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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.


 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 Systems to moving out-of-date services to new domains.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] RFC 2915 NAPTR DNS RR September 2000

 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 separation is due to several
 different 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
    in this document are to be interpreted as described in RFC 2119.
    All references to Uniform Resource Identifiers in this document
    adhere to the 'absoluteURI' production of the "Collected ABNF"
    found in RFC 2396 [9].  Specifically, the semantics of URI
    References do not apply since the concept of a Base makes no sense

2. NAPTR RR Format

 The format of the NAPTR RR is given below.  The DNS type code [1] for
 NAPTR is 35.
 Domain TTL Class Type Order Preference Flags Service Regexp
    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> 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].
    Standard DNS meaning [1].
    Standard DNS meaning [1].
    The Type Code [1] for NAPTR is 35.

Mealling & Daniel Standards Track [Page 3] RFC 2915 NAPTR DNS RR September 2000

    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).
    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.
    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.
    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 either
    an A, AAAA, or A6 record.  The "U" flag means that the next step
    is not a DNS lookup but that the output of the Regexp field is an
    URI that adheres to the 'absoluteURI' production found in the
    ABNF of RFC 2396 [9].  Since there may be applications that use
    NAPTR to also lookup aspects of URIs, implementors should be
    aware that this may cause loop conditions and should act

Mealling & Daniel Standards Track [Page 4] RFC 2915 NAPTR DNS RR September 2000

    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,AAAA, or A6 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.
    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

Mealling & Daniel Standards Track [Page 5] RFC 2915 NAPTR DNS RR September 2000

    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
  • 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.
    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
    with a URN resolver.

Mealling & Daniel Standards Track [Page 6] RFC 2915 NAPTR DNS RR September 2000

    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.
    The next NAME to query for NAPTR, SRV, or address 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

3. Substitution Expression Grammar

 The content of the regexp field is a substitution expression.  True
 sed(1) and Perl style substitution expressions are not appropriate
 for use in this application for a variety of reasons stemming from
 internationalization requirements and backref limitations, 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.

Mealling & Daniel Standards Track [Page 7] RFC 2915 NAPTR DNS RR September 2000

 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
       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.
 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.

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:

Mealling & Daniel Standards Track [Page 8] RFC 2915 NAPTR DNS RR September 2000

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

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 and ask the DNS
 if that records exists. The underscores are used to avoid collisions
 with potentially 'real' domain-names.
 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.  Also, since the NAPTR
 explicitly spells out the domain-name for which an SRV exists, that
 domain-name MUST be used in SRV queries with NO transformations.  Any
 given NAPTR record may result in a domain-name to be used for SRV
 queries that may or may not contain the SRV standardized underscore

Mealling & Daniel Standards Track [Page 9] RFC 2915 NAPTR DNS RR September 2000

 characters.  NAPTR applications that make use of SRV MUST NOT attempt
 to understand these domains or use them according to how the SRV
 specification structures its query domains.

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.

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.
 Consider a URN namespace based on MIME Content-Ids.  The URN might
 look like this:
 (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 '' then becomes the first
 'known' key in the NAPTR algorithm.  The NAPTR records for looked up and return a single record:

Mealling & Daniel Standards Track [Page 10] RFC 2915 NAPTR DNS RR September 2000
 ;;       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 "".  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 '' 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
 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 "" might look like:

;; order pref flags service regexp replacement IN NAPTR 100 50 "s" "z3950+I2L+I2C" "" IN NAPTR 100 50 "s" "rcds+I2C" "" IN NAPTR 100 50 "s" "http+I2L+I2C+I2R" ""

 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 IN SRV 0 0 1000

                       IN SRV 0    0      1000
                       IN SRV 0    0      1000
 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

Mealling & Daniel Standards Track [Page 11] RFC 2915 NAPTR DNS RR September 2000

 character, literal occurances of a backslash must be escaped by
 another backslash.  For the case of the 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

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,
 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
 software that the publisher wishes to mirror at multiple sites around
 the world:
 Using the rules specified for this application we extract the prefix,
 "http", and lookup NAPTR records for  This might
 return a record of the form 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 "".  Looking up
 NAPTR records for that might return:
   ;;       order pref flags   service  regexp     replacement
    IN NAPTR 100  100  "s"   "http+I2R"   ""
    IN NAPTR 100  100  "s"   "ftp+I2R"    ""
 Looking up SRV records for would return information
 on the hosts that has designated to be its mirror sites.  The
 client can then pick one for the user.

Mealling & Daniel Standards Track [Page 12] RFC 2915 NAPTR DNS RR September 2000

7.3 Example 3

 A non-URI example is the ENUM application which uses a NAPTR record
 to map an e.164 telephone number to a URI.  In order to convert the
 phone number to a domain name for the first iteration all characters
 other than digits are removed from the the telephone number, the
 entire number is inverted, periods are put between each digit and the
 string "" is put on the left-hand side.  For example, the
 E.164 phone number "+1-770-555-1212" converted to a domain-name it
 would be ""
 For this example telephone number we might get back the following
 NAPTR records:

$ORIGIN IN NAPTR 100 10 "u" "sip+E2U" "!^.*$!!" . IN NAPTR 102 10 "u" "mailto+E2U" "!^.*$!!" .

 This application uses the same 'u' flag as the URI Resolution
 application. This flag states that the Rule is terminal and that the
 output is a URI which contains the information needed to contact that
 telephone service.  ENUM also uses the same format for its Service
 field except that it defines the 'E2U' service instead of the 'I2*'
 services that URI resolution uses.  The example above states that the
 available protocols used to access that telephone's service are
 either the Session Initiation Protocol or SMTP mail.

8. DNS Packet Format

       The packet format for the NAPTR record is:
                                        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                  /
        /                                               /

Mealling & Daniel Standards Track [Page 13] RFC 2915 NAPTR DNS RR September 2000

 FLAGS A <character-string> which contains various flags.
 SERVICES A <character-string> which contains protocol and service
 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
 <character-string> and <domain-name> as used here are defined in
 RFC1035 [1].

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.

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 address records (A,
 AAAA, A6) 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.

Mealling & Daniel Standards Track [Page 14] RFC 2915 NAPTR DNS RR September 2000

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.

12. 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.

13. 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
 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

Mealling & Daniel Standards Track [Page 15] RFC 2915 NAPTR DNS RR September 2000

 communication with a service.  Those considerations are outside the
 scope of this document, and must be addressed by the specifications
 for particular communication protocols.

14. Acknowledgments

 The editors would like to thank Keith Moore for all his consultations
 during the development of this memo.  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.


 [1]  Mockapetris, P., "Domain names - implementation and
      specification", STD 13, RFC 1035, November 1987.
 [2]  Mockapetris, P., "Domain names - concepts and facilities", STD
      13, RFC 1034, November 1987.
 [3]  Moats, R., "URN Syntax", RFC 2141, May 1997.
 [4]  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
      specifying the location of services (DNS SRV)", RFC 2782,
      February 2000.
 [5]  Crocker, D., "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]  Daniel, R. and M. Mealling, "Resolution of Uniform Resource
      Identifiers using the Domain Name System", RFC 2168, June 1997.
 [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.T. and L. Masinter, "Uniform
      Resource Identifiers (URI): Generic Syntax", RFC 2396, August

Mealling & Daniel Standards Track [Page 16] RFC 2915 NAPTR DNS RR September 2000

Authors' Addresses

 Michael Mealling
 Network Solutions, Inc.
 505 Huntmar Park Drive
 Herndon, VA  22070
 Phone: +1 770 921 2251
 Ron Daniel
 139 Townsend Street, Ste. 100
 San Francisco, CA  94107
 Phone: +1 415 222 0100

Mealling & Daniel Standards Track [Page 17] RFC 2915 NAPTR DNS RR September 2000

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Mealling & Daniel Standards Track [Page 18]

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