one use of them from others. Absent this separation, an undifferentiated mass of these RRsets is returned to the DNS client, which then must parse through the internals of the records in the hope of finding ones that are relevant. Worse, in some cases the results are ambiguous because a record type might not adequately self-identify. With underscore-based scoping, only the relevant RRsetss are returned.

A simple example is DKIM , which uses "_domainkeys" for defining a place to hold a TXT record containing signing information for the parent domain.

This specification formally defines how underscore labels are used as "attribute" enhancements for their parent domain names. For example, domain name "_domainkey.example." acts as attribute of 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 DNS Underscore Global Scoped Entry IANA Registry for the highest-level reserved names that begin with _underscore; _underscore-based names that are farther down the hierarchy are handled within the scope of the highest-level _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.

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 node having an underscore-based name, at a defined place in the DNS tree under the 'parent' name. This constrains the use of particular RR types associated with that parent name. A direct lookup to the subordinate leaf node produces only the desired record types, at no greater cost than a typical DNS lookup.

The definition of a underscore global registry, provided in this specification, primarily attends to the top-most names used for scoping an RR type; that is the _underscore "global" names.

2. DNS Underscore Scoped Entry Registries Function

A global registry for DNS nodes names that begin with an _underscore is defined here.

The names define 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 _underscore naming.

The purpose of the Underscore Global Registry is to avoid collisions resulting from the use of the same _underscore-based name, for different applications.

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 underscore names. Semantically, that sequence represents a hierarchical model and it is theoretically reasonable to allow re-use of a subordinate underscore name in different underscore context; that is, a subordinate name is meaningful only within the scope of the right-most (top-level) underscore name. Therefore they are ignored by this DNS Underscore Global Scoped Entry Registry. This registry is for the definition of highest-level -- ie, global -- underscore node name used.

Example of Underscore Names

Only the right-most _underscore names are registered in the IANA Underscore Global table.

2.1. DNS Underscore Global Scoped Entry Registry Definition

A registry entry contains:

RR Type:
Lists an RR type that is defined for use within this scope.
_Node Name:
Specifies a single _underscore 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
Lists specification that defines a record type and its use under this Name. The organization producing the specification retains control over the registry entry for the _Node Name.

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

3. IANA Considerations

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

This section describes actions requested of IANA. The guidance in

3.1. DNS Underscore Global Scoped Entry Registry

The DNS Global Underscore Scoped Entry Registry is for DNS node names that begin with the underscore character (_) and are the right-most occurrence of any names in a domain name sequence having that form; that is they are the "top" of a DNS branch and are shown as the right-most _underscore name -- under a "parent" domain name.

Initial entries in the registry are:

Underscore Global Registry (initial entries)
OPENPGPKEY _openpgpkey [RFC7929]
SMIMEA _smimecert [RFC8162]
SRV _dccp [RFC2782]
SRV _sctp [RFC2782]
SRV _tcp [RFC2782]
SRV _udp [RFC2782]
TLSA _sctp [RFC6698]
TLSA _tcp [RFC6698]
TLSA _udp [RFC6698]
TXT _acme-challenge [ACME]
TXT _domainkey [RFC6376]
TXT _dmarc [RFC7489]
TXT _spf [RFC7208]
TXT _vouch [RFC5518]
URI _dccp [RFC7553]
URI _sctp [RFC7553]
URI _tcp [RFC7553]
URI _udp [RFC7553]

3.2. Guidance for Expert Review

This section provides guidance for expert review of registration requests in the of 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.

4. Security Considerations

This memo raises no security issues.

5. 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.
[RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.
[RFC5518] Hoffman, P., Levine, J. and A. Hathcock, "Vouch By Reference", RFC 5518, April 2009.
[RFC6335] Cotton, M., Eggert, L., Tpuch, J., Westerlund, M. and S. Cheshire, "nternet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", RFC 6335, Aug 2011.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, Sept 2011.
[RFC6698] Hoffman, J. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, August .
[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., 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.
[RFC8162] Hoffman, P. and J. Schlyter, "Using Secure DNS to Associate Certificates with Domain Names for S​/​MIME", RFC 8162, May 2017.
[RFC952] Harrenstien, K., Stahl, M. and E. Feinler, "DOD Internet Host Table Specification", RFC 952, October 1985.

Appendix A. Acknowledgements

Thanks go to Bill Fenner, Tony Hansen, Peter Koch, Olaf Kolkman, and Andrew Sullivan for diligent review of the (much) earlier drafts. For the later enhancements, thanks to: Stephane Bortzmeyer, Bob Harold, John Levine, Joel Jaeggli, Petr Špaček, Ondřej Surř, Tim Wicinski, and Paul Wouters.

Special thanks to Ray Bellis for more than 12 years of 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 USA Phone: +1.408.246.8253 EMail: dcrocker@bbiw.net URI: http://bbiw.net/