rfc2168.txt

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Daniel & Mealling             Experimental                      [Page 5]

RFC 2168            Resolution of URIs Using the DNS           June 1997


   Note that the client applies all the substitutions and performs all
   lookups, they are not performed in the DNS servers. Note also that it
   is the belief of the developers of this document that regexps should
   rarely be used. The replacement field seems adequate for the vast
   majority of situations. Regexps are only necessary when portions of a
   namespace are to be delegated to different resolvers. Finally, note
   that the regexp and replacement fields are, at present, mutually
   exclusive. However, developers of client software should be aware
   that a new flag might be defined which requires values in both
   fields.

Example 1
---------

   Consider a URN that uses the hypothetical DUNS namespace. DUNS
   numbers are identifiers for approximately 30 million registered
   businesses around the world, assigned and maintained by Dunn and
   Bradstreet. The URN might look like:

                   urn:duns:002372413:annual-report-1997

   The first step in the resolution process is to find out about the
   DUNS namespace. The namespace identifier, "duns", is extracted from
   the URN, prepended to urn.net, and the NAPTRs for duns.urn.net looked
   up. It might return records of the form:

duns.urn.net
;;      order pref flags service          regexp        replacement
 IN NAPTR 100  10  "s" "dunslink+N2L+N2C" ""  dunslink.udp.isi.dandb.com
 IN NAPTR 100  20  "s" "rcds+N2C"         ""  rcds.udp.isi.dandb.com
 IN NAPTR 100  30  "s" "http+N2L+N2C+N2R" ""  http.tcp.isi.dandb.com

   The order field contains equal values, indicating that no name
   delegation order has to be followed. The preference field indicates
   that the provider would like clients to use the special dunslink
   protocol, followed by the RCDS protocol, and that HTTP is offered as
   a last resort. All the records specify the "s" flag, which will be
   explained momentarily.  The service fields say that if we speak
   dunslink, we will be able to issue either the N2L or N2C requests to
   obtain a URL or a URC (description) of the resource. The Resource
   Cataloging and Distribution Service (RCDS)[7] could be used to get a
   URC for the resource, while HTTP could be used to get a URL, URC, or
   the resource itself.  All the records supply the next domain name to
   query, none of them need to be rewritten with the aid of regular
   expressions.






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RFC 2168            Resolution of URIs Using the DNS           June 1997


   The general case might require multiple NAPTR rewrites to locate a
   resolver, but eventually we will come to the "terminal NAPTR". Once
   we have the terminal NAPTR, our next probe into the DNS will be for a
   SRV or A record instead of another NAPTR. Rather than probing for a
   non-existent NAPTR record to terminate the loop, the flags field is
   used to indicate a terminal lookup. If it has a value of "s", the
   next lookup should be for SRV RRs, "a" denotes that A records should
   sought.  A "p" flag is also provided to indicate that the next action
   is Protocol-specific, but that looking up another NAPTR will not be
   part of it.

   Since our example RR specified the "s" flag, it was terminal.
   Assuming our client does not know the dunslink protocol, our next
   action is to lookup SRV RRs for rcds.udp.isi.dandb.com, which will
   tell us hosts that can provide the necessary resolution service. That
   lookup might return:

    ;;                          Pref Weight Port Target
    rcds.udp.isi.dandb.com IN SRV 0    0    1000 defduns.isi.dandb.com
                           IN SRV 0    0    1000 dbmirror.com.au
                           IN SRV 0    0    1000 ukmirror.com.uk

   telling us three hosts that could actually do the resolution, and
   giving us the port we should use to talk to their RCDS server.  (The
   reader is referred to the SRV proposal [4] for the interpretation of
   the fields above).

   There is opportunity for significant optimization here. We can return
   the SRV records as additional information for terminal NAPTRs (and
   the A records as additional information for those SRVs). While this
   recursive provision of additional information is not explicitly
   blessed in the DNS specifications, it is not forbidden, and BIND does
   take advantage of it [8]. This is a significant optimization. In
   conjunction with a long TTL for *.urn.net records, the average number
   of probes to DNS for resolving DUNS URNs would approach one.
   Therefore, DNS server implementors SHOULD provide additional
   information with NAPTR responses. The additional information will be
   either SRV or A records.  If SRV records are available, their A
   records should be provided as recursive additional information.

   Note that the example NAPTR records above are intended to represent
   the reply the client will see. They are not quite identical to what
   the domain administrator would put into the zone files. For one
   thing, the administrator should supply the trailing '.' character on
   any FQDNs.






Daniel & Mealling             Experimental                      [Page 7]

RFC 2168            Resolution of URIs Using the DNS           June 1997


Example 2
---------

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

                 urn:cid:[e-mail]

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

   The first step in the resolution process is to find out about the CID
   namespace. The namespace identifier, cid, is extracted from the URN,
   prepended to urn.net, and the NAPTR for cid.urn.net looked up. It
   might return records of the form:

 cid.urn.net
  ;;       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 "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:
   lookup(query=NAPTR, "gatech.edu").

   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 "gatech.edu" might look like:

gatech.edu IN NAPTR
;;       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







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RFC 2168            Resolution of URIs Using the DNS           June 1997


   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
   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
   seperating 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 zone 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".

Example 3
---------

   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 in [1] to 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
   software that the publisher wishes to mirror at multiple sites around
   the world:

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







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RFC 2168            Resolution of URIs Using the DNS           June 1997


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

   http.urn.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.

NAPTR RR Format
===============

   The format of the NAPTR RR is given below. The DNS type code for
   NAPTR is 35.

       Domain TTL Class Order Preference Flags Service Regexp
       Replacement

   where:

   Domain
          The domain name this resource record refers to.
   TTL
          Standard DNS Time To Live field
   Class
          Standard DNS meaning








Daniel & Mealling             Experimental                     [Page 10]