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.







 
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.







 
RFC 2168            Resolution of URIs Using the DNS           June 1997


Example 2
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