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 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








 
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