Network Working Group                                     M. Mealling
draft-ietf-urn-naptr-rr-00.txt                Network Solutions, Inc.
Category: Standards Track                                   R. Daniel
Expires: May, 1999                                   DATAFUSION, Inc.
                                                             Nov 1998

      The Naming Authority Pointer (NAPTR) DNS Resource Record

Status of this Memo

     This document is an Internet-Draft.  Internet-Drafts are working
     documents of the Internet Engineering Task Force (IETF), its
     areas, and its working groups.  Note that other groups may also
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     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other
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     Drafts as reference material or to cite them other than as
     "work in progress."

     To view the entire list of current Internet-Drafts, please check
     the "1id-abstracts.txt" listing contained in the Internet-Drafts
     Shadow Directories on (Africa),
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   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   vary from URN Resource Discovery Systems to moving out of date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.


   This RR was originally produced by the URN [3] Working Group 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 which 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 the
   record has usefullness in several different applications. This
   document defines the basic assumptions that all of those applications
   must adhere to. It does not define the reasons for why the rewrite
   is used or 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.

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   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 which interact
   in a non-trivial but implementable way. This document specifies
   those fields and their values.

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


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

       Domain TTL Class Order Preference Flags Service Regexp

          The domain name this resource record refers to. 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]

          A 16-bit 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 integer which 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 no understanding the preferred protocol or service.

          A String giving 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 three flags, "S", "A", and "P", are defined.
          "S" means that the next lookup should be for SRV [4] records
          instead of NAPTR records. "A" means that the next lookup
          should be for A records. 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 [17].

          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, 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). We anticipate 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" and "A" flags are called 'terminal' flags since they
          halt any looping rewrite algorithms. If those flags are
          not present then 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 no 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 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

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

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           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.

          i.e. 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 RFCXXXX, "Resolution of Uniform Resource
          Identifiers using the Domain Name System" [7], from which
          this document was extracted.

          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.

          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 A records depending
          on the value of the flags field. This MUST be DNS compressed.

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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 (see [6], section
repl         = dns_str /  backref / repl dns_str  / repl backref
dns_str      = 1*DNS_CHAR
backref      = "\" 1POS_DIGIT
flags        = "i"
DNS_CHAR     = "-" / "0" / ... / "9" / "a" / ... / "z" / "A" / ... / "Z"
POS_DIGIT    = "1" / "2" / ... / "9"  ; 0 is not an allowed backref
value domain name (see RFC-1123 [7]).

   The result of applying the substitution expression to the original
   URI MUST result in a string that obeys the syntax for DNS host names
   [7]. Since it is possible for the regexp field to be improperly
   specified, such that a non-conforming host name can be constructed,
   client software SHOULD verify that the result is a legal host 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
   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.

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) 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, then 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 lookup the NATPR 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.

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
   applicatiions the NAPTR record and algorithm are used for. Any
   document that defines such an application must define the following:

         * The first known key or how to build it
         * The valid Services and Protocols
         * What the expected use is for the output of the last rewrite
         * The validity and/or behavior of any 'P' flag protocols.
         * The general semantics surrounding why and how NAPTR and its
           algorithm are being used.

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

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   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.

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

   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 for looked
   up and returns a record:
   ;;       order pref flags service        regexp           replacement
   IN NAPTR 100   10   ""  ""  "/urn:cid:.+@([^\.]+\.)(.*)$/\2/i"    .

   We have only one NAPTR response, so ordering the responses is not a
   problem.  The replacement field is empty, so we check the regexp
   field and use the pattern provided there. 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.

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   The record returned from the query on "" might look like: IN NAPTR
   ;;       order pref flags service           regexp  replacement
   IN NAPTR 100  50  "s"  "z3950+N2L+N2C"     ""
   IN NAPTR 100  50  "s"  "rcds+N2C"          ""
   IN NAPTR 100  50  "s"  "http+N2L+N2C+N2R"  ""

   Continuing with our example, we 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
   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 zone file should be
   "/urn:cid:.+@([^\\.]+\\.)(.*)$/\\2/i".  When the client code actually
   receives the record, the pattern will have been converted to

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:

   We extract the prefix, "http", and lookup NAPTR records for This might return a record of the form

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RFC nnnn            The NAPTR Resource Record Type         November 1998 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

   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+L2R"   ""
    IN NAPTR 100  100  "s"   "ftp+L2R"    ""

   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.

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.
       IN NAPTR 100 10 "s" "h323call+N2R" ""
       IN NAPTR 102 10 "s" "potscall+N2R" ""
       IN NAPTR 102 10 "s" "smtp+N2R"     ""

   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 "" is the target that provides those
   services. Since the flag is "s" then the next step should be a
   query for an SRV record which will contain specific information
   about the "" domain.

Advice to domain administrators:

   Beware of regular expressions. Not only are they a pain 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.

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   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.


     -  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.
     -  When multiple RRs have the same "order", the client should use
        the value of the preference field to select the next NAPTR to
        consider. However, because of preferred protocols or services,
        estimates of network distance and bandwidth, etc. clients may
        use different criteria to sort the records.
     -  If the lookup after a rewrite fails, clients are strongly
        encouraged to report a failure, rather than backing up to pursue
        other rewrite paths.
     -  Note that SRV RRs impose additional requirements on clients.


   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.


   [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, Ryan, "URN Syntax", RFC-2141, May 1997.

   [4]  Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying
        the location of services (DNS SRV)", RFC-2052, October 1996.

   [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".
        RFC2169. June 1997.

   [7] Mealling, M., Daniel, R., "Resolution of Uniform Resource
       Identifiers using the Domain Name System". RFCXXXX. November 1998.

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   [8] IEEE Standard for Information Technology - Portable Operating
        System Interface (POSIX) - Part 2: Shell and Utilities (Vol. 1);
        IEEE Std 1003.2-1992; The Institute of Electrical and
        Electronics Engineers; New York; 1993. ISBN:1-55937-255-9

   [9] Braden, R., "Requirements for Internet Hosts - Application and
        and Support", RFC-1123, Oct. 1989.

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.

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
   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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Author Contact Information:

   Michael Mealling
   Network Solutions
   505 Huntmar Park Drive
   Herndon, VA  22070
   voice: (703) 742-0400
   fax: (703) 742-9552

   Ron Daniel Jr.
   139 Townsend Street, Ste. 100
   San Francisco, CA  94107
   415.222.0100 fax 415.222.0150

Mealling & Daniel                                              [Page 12]