dnsop D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Best Current Practice March 29, 2017
Expires: September 30, 2017

DNS Scoped Data Through Global '_Underscore' Naming of Attribute Leaves


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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Table of Contents

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].

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.

Example of Underscore Names

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:

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:

DNS Global Underscore Scoped Entry Registry (with initial values)
SRV _srv SRV [RFC2782] SRV template -- pro forma entry, not directly usable
SRV TCP _tcp SRV [RFC2782] Use of SRV for a TCP-based service
SRV UDP _udp SRV [RFC2782] Use of SRV for a UDP-based service
LDAP _ldap SRV [RFC2782] LDAP server
SIP _sip NAPTR [RFC3263] [RFC6011] Locating SIP Servers and UA configuration
SPF _spf TXT [RFC7372] Authorized IP addresses for sending mail
DKIM _domainkey TXT [RFC6376] Public key for verifying DKIM signature.
VBR _vouch TXT [RFC5518] Vouch-by-refererence domain assertion
DDDS --???!-- SRV [RFC3404] Mapping DDDS query to DNS records
SOAP BEEP _soap-beep SRV [RFC4227] SOAP over BEEP lookup, when no port specified
XMLRPC BEEP _xmlrpc-beep SRV [RFC3529] Resolve url for XML-RPC using BEEP
Diameter _diameter SRV [RFC6733] Diameter rendezvous
Tunnel _tunnel SRV [RFC3620] Finding the appropriate address for tunneling into a particular domain
SLP _slpda SRV [RFC3832] Discovering desired services in given DNS domains
Msg Track _mtqp SRV [RFC3887] Assist in determining the path that a particular message has taken through a messaging system
XMPP Client _xmpp-client SRV [RFC6120] XMPP client lookup of server
XMPP Server _xmpp-server SRV [RFC6120] XMPP server-server lookup
DDDS SRV _??? SRV (and NAPTR?) [RFC3958] Map domain name, application service name, and application protocol dynamically to target server and port
Kerberos _kerberos SRV [RFC4120] purpose
PKI _pkixrep SRV [RFC4386] Enables certificate-using systems to locate PKI repositories
Certificates _certificates SRV [RFC4387] Obtain certificates and certificate revocation lists (CRLs) from PKI repositories
PGP Key Store _pgpkeys SRV [RFC4387] Obtain certificates and certificate revocation lists (CRLs) from PKI repositories
MSRP Relay Locator _msrp SRV [RFC4976] purpose
Mobile IPv6 Bootstrap _mip6 SRV [RFC5026] [RFC5555] Bootstrap Mobile IPv6 Home Agent information from non-topological information
Digital Video Broadcasting _dvbservdsc SRV [RFC5328] Discover non-default DVB entry points addresses
CAPWAP AC _capwap-control rrs [RFC5415] Discover the CAPWAP AC address(es)
IEEE 802.21 Mobility _mihis NAPTR, SRV [RFC5679] Discovering servers that provide IEEE 802.21-defined Mobility Services
STUN Client/Server _stun SRV [RFC5389] Find a STUN server
TURN _turn SRV [RFC5766] [RFC5928] Control the operation of a relay to bypass NAT
STUN NAT Behavior Discovery _stun-behavior SRV [RFC5780] Discover the presence and current behavior of NATs and firewalls between the STUN client and the STUN server
Sieve Management _sieve SRV [RFC5804] Manage Sieve scripts on a remote server
AFS VLDB _afs3-vlserver SRV [RFC5864] Locate services for the AFS distributed file system
AFS PTS _afs3-prserver SRV [RFC5864] Locate services for the AFS distributed file system
Mail MSA Submission _submission SRV [RFC6186] Locate email services
IMAP _imap SRV [RFC6186] Locate email services
POP _pop3 SRV [RFC6186] Locate email services
POP TLS _pop3s SRV [RFC6186] Locate email services

6. Related and Updated Registries

Numerous specifications have defined their own, independent registries for use of underscore names. It is likely that adoption of the proposed, integrated registry should render these piecemeal registries obsolete

Registries that are candidates for replacement include:

7. Security Considerations

This memo raises no security issues.

8. References

8.1. Normative References

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, May 2008.

8.2. References -- Informative

, "
[RFC0974] Partridge, C., "Mail routing and the domain system", RFC 974, January 1986.
[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.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol (SIP): Locating SIP Servers", RFC 3263, June 2002.
[RFC3404] MMealling, M., "Dynamic Delegation Discovery System (DDDS) Part Four: The Uniform Resource Identifiers (URI) Resolution Application", RFC 3404, October 2002.
[RFC3529] Harold, W., "Using Extensible Markup Language-Remote Procedure Calling (XML-RPC) in Blocks Extensible Exchange Protocol (BEEP)", RFC 3529, April 2003.
[RFC3620] New, D., "The TUNNEL Profile", RFC 3620, October 2003.
[RFC3832] , , , , , , , and , Remote Service Discovery in the Service Location Protocol (SLP) via DNS SRV", RFC 3832, July 2004.
[RFC3887]Message Tracking Query Protocol", RFC 3887, September 2007.
[RFC3958] Daigle, L. and A. Newton, "Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS)", RFC 3958, January 2005.
[RFC4120] , , , , , and , "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005.
[RFC4227] O'Tuathail, E. and M. Rose, "Using the Simple Object Access Protocol (SOAP) in Blocks Extensible Exchange Protocol (BEEP)", RFC 4227, January 2006.
[RFC4386] Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key Infrastructure: Repository Locator Service", RFC 4386, February 2006.
[RFC4387] Gutmann, P., "Internet X.509 Public Key Infrastructure Operational Protocols: Certificate Store Access via HTTP", RFC 4387, February 2006.
[RFC4976] Jennings, C., Mahy, R. and Roach, "Relay Extensions for the Message Session Relay Protocol (MSRP)", RFC 4976, September 2007.
[RFC5026] Giaretta, G., Kempf, J. and V. Devarapalli, "Mobile IPv6 Bootstrapping in Split Scenario", RFC 5026, October 2007.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, Oct 2008.
[RFC5328] Adolf, A. and P. MacAvock, "A Uniform Resource Name (URN) Namespace for the Digital Video Broadcasting Project (DVB)", RFC 5328, September 2008.
[RFC5389] Rosenberg, , Mahy, , Matthews, and Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, October 2008.
[RFC5415] Calhoun, P., Montemurro, M. and D. Stanley, "Control And Provisioning of Wireless Access Points (CAPWAP) Protocol Specification", RFC 5415, March 2009.
[RFC5507] Faltstrom, P. and R. Austein, "Design Choices When Expanding the DNS", RFC 5507, April 2009.
[RFC5518] Hoffman, P., Levine, J. and A. Hathcock, "Vouch By Reference", RFC 5518, April 2009.
[RFC5555] Soliman, H., "Mobile IPv6 Support for Dual Stack Hosts and Routers", RFC 5555, June 2009.
[RFC5679] Bajko, G., "Locating IEEE 802.21 Mobility Services Using DNS", RFC 5679, December 2009.
[RFC5766] Mahy, R., Matthews, P. and J. Rosenberg, "Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC5780] MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery Using Session Traversal Utilities for NAT (STUN)", RFC 5780, May 2010.
[RFC5804] Melnikov, A. and T. Martin, "A Protocol for Remotely Managing Sieve Scripts", RFC 5804, July 2010.
[RFC5864] Allbery, R., "NS SRV Resource Records for AFS", RFC 5864, April 2010.
[RFC5928] Petit-Huguenin, M., "Traversal Using Relays around NAT (TURN) Resolution Mechanism", RFC 5928, August 2010.
[RFC6011] Lawrence, S. and J. Elwell, "Session Initiation Protocol (SIP) User Agent Configuration", RFC 6011, October 2010.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email Submission/Access Services", RFC 6186, March 2011.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, Sept 2011.
[RFC6733] Fajardo, V., Arkko, J., Loughney, J. and G. Zorn, "Diameter Base Protocol", RFC 6733, October 2012.
[RFC7372] Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 7372, April 2014.
[RFC7553] Falstrom, P. and O. Kolkman, "The Uniform Resource Identifier (URI) DNS Resource Record", RFC RFC7553, 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: Tim Wicinski, John Levine, Bob Harold, Joel Jaeggli, Ondřej Surý and Paul Wouters. Special thanks to Ray Bellis for more than 10 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/