Network Working Group D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Best Current Practice October 14, 2011
Expires: April 16, 2012

DNS Scoped Data Through Attribute Leaves


Historically, any DNS RR may occur for any domain name. Recent additions have defined DNS leaf nodes that contain a reserved node name, beginning with an underscore. The underscore construct is used to define a semantic scope for DNS records associated with the parent domain. This note explores the nature of this DNS usage and defines the "underscore names" registry with IANA.

Status of this Memo

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This Internet-Draft will expire on April 16, 2012.

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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) may 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.

Some recent service enhancements have defined a restricted scope for the occurrence of particular resource records. 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 host names are not allowed to use the underscore character, this distinguishes the name from all legal host name. Effectively, this convention creates a space for attributes that are associated with the parent domain, one level up.

An established example is the SRV record [RFC2782] which generalizes concepts long-used for email routing by the MX record [RFC0974][RFC2821]. The use of 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 and SRV. It provides efficient separation of one use of them from another. Absent this separation, an undifferentiated mass of these RRs are returned to the client which then must parse through the internals of the records in the hope of finding ones that are relevant. With underscore-based scoping, only the relevant RRs are returns.

This specification discusses the underscore "attribute" enhancement, provides an explicit definition of it, and establishes an IANA registry for the reserved names that begin with underscore.

Discussion Venue:
Discussion about this draft is directed to the mailing list of the IETF DNSOP Working Group.

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 the 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 to a define place in the DNS that is constrained to particular uses for particular RRs. This means that a direct lookup produces only the desired records, at no greater cost than a typical 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 internal inspection might not be a reliable means of distinguishing among the different uses. Underscore-based names therefore provide 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 distinguishing among different types of use was part of the design. [RFC2782] The SRV specification serves as a template, defining an RR that may only be used for specific applications when there is an additional specification. The template definition includes reference to tables of names from which underscore-names should be drawn. The set of <service> names is defined in terms of other IANA tables, namely any table with symbolic names. The other SRV naming field is <proto>, although its pool of names is not explicitly defined.

3. Underscore DNS Registry Function

This specification defines a registry for DNS nodes names, used to define scope of use for specific resource records (RR). A given name defines a specific, constrained context for the use of such records. This does not constrain the use of other resource records that are not specified. The purpose of the registry is to avoid collisions resulting from the use of the same underscore 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 names. Semantically, this is a hierarchical model, thereby making a flat registry unexpected.

The registry requires such hierarchies to be registered as a combinatorial case analysis set, with each entry being a full sequence of underscore names. Given a scheme that is actually structured, this flat design is inelegant. However it has the benefit of being extremely simple, with the added advantage of being easier for readers to understand, as long as these cases are small and few.

Example of Underscore Names

The flat registry design:

  • provides significantly simpler administration than is needed for hierarchical tables, simples, and
  • is significantly simpler for readers to understand and is likely to produce fewer programming or administration errors.

4. DNS Underscore Registry Definition

A registry entry MUST contain:

  • 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(s):
    Specifies a sequence of one or more underscore names that define a single name reservation.
    Specifies any restrictions on use of the name.
    Lists the RRs that are defined for use within this scope.
    Lists specifications that define the records and their use under this Name.
    Specifies the particular purpose/use for specific RR(s), defined for use within the scope of the registered underscore name.

5. IANA Considerations

Per [RFC2434], IANA is requested to establish a DNS Underscore Name 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.

DNS Underscore SCOPE Name Registry (with initial values)
SRV TCP _srv._tcp SRV [RFC2782] SRV template
SRV UDP _srv._udp SRV [RFC2782] SRV template
LDAP SRV _ldap._tcp SRV [RFC2782] LDAP server
SIP TCP _sip._tcp NAPTR [RFC3263], [RFC6011] Locating SIP Servers and UA configuration
SIPS TCP _sips._tcp NAPTR [RFC3263], [RFC6011] Locating SIP Servers and UA configuration
SIP UDP _sip._udp SRV [RFC3263], [RFC6011] Locating SIP servers and UA configuration
SPF _spf TXT [RFC4408] Authorized IP addresses for sending mail
DKIM _domainkey TXT [RFC4871] Public key for verifying DKIM signature.
ADSP _adsp._domainkey TXT [RFC5617] Published DKIM usage practices
PKI LDAP _PKIXREP._ldap SRV [RFC4386] LDAP PKI Repository
PKI HTTP _PKIXREP._http SRV [RFC4386] HTTP PKI Repository
PKI OCSP _PKIXREP._ocsp SRV [RFC4386] OCSP PKI Repository
VBR _vouch TXT [RFC5518] Vouch-by-refererence domain assertion
DDDS --unknown!-- SRV [RFC3404] Mapping DDDS query to DNS records
SOAP BEEP _soap-beep._tcp SRV [RFC4227] SOAP over BEEP lookup, when no port specified
XMLRPC BEEP _xmlrpc-beep._tcp SRV [RFC3529] Resolve url for XML-RPC using BEEP
Diameter SCTP _diameter._sctp SRV [RFC3588] Diameter rendezvous over SCTP
Diameter TCP _diameter._tcp SRV [RFC3588] Diameter rendezvous over TCP
Tunnel _tunnel._tcp SRV [RFC3620] Finding the appropriate address for tunneling into a particular domain
SLP TCP _slpda._tcp SRV [RFC3832] Discovering desired services in given DNS domains
SLP UDP _slpda._udp SRV [RFC3832] Discovering desired services in given DNS domains
IM _im SRV [RFC3861] Instant Messaging address resolution
Pres _pres SRV [RFC3861] Presence address resolution
Msg Track _mtqp._tcp SRV [RFC3887] Assist in determining the path that a particular message has taken through a messaging system
XMPP Client _xmpp-client._tcp SRV [RFC6120] XMPP client lookup of server
XMPP Server _xmpp-server._tcp SRV [RFC6120] XMPP server-server lookup
DDDS SRV _??? (unable to discern details. /dcrocker) SRV (and NAPTR?) [RFC3958] Map domain name, application service name, and application protocol dynamically to target server and port
Kerberos TCP _kerberos._tcp SRV [RFC4120] purpose
Kerberos UDP _kerberos._udp SRV [RFC4120] purpose
PKI LDAP _pkixrep._ldap SRV [RFC4386] Enables certificate-using systems to locate PKI repositories
PKI HTTP _pkixrep._http SRV [RFC4386] Enables certificate-using systems to locate PKI repositories
PKI OCSP _pkixrep._ocsp SRV [RFC4386] Enables certificate-using systems to locate PKI repositories
Cert Store _certificates._tcp SRV [RFC4387] Obtain certificates and certificate revocation lists (CRLs) from PKI repositories
Cert Revocation Store _crls._tcp SRV [RFC4387] Obtain certificates and certificate revocation lists (CRLs) from PKI repositories
PGP Key Store pgpkeys._tcp SRV [RFC4387] Obtain certificates and certificate revocation lists (CRLs) from PKI repositories
MSRP Relay Locator _msrp._tcp SRV [RFC4976] purpose
Mobile IPv6 Bootstrap _mip6._ipv6 SRV [RFC5026], [RFC5555] Bootstrap Mobile IPv6 Home Agent information from non-topological information
Digital Video Broadcasting TCP _dvbservdsc._tcp SRV [RFC5328] Discover non-default DVB entry points addresses
Digital Video Broadcasting UDP _dvbservdsc._udp SRV [RFC5328] Discover non-default DVB entry points addresses
CAPWAP AC _capwap-control._udp rrs [RFC5415] Discover the CAPWAP AC address(es)
IM SIP _im._sip SRV [RFC5509] For resolving Instant Messaging and Presence services with SIP
Pres SIP _pres._sip SRV [RFC5509] For resolving Instant Messaging and Presence services with SIP
IEEE 802.21 Mobility TCP _mihis._tcp NAPTR, SRV [RFC5679] Discovering servers that provide IEEE 802.21-defined Mobility Services
IEEE 802.21 Mobility UDP _mihis._udp NAPTR, SRV [RFC5679] Discovering servers that provide IEEE 802.21-defined Mobility Services
STUN Client/Server TCP _stun._.tcp SRV [RFC5389] Find a STUN server
STUN Client/Server UDP _stun._.udp SRV [RFC5389] Find a STUN server
STUN Client/Server TLS _stuns._.tcp SRV [RFC5389] Find a STUN server
TURN TCP _turn._tcp SRV [RFC5766], [RFC5928] Control the operation of a relay to bypass NAT
TURN UDP _turn._udp SRV [RFC5766], [RFC5928] Control the operation of a relay to bypass NAT
TURN TLS _turns._tcp SRV [RFC5766], [RFC5928] Control the operation of a relay to bypass NAT
STUN NAT Behavior Discovery TCP _stun-behavior._tcp SRV [RFC5780] Discover the presence and current behavior of NATs and firewalls between the STUN client and the STUN server
STUN NAT Behavior Discovery UDP _stun-behavior._udp SRV [RFC5780] Discover the presence and current behavior of NATs and firewalls between the STUN client and the STUN server
STUN NAT Behavior Discovery TLS _stun-behaviors._tcp SRV [RFC5780] Discover the presence and current behavior of NATs and firewalls between the STUN client and the STUN server
Sieve Management _sieve._tcp SRV [RFC5804] Manage Sieve scripts on a remote server
AFS VLDB _afs3-vlserver._udp SRV [RFC5864] Locate services for the AFS distributed file system
AFS PTS _afs3-prserver._udp SRV [RFC5864] Locate services for the AFS distributed file system
Mail MSA Submission _submission._tcp SRV [RFC6186] Locate email services
IMAP _imap._tcp SRV [RFC6186] Locate email services
IMAP TLS _imaps._tcp SRV [RFC6186] Locate email services
POP _pop3._tcp SRV [RFC6186] Locate email services
POP TLS _pop3s._tcp SRV [RFC6186] Locate email services

6. Related 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:

  • Instant Messaging SRV Protocol Label Registry
  • Public Key Infrastructure using X.509 (PKIX) Parameters
  • Presence SRV Protocol Label Registry

7. Security Considerations

This memo raises no security issues.

8. References

8.1. Normative References

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, October 1998.

8.2. References -- Informative

, "
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001.
[RFC0974] Partridge, C., "Mail routing and the domain system", RFC 974, January 1986.
[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.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J. Arkko, "Diameter Base Protocol ", September 2003.
[RFC3620] New, D., "The TUNNEL Profile", RFC 3620, October 2003.
[RFC3861] Peterson, J., "Address Resolution for Instant Messaging and Presence", RFC 3861, August 2004.
[RFC3832] , , , , , , , and , "Remote Service Discovery in the Service Location Protocol (SLP) via DNS SRV", July 2004.
[RFC3887] Message Tracking Query Protocol", 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", February 2006.
[RFC4387] Gutmann, P., "Internet X.509 Public Key Infrastructure Operational Protocols: Certificate Store Access via HTTP", RFC 4387, February 2006.
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 4408, April 2006.
[RFC5617] , , , , , and , "DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)", August 2009.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J. and M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures", RFC 4871, May 2007.
[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.
[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.
[RFC5507] Faltstrom, P. and R. Austein, "", RFC 5507, April 2009.
[RFC5415] Calhoun, P., Montemurro, M. and D. Stanley, "Control And Provisioning of Wireless Access Points (CAPWAP) Protocol Specification", RFC 5415, March 2009.
[RFC5509] Loreto, S., "Internet Assigned Numbers Authority (IANA) Registration of Instant Messaging and Presence DNS SRV RRs for the Session Initiation Protocol (SIP)", RFC 5509, April 2009.
[RFC5518] Hoffman, P., Levine, J. and A. Hathcock, "Vouch By Reference", RFC5 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.

Appendix A. Acknowledgements

Thanks go to Bill Fenner, Tony Hansen, Peter Koch, Olaf Kolkman, and Andrew Sullivan for diligent review of the earlier drafts.

Author's Address

Dave Crocker Brandenburg InternetWorking 675 Spruce Dr. Sunnyvale, CA 94086 USA Phone: +1.408.246.8253 EMail: URI: