dnsop D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Best Current Practice July 15, 2018
Expires: January 16, 2019

DNS Scoped Data Through 'underscore' Naming of Attribute Leaves


Formally, any DNS resource record may occur under 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 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

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This Internet-Draft will expire on January 16, 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 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) do not allow use of the underscore character, this distinguishes the underscored 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 its specific purpose. With underscore-based scoping, only the relevant RRsetss 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.

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 or [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.

2. DNS Underscore Scoped Entry Registries Function

A registry for 'global' DNS nodes 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 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 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.

Example of Underscore Names

Only global underscored names are registered in the IANA Underscore Global table.

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.

Note to RFC Editor:
Please replace the above "{RFC Attrleaf}" text with a reference to this document's RFC number. /d
Underscore Global Registry Entry
{RRTYPE} _{DNS global node name} {citation for the document making the addition.}

4. IANA Considerations

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 (_) 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:

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
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.

4.3. Initial entries

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 _mta-sts [MTA-STS]
TXT _acme-challenge [ACME]
TXT _dmarc [RFC7489]
TXT _domainkey [RFC6376]
TXT _spf [RFC7208]
TXT _vouch [RFC5518]
URI _iax [RFC7553]
URI _acct [RFC7553]
URI _dccp [RFC7553]
URI _email [RFC7553]
URI _ems [RFC7553]
URI _fax [RFC7553]
URI _ft [RFC7553]
URI _h323 [RFC7553]
URI _ical-sched [RFC7553]
URI _ical-access [RFC7553]
URI _ifax [RFC7553]
URI _im [RFC7553]
URI _mms [RFC7553]
URI _pres [RFC7553]
URI _pstn [RFC7553]
URI _sctp [RFC7553]
URI _sip [RFC7553]
URI _sms [RFC7553]
URI _tcp [RFC7553]
URI _udp [RFC7553]
URI _unifmsg [RFC7553]
URI _vcard [RFC7553]
URI _videomsg [RFC7553]
URI _voice [RFC7553]
URI _voicemsg [RFC7553]
URI _vpim [RFC7553]
URI _xmp [RFC7553]

5. 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.

6. Security Considerations

This memo raises no security issues.

6.1. 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.

7. 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 SSpecification", 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, Dick Franks, Tony Hansen, Martin Hoffmann, 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, Warren Kumari, John Levine, Joel Jaeggli, Petr Špaček, Ondřej Surř, 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 USA Phone: +1.408.246.8253 EMail: dcrocker@bbiw.net URI: http://bbiw.net/