DNS Scoped Data Through Global '_Underscore' Naming of Attribute Leaves
draft-ietf-dnsop-attrleaf-02

Abstract

Formally, any DNS RR may occur for any domain name. However some services have defined an operational convention that applies to DNS leaf nodes that have a reserved node name, beginning with an underscore. The underscore construct is used to define a semantic scope for DNS records that are associated with the parent domain. This specification explores the nature of this DNS usage and defines the "DNS Global Underscore Scoped Entry Registry" registry with IANA.

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 September 30, 2017.

Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Scaling Benefits and TXT and SRV Resource Records
• 3. DNS Global Underscore Scoped Entry Registry Function
• 4. DNS Global Underscore Scoped Entry Registry Definition
• 5. IANA Considerations
• 6. Related and Updated Registries
• 7. Security Considerations
• 8. References
• 8.1. Normative References
• 8.2. References -- Informative
• Appendix A. Acknowledgements
• Author's Address

1. Introduction

The core 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. 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. Data semantics have been limited to the specification of particular resource records, on the expectation that new ones would be added as needed.

As an alternative to defining new RRs, some DNS service enhancements reuse an existing resource record, but have specified 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 has a distinguished naming convention: It uses a reserved DNS node name that begins 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 attributes that are associated with the parent domain, one level up.

One example is the SRV record [RFC2782] which generalizes concepts long-used for email routing by the MX record [RFC0974][RFC5321]. An equivalent usage to SRV is the URI RR [RFC7553]. Relying on special DNS names has significant benefits and detriments. Some of these are explored in [RFC5507].

[Comment]:

The terms "resolution context" and "scoping rules" have been suggested, in place of "semantic scope". In order to avoid concern for matters of semantics, this specification uses the term "scoping rules", to create a focus on the mechanics being defined, rather than nuances of interpretation for the mechanism.

The scoping feature is particularly useful when generalized resource records are used -- notably TXT, SRV and URI. 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.

This specification discusses the underscore "attribute" enhancement, provides an explicit definition of it, and establishes an IANA registry for the highest-level reserved names that begin with _underscore; underscore-based names that are farther down the hierarchy is handled within the scope of the highest-level _underscore name. It updates the many existing specifications that have defined underscore names, in order to aggregate the references to a single IANA table.

Discussion Venue:

Discussion about this draft should be directed to the dnsop@ietf.org mailing list.

2. Scaling Benefits and TXT and SRV 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 and SRV records are notable examples. Used freely, some of these approaches 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, uses an underscore-based name, at a defined place in the DNS tree, so as to constrain to particular uses for 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.

In the case of TXT records, different uses have developed largely without coordination. One side-effect is that there is no consistently distinguishable internal syntax for the record; even the inefficiencies of internal inspection might not provide a reliable means of distinguishing among the different uses. Underscore-based names therefore define an administrative way of separating TXT records that might have different uses, but otherwise would have no syntactic markers for distinguishing among them.

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 includes 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 current definition of a global underscore registry attends only to the "upper-level" names used for these RRs, that is the _proto names.

3. DNS Global Underscore Scoped Entry Registry Function

This specification creates a registry for DNS nodes names that begin with an underscore and are used to define scope of use for specific resource records. A given name defines a specific, constrained context for the use of such records. Within this scope, use of other resource records that are not specified is permitted. The purpose of the Underscore 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 SRV, an underscore 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 an underscore name in different underscore context; a subordinate name is meaningful only within the scope of the first (parent) underscore name. As such, they can be ignored by this DNS Global Underscore Scoped Entry Registry. That is, the registry is for the definition of highest-level underscore node name used.

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

4. DNS Global Underscore Scoped Entry Registry Definition

A registry entry contains:

• Name:

  Specifies a textual name for a scoped portion of the DNS. The name will usually be taken from the specification cited in the "Purpose" column and is intended for use in discussions about the entry.

  DNS Label:

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

  Constraints:

  Specifies any restrictions on use of the name.

  RR(s):

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

  References

  Lists specifications that define the records and their use under this Name.

  Purpose:

  Specifies the particular purpose/use for specific RR(s), defined for use within the scope of the registered underscore name.

5. IANA Considerations

Per [RFC5226], IANA is requested to establish a DNS Global Underscore Scoped Entry Registry, for DNS node names that begin with the underscore character (_) and have been specified in any published RFC, or are documented by a specification published by another standards organization. The contents of each entry are defined in Section 4.

Initial entries in the registry are:

• { Enhancement of this table to include all underscore name reservations in effect at the time this document is published is left as an exercise to the readers... /d }