The Naming Authority Pointer (NAPTR) DNS Resource Record
RFC 2915

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Network Working Group                                         M. Mealling
Request for Comments: 2915                        Network Solutions, Inc.
Updates: 2168                                                   R. Daniel
Category: Standards Track                                DATAFUSION, Inc.
                                                           September 2000

        The Naming Authority Pointer (NAPTR) DNS Resource Record

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000). All Rights Reserved.

Abstract

   This document describes a Domain Name System (DNS) resource record
   which specifies a regular expression based rewrite rule that, when
   applied to an existing string, will produce a new domain label or
   Uniform Resource Identifier (URI).  Depending on the value of the
   flags field of the resource record, the resulting domain label or URI
   may be used in subsequent queries for the Naming Authority Pointer
   (NAPTR) resource records (to delegate the name lookup) or as the
   output of the entire process for which this system is used (a
   resolution server for URI resolution, a service URI for ENUM style
   e.164 number to URI mapping, etc).

   This allows the DNS to be used to lookup services for a wide variety
   of resource names (including URIs) which are not in domain name
   syntax.  Reasons for doing this range from URN Resource Discovery
   Systems to moving out-of-date services to new domains.

   This document updates the portions of RFC 2168 specifically dealing
   with the definition of the NAPTR records and how other, non-URI
   specific applications, might use NAPTR.

Mealling & Daniel           Standards Track                     [Page 1]
RFC 2915                      NAPTR DNS RR                September 2000

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  NAPTR RR Format  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Substitution Expression Grammar  . . . . . . . . . . . . . .   7
   4.  The Basic NAPTR Algorithm  . . . . . . . . . . . . . . . . .   8
   5.  Concerning How NAPTR Uses SRV Records  . . . . . . . . . . .   9
   6.  Application Specifications . . . . . . . . . . . . . . . . .  10
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.1 Example 1  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.2 Example 2  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.3 Example 3  . . . . . . . . . . . . . . . . . . . . . . . . .  13
   8.  DNS Packet Format  . . . . . . . . . . . . . . . . . . . . .  13
   9.  Master File Format . . . . . . . . . . . . . . . . . . . . .  14
   10. Advice for DNS Administrators  . . . . . . . . . . . . . . .  14
   11. Notes  . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . .  15
   13. Security Considerations  . . . . . . . . . . . . . . . . . .  15
   14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  16
       References . . . . . . . . . . . . . . . . . . . . . . . . .  16
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  17
       Full Copyright Statement . . . . . . . . . . . . . . . . . .  18

1. Introduction

   This RR was originally produced by the URN Working Group [3] as a way
   to encode rule-sets in DNS so that the delegated sections of a URI
   could be decomposed in such a way that they could be changed and re-
   delegated over time.  The result was a Resource Record that included
   a regular expression that would be used by a client program to
   rewrite a string into a domain name.  Regular expressions were chosen
   for their compactness to expressivity ratio allowing for a great deal
   of information to be encoded in a rather small DNS packet.

   The function of rewriting a string according to the rules in a record
   has usefulness in several different applications.  This document
   defines the basic assumptions to which all of those applications must
   adhere to.  It does not define the reasons the rewrite is used, what
   the expected outcomes are, or what they are used for.  Those are
   specified by applications that define how they use the NAPTR record
   and algorithms within their contexts.

   Flags and other fields are also specified in the RR to control the
   rewrite procedure in various ways or to provide information on how to
   communicate with the host at the domain name that was the result of
   the rewrite.

Mealling & Daniel           Standards Track                     [Page 2]