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Versions: 00 01 02 03 RFC 2915

Network Working Group                                        M. Mealling
Internet-Draft                                   Network Solutions, Inc.
Expires: December 23, 1999                                     R. Daniel
                                                        DATAFUSION, Inc.
                                                           June 24, 1999

        The Naming Authority Pointer (NAPTR) DNS Resource Record
                       draft-ietf-urn-naptr-rr-03.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To view the entire list of Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on December 23, 1999.

Abstract

   This document describes a DNS Resource Record which specifies a
   rewrite rule that, when applied to an existing string will produce a
   new domain. Reasons for rewriting a domain vary from URN Resource
   Discovery Systems to moving out of date services to new domains.

   This document updates the portions of RFC2168 specifically dealing
   with the definition of the NAPTR record.

Copyright Notice

   Copyright (C) The Internet Society (1999). All Rights Reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  NAPTR RR Format  . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Substitution Expression Grammar  . . . . . . . . . . . . . . .  8
   4.  The Basic NAPTR Algorithm  . . . . . . . . . . . . . . . . . . 10
   5.  Concerning How NAPTR Uses SRV Records  . . . . . . . . . . . . 11
   6.  Application Specifications . . . . . . . . . . . . . . . . . . 12
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.1 Example 1  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.2 Example 2  . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.3 Example 3  . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  DNS Packet Format  . . . . . . . . . . . . . . . . . . . . . . 17
   9.  Master File Format . . . . . . . . . . . . . . . . . . . . . . 18
   10. Advice for DNS Administrators  . . . . . . . . . . . . . . . . 19
   11. Notes  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 22
   A.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   B.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25

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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.

   The final result is a RR that has several fields that interact in a
   non-trivial but implementable way. This document specifies those
   fields and their values.

   This document does not define applications that utilizes this
   rewrite functionality. Instead it specifies just the mechanics of
   how it is done. Why its done, what the rules concerning the inputs,
   and the types of rules used are reserved for other documents that
   fully specify a particular application. This seperation is due to
   several differrent applications all wanting to take advantage of the
   rewrite rule lookup process. Each one has vastly different reasons
   for why and how it uses the service, thus requiring that the
   definition of the service be generic.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC 2119.

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2. NAPTR RR Format

   The format of the NAPTR RR is given below. The DNS[1]type code[2]
   for NAPTR is 35.

   Domain TTL Class Order Preference Flags Service Regexp Replacement

   Domain
      The domain name to which this resource record refers. This is the
      'key' for this entry in the rule database. This value will either
      be the first well known key (<something>.uri.net for example) or
      a new key that is the output of a replacement or regexp rewrite.
      Beyond this, it has the standard DNS requirements[1].

   TTL
      Standard DNS meaning.[1]

   Class
      Standard DNS meaning[1].

   Order
      A 16-bit unsigned integer specifying the order in which the NAPTR
      records MUST be processed to ensure the correct ordering of
      rules. Low numbers are processed before high numbers, and once a
      NAPTR is found whose rule "matches" the target, the client MUST
      NOT consider any NAPTRs with a higher value for order (except as
      noted below for the Flags field).

   Preference
      A 16-bit unsigned integer that specifies the order in which NAPTR
      records with equal "order" values SHOULD be processed, low
      numbers being processed before high numbers. This is similar to
      the preference field in an MX record, and is used so domain
      administrators can direct clients towards more capable hosts or
      lighter weight protocols. A client MAY look at records with
      higher preference values if it has a good reason to do so such as
      not understanding the preferred protocol or service.

      The important difference between Order and Preference is that
      once a match is found the client MUST NOT consider records with a
      different Order but they MAY process records with the same Order
      but different Preferences. I.e. Preference is used to give weight
      to rules that are considered the same from an authority
      standpoint but not from a simple load balancing standpoint.

   Flags
      A <character-string> containing flags to control aspects of the
      rewriting and interpretation of the fields in the record. Flags
      are single characters from the set [A-Z0-9]. The case of the
      alphabetic characters is not significant.

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      At this time only four flags, "S", "A", "U", and "P", are
      defined. The "S", "A" and "U" flags denote a terminal lookup.
      This means that this NAPTR record is the last one and that the
      flag determines what the next stage should be.  The "S" flag
      means that the next lookup should be for SRV records[4]. See
      Section 5 for additional information on how NAPTR uses the SRV
      record type. "A" means that the next lookup should be for A
      records. The "U" flag means that the next step is not a DNS
      lookup but that the output of the Regexp field is a URL[9].

      The "P" flag says that the remainder of the application side
      algorithm shall be carried out in a Protocol-specific fashion.
      The new set of rules is identified by the Protocol specified in
      the Services field.  The record that contains the 'P' flag is the
      last record that is interpreted by the rules specified in this
      document.  The new rules are dependent on the application for
      which they are being used and the protocol specified. For
      example, if the application is a URI RDS and the protocol is WIRE
      then the new set of rules are governed by the algorithms
      surrounding the WIRE HTTP specification and not this document.

      The remaining alphabetic flags are reserved for future versions
      of the NAPTR specification. The numeric flags may be used for
      local experimentation. The S, A, U and P flags are all mutually
      exclusive, and resolution libraries MAY signal an error if more
      than one is given. (Experimental code and code for assisting in
      the creation of NAPTRs would be more likely to signal such an
      error than a client such as a browser). It is anticipated that
      multiple flags will be allowed in the future, so implementers
      MUST NOT assume that the flags field can only contain 0 or 1
      characters. Finally, if a client encounters a record with an
      unknown flag, it MUST ignore it and move to the next record. This
      test takes precedence even over the "order" field. Since flags
      can control the interpretation placed on fields, a novel flag
      might change the interpretation of the regexp and/or replacement
      fields such that it is impossible to determine if a record
      matched a given target.

      The "S", "A", and "U"  flags are called 'terminal' flags since
      they halt the looping rewrite algorithm. If those flags are not
      present, clients may assume that another NAPTR RR exists at the
      domain name produced by the current rewrite rule. Since the "P"
      flag specifies a new algorithm, it may or may not be 'terminal'.
      Thus, the client cannot assume that another NAPTR exists since
      this case is determined elsewhere.

      DNS servers MAY interpret these flags and values and use that
      information to include appropriate SRV and A records in the
      additional information portion of the DNS packet. Clients are
      encouraged to check for additional information but are not
      required to do so.

   Service

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      Specifies the service(s) available down this rewrite path. It may
      also specify the particular protocol that is used to talk with a
      service. A protocol MUST be specified if the flags field states
      that the NAPTR is terminal. If a protocol is specified, but the
      flags field does not state that the NAPTR is terminal, the next
      lookup MUST be for a NAPTR. The client MAY choose not to perform
      the next lookup if the protocol is unknown, but that behavior
      MUST NOT be relied upon.

      The service field may take any of the values below (using the
      Augmented BNF of RFC 2234[5]):

              service_field = [ [protocol] *("+" rs)]
              protocol      = ALPHA *31ALPHANUM
              rs            = ALPHA *31ALPHANUM
              ; The protocol and rs fields are limited to 32
              ; characters and must start with an alphabetic.

      For example, an optional protocol specification followed by 0 or
      more resolution services. Each resolution service is indicated by
      an initial '+' character.

      Note that the empty string is also a valid service field. This
      will typically be seen at the beginning of a series of rules,
      when it is impossible to know what services and protocols will be
      offered by a particular service.

      The actual format of the service request and response will be
      determined by the resolution protocol, and is the subject for
      other documents. Protocols need not offer all services. The
      labels for service requests shall be formed from the set of
      characters [A-Z0-9]. The case of the alphabetic characters is not
      significant.

      The list of "valid" protocols for any given NAPTR record is any
      protocol that implements some or all of the services defined for
      a NAPTR application.  Currently, THTTP[6] is the only protocol
      that is known to make that claim at the time of publication. Any
      other protocol that is to be used must have documentation
      specifying:

      *  how it implements the services of the application

      *  how it is to appear in the NAPTR record (i.e., the string id
         of the protocol)

      The list of valid Resolution Services is defined by the documents
      that specify individual NAPTR based applications. One example is
      RFC-XXXX, "Resolution of Uniform Resource Identifiers using the
      Domain Name System"[7].

      It is worth noting that the interpretation of this field is
      subject to being changed by new flags, and that the current
      specification is oriented towards telling clients how to talk

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      with a URN resolver.

   Regexp
      A STRING containing a substitution expression that is applied to
      the original string held by the client in order to construct the
      next domain name to lookup. The grammar of the substitution
      expression is given in the next section.

      The regular expressions MUST NOT be used in a cumulative fashion,
      that is, they should only be applied to the original string held
      by the client, never to the domain name produced by a previous
      NAPTR rewrite. The latter is tempting in some applications but
      experience has shown such use to be extremely fault sensitive,
      very error prone, and extremely difficult to debug.

   Replacement
      The next NAME to query for NAPTR, SRV, or A records depending on
      the value of the flags field. This MUST be a fully qualified
      domain-name. Unless and until permitted by future standards
      action, name compression is not to be used for this field.

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3. Substitution Expression Grammar

   The content of the regexp field is a substitution expression. True
   sed(1) substitution expressions are not appropriate for use in this
   application for a variety of reasons, therefore the contents of the
   regexp field MUST follow the grammar below:

   subst_expr   = delim-char  ere  delim-char  repl  delim-char  *flags
   delim-char   = "/" / "!" / ... <Any non-digit or non-flag character
                  other than backslash '\'. All occurances of a delim_char
                  in a subst_expr must be the same character.>
   ere          = POSIX Extended Regular Expression
   repl         = 1 * ( OCTET /  backref )
   backref      = "\" 1POS_DIGIT
   flags        = "i"
   POS_DIGIT    = %x31-39                 ; 0 is not an allowed backref

   The definition of a POSIX Extended Regular Expression can be found
   in [8], section 2.8.4.

   The result of applying the substitution expression to the original
   URI MUST result in either a string that obeys the syntax for DNS
   domain-names[1]  or a URI[9] if the Flags field contains a 'U'.
   Since it is possible for the regexp field to be improperly
   specified, such that a non-conforming domain-name can be
   constructed, client software SHOULD verify that the result is a
   legal DNS domain-name before making queries on it.

   Backref expressions in the repl portion of the substitution
   expression are replaced by the (possibly empty) string of characters
   enclosed by '(' and ')' in the ERE portion of the substitution
   expression. N is a single digit from 1 through 9, inclusive. It
   specifies the N'th backref expression, the one that begins with the
   N'th '(' and continues to the matching ')'.  For example, the ERE

                         (A(B(C)DE)(F)G)

      has backref expressions:

                         \1  = ABCDEFG
                         \2  = BCDE
                         \3  = C
                         \4  = F
                         \5..\9  = error - no matching subexpression

   The "i" flag indicates that the ERE matching SHALL be performed in a
   case-insensitive fashion. Furthermore, any backref replacements MAY
   be normalized to lower case when the "i" flag is given.

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   The first character in the substitution expression shall be used as
   the character that delimits the components of the substitution
   expression.  There must be exactly three non-escaped occurrences of
   the delimiter character in a substitution expression. Since escaped
   occurrences of the delimiter character will be interpreted as
   occurrences of that character, digits MUST NOT be used as
   delimiters. Backrefs would be confused with literal digits were this
   allowed. Similarly, if flags are specified in the substitution
   expression, the delimiter character must not also be a flag
   character.

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4. The Basic NAPTR Algorithm

   The behavior and meaning of the flags and services assume an
   algorithm where the output of one rewrite is a new key that points
   to another rule. This looping algorithm allows NAPTR records to
   incrementally specify a complete rule. These incremental rules can
   be delegated which allows other entities to specify rules so that
   one entity does not need to understand _all_ rules.

   The algorithm starts with a string and some known key (domain).
   NAPTR records for this key are retrieved, those with unknown Flags
   or inappropriate Services are discarded and the remaining records
   are sorted by their Order field. Within each value of Order, the
   records are further sorted by the Preferences field.

   The records are examined in sorted order until a matching record is
   found. A record is considered a match iff:

   1.  it has a Replacement field value instead of a Regexp field value.

   2.  or the Regexp field matches the string held by the client.

   The first match MUST be the match that is used. Once a match is
   found, the Services field is examined for whether or not this rule
   advances toward the desired result. If so, the rule is applied to
   the target string. If not, the process halts. The domain that
   results from the regular expression is then used as the domain of
   the next loop through the NAPTR algorithm. Note that the same target
   string is used throughout the algorithm.

   This looping is extremely important since it is the method by which
   complex rules are broken down into manageable delegated chunks. The
   flags fields simply determine at which point the looping should stop
   (or other specialized behavior).

   Since flags are valid at any level of the algorithm, the
   degenerative case is to never loop but to look up the NAPTR and then
   stop. In many specialized cases this is all that is needed.
   Implementors should be aware that the degenerative case should not
   become the common case.

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5. Concerning How NAPTR Uses SRV Records

   When the SRV record type was originally specified it assumed that
   the client did not know the specific domain-name before hand. The
   client would construct a domain-name more in the form of a question
   than the usual case of knowing ahead of time that the domain-name
   should exist. I.e., if the client wants to know if there is a TCP
   based HTTP server running at a particular domain, the client would
   construct the domain-name _http._tcp.somedomain.com and ask the DNS
   if that records exists.

   In the case of NAPTR, the actual domain-name is specified by the
   various fields in the NAPTR record. In this case the client isn't
   asking a question but is instead attempting to get at information
   that it has been told exists in an SRV record at that particular
   domain-name. While this usage of SRV is slightly different than the
   SRV authors originally intended it does not break any of the
   assumptions concerning what SRV contains. Since NAPTR specifies the
   entire domain a priori it could have ignored the underscore
   collission avoidance convention. It was determined that such a
   radical departure from how SRV was assumed to work would cause
   problems in the long run. Thus the underscores were kept.

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6. Application Specifications

   It should be noted that the NAPTR algorithm is the basic assumption
   about how NAPTR works. The reasons for the rewrite and the expected
   output and its use are specified by documents that define what
   applications the NAPTR record and algorithm are used for. Any
   document that defines such an application must define the following:

   o  The first known domain-name or how to build it

   o  The valid Services and Protocols

   o  What the expected use is for the output of the last rewrite

   o  The validity and/or behavior of any 'P' flag protocols.

   o  The general semantics surrounding why and how NAPTR and its
      algorithm are being used.

   Currently the only example of such a document is RFC-XXXX,
   "Resolution of Uniform Resource Identifiers using the Domain Name
   System"[7].

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7. Examples

   NOTE: These are examples only. They are taken from ongoing work and
   may not represent the end result of that work. They are here for
   pedagogical reasons only.

7.1 Example 1

   NAPTR was originally specified for use with the a Uniform Resource
   Name Resolver Discovery System. This example details how a
   particular URN would use the NAPTR record to find a resolver
   service. The document that actually specifies this fully is
   RFCXXXX[7].

   Consider a URN namespace based on MIME Content-Ids. The URN might
   look like this:

      urn:cid:199606121851.1@mordred.gatech.edu

   (Note that this example is chosen for pedagogical purposes, and does
   not conform to the CID URL scheme.)

   The first step in the resolution process is to find out about the
   CID namespace. The namespace identifier[3], 'cid', is extracted from
   the URN, prepended to urn.net. 'cid.urn.net' then becomes the first
   'known' key in the NAPTR algorithm. The process of determining the
   first known key is defined by the URN RDS application document (
   RFCXXXX[7]). The NAPTR records for cid.urn.net looked up and return
   a single record:

      cid.urn.net.
      ;;       order pref flags service        regexp           replacement
      IN NAPTR 100   10   ""  ""  "/urn:cid:.+@([^\.]+\.)(.*)$/\2/i"    .

   There is only one NAPTR response, so ordering the responses is not a
   problem.  The replacement field is empty, so the pattern provided in
   the regexp field is used . We apply that regexp to the entire URN to
   see if it matches, which it does.  The \2 part of the substitution
   expression returns the string "gatech.edu". Since the flags field
   does not contain "s" or "a", the lookup is not terminal and our next
   probe to DNS is for more NAPTR records where the new domain is
   'gatech.edu' and the string is the same string as before.

   Note that the rule does not extract the full domain name from the
   CID, instead it assumes the CID comes from a host and extracts its
   domain.  While all hosts, such as mordred, could have their very own
   NAPTR, maintaining those records for all the machines at a site as
   large as Georgia Tech would be an intolerable burden. Wildcards are

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   not appropriate here since they only return results when there is no
   exactly matching names already in the system.

   The record returned from the query on "gatech.edu" might look like:

    gatech.edu.
    ;;       order pref flags service           regexp  replacement
    IN NAPTR 100  50  "s"  "z3950+N2L+N2C"     ""    _z3950._tcp.gatech.edu.
    IN NAPTR 100  50  "s"  "rcds+N2C"          ""    _rcds._udp.gatech.edu.
    IN NAPTR 100  50  "s"  "http+N2L+N2C+N2R"  ""    _http._tcp.gatech.edu.

   Continuing with the example, note that the values of the order and
   preference fields are equal in all records, so the client is free to
   pick any record. The flags field tells us that these are the last
   NAPTR patterns we should see, and after the rewrite (a simple
   replacement in this case) we should look up SRV records to get
   information on the hosts that can provide the necessary service.

   Assuming we prefer the Z39.50 protocol, our lookup might return:

     ;;                        Pref Weight   Port Target
     _z3950._tcp.gatech.edu. IN SRV 0    0      1000 z3950.gatech.edu.
                             IN SRV 0    0      1000 z3950.cc.gatech.edu.
                             IN SRV 0    0      1000 z3950.uga.edu.

   telling us three hosts that could actually do the resolution, and
   giving us the port we should use to talk to their Z39.50 server.

   Recall that the regular expression used \2 to extract a domain name
   from the CID, and \. for matching the literal '.' characters
   separating the domain name components. Since '\' is the escape
   character, literal occurances of a backslash must be escaped by
   another backslash. For the case of the cid.urn.net record above, the
   regular expression entered into the master file should be
   "/urn:cid:.+@([^\\.]+\\.)(.*)$/\\2/i".  When the client code
   actually receives the record, the pattern will have been converted
   to "/urn:cid:.+@([^\.]+\.)(.*)$/\2/i".

7.2 Example 2

   Even if URN systems were in place now, there would still be a
   tremendous number of URLs.  It should be possible to develop a URN
   resolution system that can also provide location independence for
   those URLs.  This is related to the requirement that URNs be able to
   grandfather in names from other naming systems, such as ISO Formal
   Public Identifiers, Library of Congress Call Numbers, ISBNs, ISSNs,
   etc.

   The NAPTR RR could also be used for URLs that have already been
   assigned.  Assume we have the URL for a very popular piece of

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   software that the publisher wishes to mirror at multiple sites
   around the world:

      http://www.foo.com/software/latest-beta.exe

   Using the rules specified in RFCXXXX[7] we extract the prefix,
   "http", and lookup NAPTR records for http.uri.net. Note again that
   RFCXXXX defines the first known key as the URI scheme appended with
   the string ".uri.net". This might return a record of the form

      http.uri.net. IN NAPTR
      ;;  order   pref flags service      regexp             replacement
           100     90   ""      ""   "!http://([^/:]+)!\1!i"       .

   This expression returns everything after the first double slash and
   before the next slash or colon. (We use the '!' character to delimit
   the parts of the substitution expression. Otherwise we would have to
   use backslashes to escape the forward slashes and would have a
   regexp in the zone file that looked like
   "/http:\\/\\/([^\\/:]+)/\\1/i".).

   Applying this pattern to the URL extracts "www.foo.com". Looking up
   NAPTR records for that might return:

      www.foo.com.
      ;;       order pref flags   service  regexp     replacement
       IN NAPTR 100  100  "s"   "http+L2R"   ""    _http._tcp.foo.com.
       IN NAPTR 100  100  "s"   "ftp+L2R"    ""    _ftp._tcp.foo.com.

   Looking up SRV records for http.tcp.foo.com would return information
   on the hosts that foo.com has designated to be its mirror sites. The
   client can then pick one for the user.

7.3 Example 3

   A non-URI example is where a NAPTR is used to specify the available
   mappings from a domain-name to telephony based endpoints. In this
   example the regular expression field is not used since the important
   information is encoded within the services field.

      0.0.0.4.6.2.6.5.8.6.4.e164.int.
          IN NAPTR 100 10 "s" "h323call+N2R" "" _h323._udp.tele2.se.
          IN NAPTR 102 10 "s" "potscall+N2R" "" _potscall._tcp.tele2.se.
          IN NAPTR 102 10 "s" "smtp+N2R"     "" _smtp._tcp.tele2.se.

   In these examples the domain is an encoded E164 telephone number.
   The services field specifies that, for this particular telephone
   number, the services that are available are h323call, potscall and
   smtp; and that "tele2.se" is the target that provides those

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   services. Since the flag is "s", the next step should be a query for
   an SRV record which will contain specific information about the
   "tele2.se" domain.

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8. DNS Packet Format

      The packet format for the NAPTR record is as follows
                                       1  1  1  1  1  1
         0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
       |                     ORDER                     |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
       |                   PREFERENCE                  |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
       /                     FLAGS                     /
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
       /                   SERVICES                    /
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
       /                    REGEXP                     /
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
       /                  REPLACEMENT                  /
       /                                               /
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

   where:

   FLAGS A <character-string> which contains various flags.

   SERVICES A <character-string> which contains protocol and service
      identifiers.

   REGEXP A <character-string> which contains a regular expression.

   REPLACEMENT A <domain-name> which specifies the new value in the
      case where the regular expression is a simple replacement
      operation.

   <character-string> and <domain-name> as used here are defined in
   RFC1035[1].

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9. Master File Format

   The master file format follows the standard rules in RFC-1035[1].
   Order and preference, being 16-bit unsigned integers, shall be an
   integer between 0 and 65535. The Flags and Services and Regexp
   fields are all quoted <character-string>s.  Since the Regexp field
   can contain numerous backslashes and thus should be treated with
   care. See Section 10 for how to correctly enter and escape the
   regular expression.

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10. Advice for DNS Administrators

   Beware of regular expressions. Not only are they difficult to get
   correct on their own, but there is the previously mentioned
   interaction with DNS. Any backslashes in a regexp must be entered
   twice in a zone file in order to appear once in a query response.
   More seriously, the need for double backslashes has probably not
   been tested by all implementors of DNS servers.

   The "a" flag allows the next lookup to be for A records rather than
   SRV records. Since there is no place for a port specification in the
   NAPTR record, when the "A" flag is used the specified protocol must
   be running on its default port.

   The URN Syntax draft defines a canonical form for each URN, which
   requires %encoding characters outside a limited repertoire. The
   regular expressions MUST be written to operate on that canonical
   form. Since international character sets will end up with extensive
   use of %encoded characters, regular expressions operating on them
   will be essentially impossible to read or write by hand.

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11. Notes

   o  A client MUST process multiple NAPTR records in the order
      specified by the "order" field, it MUST NOT simply use the first
      record that provides a known protocol and service combination.

   o  When multiple RRs have the same "order" and all other criteria
      being equal, the client should use the value of the preference
      field to select the next NAPTR to consider. However, because it
      will often be the case where preferred protocols or services
      exist, clients may use this additional criteria to        sort
      the records.

   o  If the lookup after a rewrite fails, clients are strongly
      encouraged to report a failure, rather than backing up to pursue
      other rewrite paths.

   o  Note that SRV RRs impose additional requirements on clients.

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12. Acknowledgments

   The editors would like to thank Keith Moore for all his
   consultations during the development of this draft. We would also
   like to thank Paul Vixie for his assistance in debugging our
   implementation, and his answers on our questions. Finally, we would
   like to acknowledge our enormous intellectual debt to the
   participants in the Knoxville series of meetings, as well as to the
   participants in the URI and URN working groups.

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References

   [1]  Mockapetris, P., "Domain names - implementation and
        specification", RFC 1035, STD 13, November 1987.

   [2]  Mockapetris, P., "Domain names - concepts and facilities", RFC
        1034, STD 13, November 1987.

   [3]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [4]  Eastlake, D., Gulbrandsen, A., "A DNS RR for specifying the
        location of services (DNS SRV)", January 1999.

   [5]  Crocker, D., Overell, P., "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [6]  Daniel, R., "A Trivial Convention for using HTTP in URN
        Resolution", RFC 2169, June 1997.

   [7]  Mealling, M., Daniel, R., "Resolution of Uniform Resource
        Identifiers using the Domain Name System", July 1999.

   [8]  IEEE, "IEEE Standard for Information Technology - Portable
        Operating System Interface (POSIX) - Part 2: Shell and
        Utilities (Vol. 1)", IEEE Std 1003.2-1992, January 1993.

   [9]  Berners-Lee, T., Fielding, R., Masinter, L., "Uniform Resource
        Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

Authors' Addresses

   Michael Mealling
   Network Solutions, Inc.
   505 Huntmar Park Drive
   Herndon, VA  22070
   US

   Phone: +1 770 935 5492
   EMail: michaelm@netsol.com
   URI:   http://www.netsol.com

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   Ron Daniel
   DATAFUSION, Inc.
   139 Townsend Street, Ste. 100
   San Francisco, CA  94107
   US

   Phone: +1 415 222 0100
   EMail: rdaniel@datafusion.net
   URI:   http://www.datafusion.net

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Appendix A. IANA Considerations

   The only registration function that impacts the IANA is for the
   values that are standardized for the Services and Flags fields. To
   extend the valid values of the Flags field beyond what is specified
   in this document requires a published specification that is approved
   by the IESG.

   The values for the Services field will be determined by the
   application that makes use of the NAPTR record. Those values must be
   specified in a published specification and approved by the IESG.

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Appendix B. Security Considerations

   The interactions with DNSSEC are currently being studied. It is
   expected that NAPTR records will be signed with SIG records once the
   DNSSEC work is deployed.

   The rewrite rules make identifiers from other namespaces subject to
   the same attacks as normal domain names. Since they have not been
   easily resolvable before, this may or may not be considered a
   problem.

   Regular expressions should be checked for sanity, not blindly passed
   to something like PERL.

   This document has discussed a way of locating a service, but has not
   discussed any detail of how the communication with that service
   takes place. There are significant security considerations attached
   to the communication with a service. Those considerations are
   outside the scope of this document, and must be addressed by the
   specifications for particular communication protocols.

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Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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