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









 
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