rfc2168.txt

bind-3.2.


RFC 2168            Resolution of URIs Using the DNS           June 1997


   Order
          A 16-bit integer specifying the order in which the NAPTR
          records MUST be processed to ensure correct delegation of
          portions of the namespace over time. Low numbers are processed
          before high numbers, and once a NAPTR is found that "matches"
          a URN, the client MUST NOT consider any NAPTRs with a higher
          value for order.

   Preference
          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.

   Flags
          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.

          At this time only three flags, "S", "A", and "P", are defined.
          "S" means that the next lookup should be for SRV 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 resolution shall be carried out in a Protocol-specific
          fashion, and we should not do any more DNS queries.

          The remaining alphabetic flags are reserved. 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 URN.







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   Service
          Specifies the resolution service(s) available down this
          rewrite path. It may also specify the particular protocol that
          is used to talk with a resolver. 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 822[9]):

           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.
        // The current set of "known" strings are:
        // protocol      = "rcds" / "thttp" / "hdl" / "rwhois" / "z3950"
        // rs            = "N2L" / "N2Ls" / "N2R" / "N2Rs" / "N2C"
        //               / "N2Ns" / "L2R" / "L2Ns" / "L2Ls" / "L2C"

          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 top levels of a namespace, when
          it is impossible to know what services and protocols will be
          offered by a particular publisher within that name space.

          At this time the known protocols are rcds[7], hdl[10] (binary,
          UDP-based protocols),  thttp[5] (a textual, TCP-based
          protocol), rwhois[11] (textual, UDP or TCP based), and
          Z39.50[12] (binary, TCP-based). More will be allowed later.
          The names of the protocols must be formed from the characters
          [a-Z0-9]. Case of the characters is not significant.

          The service requests currently allowed will be described in
          more detail in [6], but in brief they are:
                N2L  - Given a URN, return a URL
                N2Ls - Given a URN, return a set of URLs
                N2R  - Given a URN, return an instance of the resource.
                N2Rs - Given a URN, return multiple instances of the
                       resource, typically encoded using
                       multipart/alternative.



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                N2C  - Given a URN, return a collection of meta-
                       information on the named resource. The format of
                       this response is the subject of another document.
                N2Ns - Given a URN, return all URNs that are also
                       identifers for the resource.
                L2R  - Given a URL, return the resource.
                L2Ns - Given a URL, return all the URNs that are
                       identifiers for the resource.
                L2Ls - Given a URL, return all the URLs for instances of
                       of the same resource.
                L2C  - Given a URL, return a description of the
                       resource.

          The actual format of the service request and response will be
          determined by the resolution protocol, and is the subject for
          other documents (e.g. [5]). 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.

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

   Replacement
          The next NAME to query for NAPTR, SRV, or A records depending
          on the value of the flags field. As mentioned above, this may
          be compressed.

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 [13], section
               2.8.4)
repl         = dns_str /  backref / repl dns_str  / repl backref
dns_str      = 1*DNS_CHAR
backref      = "\" 1POS_DIGIT



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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 [14]).

   The result of applying the substitution expression to the original
   URI MUST result in a string that obeys the syntax for DNS host names
   [14]. 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
                      (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.

   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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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. We anticipate that urn.net
   will be the heaviest user of regexps. Only when delegating portions
   of namespaces should the typical domain administrator need to use
   regexps.

   On a related note, beware of interactions with the shell when
   manipulating regexps from the command line. Since '\' is a common
   escape character in shells, there is a good chance that when you
   think you are saying "\\" you are actually saying "\".  Similar
   caveats apply to characters such as

   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 Sytnax 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.

Usage
=====

   For the edification of implementers, pseudocode for a client routine
   using NAPTRs is given below. This code is provided merely as a
   convience, it does not have any weight as a standard way to process
   NAPTR records. Also, as is the case with pseudocode, it has never
   been executed and may contain logical errors. You have been warned.

    //
    // findResolver(URN)
    // Given a URN, find a host that can resolve it.
    //
    findResolver(string URN) {
      // prepend prefix to urn.net
      sprintf(key, "%s.urn.net", extractNS(URN));
      do {



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