dnsop D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Best Current Practice J. Levine
Expires: September 22, 2018 Taughannock Networks
March 21, 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 that applies to DNS leaf nodes that are under a DNS branch that has one or more reserved node names that begin with an underscore. The underscore naming construct defines a semantic scope for DNS records that are associated with the parent domain, above the underscored branch. This 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 records (RRs) are permitted to be associated with particular names.[RFC1035] 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 records, on the expectation that new ones would be added as needed. Unfortunately, the addition of new resource records has proved extremely challenging, over the life of the DNS, with significant adoption and use barriers.

1.1. _Underscore Scoping

As an alternative to defining new RRs, some DNS service enhancements call for using an existing resource record, but specify a restricted scope for its occurrence. That scope is a leaf node, within which the uses of specific resource records 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 [RFC1035]. Effectively, this convention for leaf node naming creates a space for the listing of 'attributes' -- in the form of resource records -- that are associated with the parent domain, above the underscore sub-branch.

The scoping feature is particularly useful when generalized resource records are used -- notably TXT, SRV, and URI [RFC1035],[RFC2782],[RFC7553]. It provides efficient separation of one use of them from others. Absent this separation, an undifferentiated mass of these RRs 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 the records do not adequately self-identify. With underscore-based scoping, only the relevant RRs are returned.

A simple example is DKIM [RFC6376] , which uses "_domainkey" for defining a place to hold a TXT record containing signing information for the parent domain.

This specification formally defines how 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, 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 for TXT, SRV, and URI Resource Records

Some resource records are generic and support a variety of uses. Each additional use defines its own rules and, possibly, its own internal syntax and node-naming conventions to distinguish among particular types. The TXT, SRV, and URI records are notable examples. Their use can scale poorly, particularly when the same RR can be present in the same leaf node, but with different uses.

An increasingly-popular approach, with excellent scaling properties, place the RR undr a node wit an underscore-based name, at a defined place in the DNS tree, so as to constrain to the use of particular RRs farther down the branch using that name. This means that a direct lookup produces only the desired records, at no greater cost than a typical DNS lookup.

The definition of a underscore global registry, provided in this specification, primarily attends to the "upper-level" names used for RRs; that is the _underscore "global" names. For efficiency, a single, subordinate _underscore second-level table also is defined, for second level names that are not otherwise registered.

2. DNS Underscore Scoped Entry Registries Function

A global registry for DNS nodes names that begin with an _underscore is defined here. The names are used to define scope of use for specific resource records, 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 names that begin 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. The registry identifies the place where the relevant second level names are defined.

Example of Underscore Names

Only the right-most names are registered in the IANA Underscore Global table. Definition and registration of the subordinate names is the responsibility of the specification that creates the highest-level (right-most) registry entry.

For convenience, an Underscore Common Second-Level Names table is also defined, to cover some second level names that are not handled by existing registries.

2.1. DNS Underscore Global Scoped Entry Registry Definition

A registry entry contains:

2.2. DNS Common Second-Level Underscore Scoped Entry Registry Definition

A registry entry contains:

3. IANA Considerations

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

  1. DNS Underscore Global Scoped Entry Registry
  2. DNS Underscore Common Second-Level Scoped Entry Registry

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 occur at the "top" of a DNS branch -- ie, are right-most -- under a "parent" domain name. Section 2.1.

The contents of each entry in the Global registry are defined in

Initial entries in the registry are:

Underscore Global Registry (initial entries)
SRV TCP _tcp SRV Ports and Services registry [RFC6335] Use of SRV for a TCP-based service
SRV UDP _udp SRV Ports and Services registry [RFC6335] Use of SRV for a UDP-based service
SRV SCTP _sctp SRV Ports and Services registry [RFC6335] Use of SRV for a SCTP-based service
URI ICAL-ACCESS _ical-access URI Enumservices registry, subtypes [RFC6335] Use of URI for an enum service
SPF _spf TXT None [RFC7208] Authorized IP addresses for sending mail
DKIM _domainkey TXT Second level registry [RFC6376] Public key for verifying DKIM signature.
VBR _vouch TXT None [RFC5518] Vouch-by-refererence domain assertion

3.2. DNS Common Second-Level Underscore Scoped Entry Registry

A DNS Common Second-Level Underscore Scoped Entry Registry is for DNS node names that begin with the underscore character (_) and occur immediately below a Global ("top-level") node name beginning with an _underscore. Section 2.2.

The contents of each entry in the Common, Second-Level registry are defined in

Initial entries in the registry are:

Underscore 2d-Level Registry (initial entries)
ADSP _adsp TXT [RFC5617] ADSP policy for _domainkey

4. Security Considerations

This memo raises no security issues.

5. References

5.1. Normative References

[RFC8126] Cotton, M., Leiba, B. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 8126, June 2017.

5.2. References -- Informative

[IANA] M. Cotton, , B. Leiba, and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", I-D draft-leiba-cotton-iana-5226bis-11, 2017.
[RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.
[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.
[RFC5617] Allman, E., Fenton, J., Delany, M. and J. Levine, "DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617, August 2009.
[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.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, Sept 2011.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 7208, April 2014.
[RFC7553] Falstrom, P. and O. Kolkman, "The Uniform Resource Identifier (URI) DNS Resource Record", RFC 7553, ISSN 2070-1721, June 2015.

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.

Authors' Addresses

Dave Crocker Brandenburg InternetWorking 675 Spruce Dr. Sunnyvale, CA 94086 USA Phone: +1.408.246.8253 EMail: dcrocker@bbiw.net URI: http://bbiw.net/
John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 EMail: standards@taugh.com