RFC 4848

Domain-Based Application Service Location Using URIs and the Dynamic Delegation Discovery Service (DDDS)

📅 published April 2007, from ietf

RFC 4848 🦋 Domain-Based Application Service Location Using URIs and the Dynamic Delegation Discovery Service (DDDS)

Network Working Group 🌸 Author: L. Daigle, Cisco Systems 💜 Category: Standards Track 👑 Date: April 2007 📅

Status of This Memo ✨

Okay bestie, this part is the official fine print 📜. This document is an Internet standards track protocol for the Internet community, and it's asking everyone to talk about it and suggest improvements 💬. If you want the actual standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1) 🔍. And distribution of this memo is unlimited, so share it wherever you want 🌈.

Copyright (C) The IETF Trust (2007) 💅

Abstract 🦋

The goal here is to define a brand new, super straightforward Dynamic Delegation Discovery Service (DDDS) application 🧚. It lets you map domain names to URIs for specific application services and protocols, which is honestly iconic 🔥. It's called U-NAPTR, and even though it's its own DDDS application, it's basically an extension of the Straightforward NAPTR (S-NAPTR) DDDS Application 💖.

Table of Contents 📚

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Straightforward URI-Enabled NAPTR (U-NAPTR) . . . . . . . . . . 3
     2.1.  Permitted Flags . . . . . . . . . . . . . . . . . . . . . . 3
     2.2.  Permitted Regular Expressions . . . . . . . . . . . . . . . 4
   3.  Sample U-NAPTR DNS Records  . . . . . . . . . . . . . . . . . . 4
   4.  Formal Definition of U-NAPTR Application of DDDS  . . . . . . . 5
     4.1.  Application Unique String . . . . . . . . . . . . . . . . . 5
     4.2.  First Well Known Rule . . . . . . . . . . . . . . . . . . . 5
     4.3.  Expected Output . . . . . . . . . . . . . . . . . . . . . . 5
     4.4.  Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     4.5.  Service Parameters  . . . . . . . . . . . . . . . . . . . . 5
       4.5.1.  Application Services  . . . . . . . . . . . . . . . . . 6
       4.5.2.  Application Protocols . . . . . . . . . . . . . . . . . 6
     4.6.  Valid Rules . . . . . . . . . . . . . . . . . . . . . . . . 6
     4.7.  Valid Databases . . . . . . . . . . . . . . . . . . . . . . 7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 9

1. Introduction 🌷

So the purpose here is the same girlboss energy: define a new, straightforward Dynamic Delegation Discovery Service (DDDS) [7] application that maps domain names to URIs for particular application services and protocols 🎀. This is what lets you "lookup" which services are available for a given domain, for example 🔎.

Even though this is a totally new and separate DDDS Application called U-NAPTR, it's built on the same principles as the Straightforward NAPTR (S-NAPTR) application, which is specified in [2] 💅. This spec is not an update of S-NAPTR, but you should go read that doc for the deep dive on motivation and implementation stuff 📖.

S-NAPTR gives you application service location without relying on rigid domain naming conventions ✨. It's called "straightforward" partly because it bans regular expressions in NAPTR records (for the S-NAPTR DDDS Application) 🙈. The catch is that this also rules out using a URI as the target of DDDS resolution 😭. A bunch of applications, both specified (e.g., [9]) and proposed, find that restriction too limiting, so S-NAPTR is a near miss for them 🥲.

So U-NAPTR is basically a modest glow-up of S-NAPTR 💖. It lets URIs be targets, without allowing the full range of possible regular expressions in NAPTR records 🫶.

2. Straightforward URI-Enabled NAPTR (U-NAPTR) 💜

This document assumes you already know the S-NAPTR specification [2], which is totally fair 🙌. U-NAPTR is meant to do everything S-NAPTR does, with two differences: it allows the use of the "U" flag in the NAPTR record, and it allows one specific form of REGEXP 🎯.

2.1. Permitted Flags 🚩

U-NAPTR permits the same flags as S-NAPTR ("S", "A", or empty), plus the "U" Flag ✨. For the U-NAPTR DDDS Application, when the "U" Flag shows up in the NAPTR record, the REGEXP field must be populated, which means the REPLACEMENT field is empty as a result 🧩. The regular expression in the REGEXP field must be of the limited form described below, and evaluating that regular expression gives you a URI, which is the result of the DDDS resolution 🦋.

2.2. Permitted Regular Expressions 🌈

U-NAPTR permits regular expressions that completely replace the matched string with a URI, written as a constant string 💅. This is basically a workaround, because the REPLACEMENT field in NAPTR is required to produce only a fully qualified domain name, so it can't be used for a URI 🙃.

The specific allowed syntax for U-NAPTR regular expressions is:

        u-naptr-regexp = "!.*!"<URI>"!"

Here, <URI> is as defined in STD 66 [8], the URI syntax specification 📚.

With this limited form of regular expression, applications using U-NAPTR don't need to implement full regular expression parsers 🎉. Less work, more slay 💖.

3. Sample U-NAPTR DNS Records 🍓

In the sample NAPTR RRs for example.com shown below, "WP" is the imagined application service tag for "white pages" 📒, and "EM" is the application service tag for an imagined "Extensible Messaging" application service 💌.

   example.com.
   ;;       order pref flags
   IN NAPTR 100   10   ""    "WP:whois++"      ( ; service
                             ""                  ; regexp
                             bunyip.example.com. ; replacement
                                               )
   IN NAPTR 100   20   "s"   "WP:ldap"         ( ; service
                             ""                  ; regexp
                            _ldap._tcp.myldap.example.com. ; replacement
                                               )
   IN NAPTR 200   10   "u"    "EM:protA"        ( ; service
                             "!.*!prota://someisp.example.com!" ; regexp
                             ""                  ; replacement
                                               )
   IN NAPTR 200   30   "a"   "EM:protB"          ; service
                             ""                  ; regexp
                             myprotB.example.com.; replacement
                                               )

4. Formal Definition of U-NAPTR Application of DDDS 🎀

This section formally defines the DDDS Application, as described in [7] 📐. Time to get serious, but still cute 💅.

4.1. Application Unique String 🔑

The Application Unique String is a fully qualified domain name (FQDN) 🌐. The thing you're looking for is the authoritative server for a particular service, for that domain 🔍.

4.2. First Well Known Rule 🥇

The "First Well Known Rule" is identity 🪞. That means the output of the rule is the Application Unique String itself, the FQDN whose authoritative server for a particular service you're seeking 💖.

4.3. Expected Output 🎁

The expected output of this Application is the information you need to connect to the authoritative server(s) for an application service within a given domain 💌. That's the host, port, protocol, or URI, so basically everything you need to connect 🔗.

4.4. Flags 🏳️‍🌈

This DDDS Application uses only 3 of the Flags defined for the URI/URN Resolution Application [5]: "S", "A", and "U" 🎯. No other Flags are valid 🙅‍♀️. If a client gets a NAPTR RR for a U-NAPTR-using application that contains any other flag, that NAPTR RR should be ignored, and processing continues with the next record (if there is one) 🚫.

These flags are for terminal lookups 🏁. That means the Rule is the last one, and the flag decides what the next stage should be 💅. The "S" flag means the output of this Rule is a FQDN with one or more SRV [3] records 📡. "A" means the output of the Rule is a domain name, and it should be used to look up address records for that domain 🏠. "U" means the output of the Rule is a URI that should be resolved to get access to the described service 🦋.

Consistent with the DDDS algorithm, if the Flag string is empty, the next lookup is for another NAPTR record (for the replacement target) 🔁.

4.5. Service Parameters 🧁

Service Parameters for this Application take the form of a string of characters that follow this ABNF [1] 📝:

      service-parms = [ [app-service] *(":" app-protocol)]
      app-service   = experimental-service  / iana-registered-service
      app-protocol  = experimental-protocol / iana-registered-protocol
      experimental-service      = "x-" 1*30ALPHANUMSYM
      experimental-protocol     = "x-" 1*30ALPHANUMSYM
      iana-registered-service   = ALPHA *31ALPHANUMSYM
      iana-registered-protocol  = ALPHA *31ALPHANUMSYM
      ALPHA         =  %x41-5A / %x61-7A   ; A-Z / a-z
      DIGIT         =  %x30-39 ; 0-9
      SYM           =  %x2B / %x2D / %x2E  ; "+" / "-" / "."
      ALPHANUMSYM   =  ALPHA / DIGIT / SYM
      ; The app-service and app-protocol tags are limited to 32
      ; characters and must start with an alphabetic character.
      ; The service-parms are considered case-insensitive.

So the Service Parameters can be an empty string, just an app-service, or an app-service with one or more app-protocol specifications separated by the ":" symbol 🌸.

This is similar to, but not the same as, the syntax used in the URI DDDS application [5] 👯. The DDDS DNS database needs each DDDS application to define the syntax of its allowable service strings 📋. The syntax here is expanded to allow the characters that are valid in any URI scheme name (see [8]) 🔤. And since "+" (the separator used in the RFC3404 service parameter string) is an allowed character for URI scheme names, ":" is chosen as the separator here ✂️. Smart, right? 🧠

4.5.1. Application Services 🛎️

The "app-service" must be an IANA-registered service. See Section 5 for instructions on registering new application service tags 📮.

4.5.2. Application Protocols 🔌

The protocol identifiers that are valid for the "app-protocol" production are standard, registered protocols. See Section 5 for instructions on registering new application protocol tags 🏷️.

4.6. Valid Rules ✅

Permitted rules are substitution rules and regular expressions of the following syntax (i.e., a regular expression that replaces the domain name with a URI) 🔁:

           u-naptr-regexp = "!.*!"<URI>"!"

Here, <URI> is as defined in STD 66 [8], the URI syntax specification 📚.

4.7. Valid Databases 🗄️

Right now, only one DDDS Database is specified for this Application 💁‍♀️. [4] specifies a DDDS Database that uses the NAPTR DNS resource record to contain the rewrite rules 📄. The Keys for this database are encoded as domain names 🌐.

The First Well Known Rule produces a domain name, and that's the Key used for the first lookup, where the NAPTR records for that domain are requested 🔑.

DNS servers MAY interpret Flag values and use that information to include appropriate NAPTR, SRV, or A records in the Additional Information portion of the DNS packet 📦. Clients are encouraged to check for additional information, but they aren't required to do so 🤷‍♀️. For more on NAPTR records and the Additional Information section of a DNS response packet, see the Additional Information Processing section of [4] 🔍.

5. IANA Considerations ✨

okay bestie, so this one's kinda chill for IANA 🦋 This document doesn't actually put any requirements on IANA itself. What it does do is set up the foundation that U-NAPTR services can use to tap into the existing IANA registries for application service tags and application protocol tags (the ones defined in RFC 3958 [2]). Iconic, right? 💅

Okay now the "x-" thing, this is giving experimental vibes 🧪 All application service and protocol tags that start with x- are considered experimental, and there's NO provision to stop people from using the same string twice. So like... if two people pick the same x- tag, nobody's stopping them 💀 Use them at your own risk, bestie 🙈

Everything else (so anything that doesn't start with x-) gets registered using the "specification required" option from [6], and the "specification" has to be an RFC of any category 📜

There are no further restrictions on the tags, except they MUST conform to the syntax defined above (Section 4.5). Like, that's the only rule 💖

The defining RFC MUST clearly identify and describe, for each tag being registered:

  • Application protocol or service tag 🏷️
  • Intended usage 🎯
  • Interoperability considerations 🤝
  • Security considerations (see Section 6 of this document for further discussion of the types of considerations that are applicable) 🔐
  • Any relevant related publications 📚

The defining RFC MAY also include further application-specific restrictions, like limitations on the types of URIs that can be returned for the application service 🌈

6. Security Considerations 🔐

okay this section is SO important, pls pay attention 😭

U-NAPTR has the same security considerations as S-NAPTR, so check out Section 8 of [2] for those 🫶 U-NAPTR also has an extra consideration: resolving the URIs you get back from the DDDS resolution has its own set of security implications. That stuff is covered in the URI spec (especially Section 7 of [8]) ✨

Here's the big picture: using DNSSEC, client software can be confident that the URI it got via U-NAPTR is the exact one the domain's administrator specified, and it came from that domain 🔒 BUT the validity of the service you reach by resolving that URI is a totally separate matter, and it's down to URI resolution security practices 🚨 So DNSSEC covers the URI, not the service behind it, babe 💀

7. Acknowledgements 🌸

Huge thanks to Martin Thomson, John Klensin, Bernard Aboba, Alfred Hoenes, Dan Romascanu, Suresh Krishnan, and Lars Eggert for reviewing earlier versions and catching errors! 🫶💖 You all are literally the MVPs 👑

8. References 📚

8.1. Normative References

  • [1] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. 🧩
  • [2] Daigle, L. and A. Newton, "Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS)", RFC 3958, January 2005. 🦋
  • [3] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. 🎀
  • [4] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Three: The Domain Name System (DNS) Database", RFC 3403, October 2002. 🌐
  • [5] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Four: The Uniform Resource Identifiers (URI)", RFC 3404, October 2002. 🌈
  • [6] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. 📋

8.2. Informative References

  • [7] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS", RFC 3401, October 2002. 💜
  • [8] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", RFC 3986, STD 66, January 2005. ✨
  • [9] Malamud, C., "Attaching Meaning to Solicitation Class Keywords", RFC 4095, May 2005. 🍓

Author's Address 🖊️

Leslie L. Daigle Cisco Systems 13615 Dulles Technology Drive Herndon, VA 20171 US

EMail: ledaigle@cisco.com; leslie@thinkingcat.com 💌

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