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 {




 
RFC 2168            Resolution of URIs Using the DNS           June 1997


        rewrite_flag = false;
        terminal = false;
        if (key has been seen) {
          quit with a loop detected error
        }
        add key to list of "seens"
        records = lookup(type=NAPTR, key); // get all NAPTR RRs for 'key'

        discard any records with an unknown value in the "flags" field.
        sort NAPTR records by "order" field and "preference" field
            (with "order" being more significant than "preference").
        n_naptrs = number of NAPTR records in response.
        curr_order = records[0].order;
        max_order = records[n_naptrs-1].order;

        // Process current batch of NAPTRs according to "order" field.
        for (j=0; j < n_naptrs && records[j].order <= max_order; j++) {
          if (unknown_flag) // skip this record and go to next one
             continue;
          newkey = rewrite(URN, naptr[j].replacement, naptr[j].regexp);
          if (!newkey) // Skip to next record if the rewrite didn't
             match continue;
          // We did do a rewrite, shrink max_order to current value
          // so that delegation works properly
          max_order = naptr[j].order;
          // Will we know what to do with the protocol and services
          // specified in the NAPTR? If not, try next record.
          if(!isKnownProto(naptr[j].services)) {
            continue;
          }
          if(!isKnownService(naptr[j].services)) {
            continue;
          }

          // At this point we have a successful rewrite and we will
          // know how to speak the protocol and request a known
          // resolution service. Before we do the next lookup, check
          // some optimization possibilities.

          if (strcasecmp(flags, "S")
           || strcasecmp(flags, "P"))
           || strcasecmp(flags, "A")) {
             terminal = true;
             services = naptr[j].services;
             addnl = any SRV and/or A records returned as additional
                     info for naptr[j].
          }
          key = newkey;




 
RFC 2168            Resolution of URIs Using the DNS           June 1997