DNS Scoped Data Through '_Underscore' Naming of Attribute Leaves
draft-ietf-dnsop-attrleaf-05

Abstract

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 records 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 September 26, 2018.

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Table of Contents

• 1. Introduction
• 1.1. _Underscore Scoping
• 1.2. Scaling Benefits for TXT, SRV, and URI Resource Records
• 2. DNS Underscore Scoped Entry Registries Function
• 2.1. DNS Underscore Global Scoped Entry Registry Definition
• 3. IANA Considerations
• 3.1. DNS Underscore Global ScopedEntry Registry
• 4. Security Considerations
• 5. References
• 5.1. Normative References
• 5.2. References -- Informative
• Appendix A. Acknowledgements
• Author's Address

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],[RFC6335], [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 , 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, 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.

NOTE TO RFC EDITOR:

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 under a node with an underscore-based name, at a defined place in the DNS tree, so as to constrain the use of particular RRs farther down the branch with 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 top-most names used for RRs; 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 'global' (right-most) node name that uses an _underscore prefix MUST be entered into this registry.

The names define scope of use for specific resource records, which are associated with the domain name that is the "parent" to the branch defined by the _underscore naming.

• A given name defines a specific, constrained context for one or more RR records, in which use of such records MUST conform to the defined constraints. Within this scope, other resource records that are not specified MAY be used.

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. 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 first (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.

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

• Definition and registration of the subordinate underscore node names is the responsibility of the specification that creates the highest-level (right-most) global registry entry.
• That is, if a scheme using a global underscore node name also has one or more subordinate levels of underscore node naming, the namespaces from which names for those lower levels is chosen is controlled by the parent underscore node name. Each globally-registered underscore name owns a distinct, subordinate name space.

2.1. DNS Underscore Global Scoped Entry Registry Definition

A registry entry contains:

_Node Name:

Specifies a single _underscore name that defines a reserved name; this name is the "global" entry name for the scoped resource records that are associated with that name.

RR(s):

Lists the RRs that are defined for use within this scope.

References

Lists specification that define the records and their use under this Name. The organization producing the specification retains control over the registry entry for the _Node Name.

3. IANA Considerations

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

• DNS Underscore Global Scoped Entry Registry

This section describes actions requested of IANA. The guidance in

3.1. DNS Underscore Global ScopedEntry Registry

The DNS Global Underscore Scoped Entry Registry is for DNS node names that begin with the underscore character (_) and are the first 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.

• This registry is to operate under the IANA rules for "First Come First Served" registration.
• The contents of each entry in the Global registry are defined in Section 2.1.
• The required Reference for an entry MUST have a stable resolution to the organization controlling that registry entry

Initial entries in the registry are:

4. Security Considerations

This memo raises no security issues.

5. References

5.1. Normative References

5.2. References -- Informative

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