Network Working Group                                     M. Mealling
Internet Draft                                Network Solutions, Inc.
draft-ietf-urn-dns-rds-00.txt                               R. Daniel
Category: Standards Track                            DATAFUSION, Inc.
Expires: May, 1999

               Resolution of Uniform Resource Identifiers
                      using the Domain Name System

Status of this Memo

     This document is an Internet-Draft.  Internet-Drafts are working
     documents of the Internet Engineering Task Force (IETF), its
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   The architectural principles laid out in RFC2276 [15] defines the
   concept of a "resolver discovery service". This document describes
   an immediately-deployable RDS. It is implemented by a new DNS Resource
   Record, NAPTR (Naming Authority PoinTeR) [16], that provides a method
   for encoding incrementally discovered rules within DNS. By using
   these incrementally discovered rules to re-map parts of a URI, we
   can change the host that is contacted to resolve a URI.  This will
   allow a more graceful handling of URLs over long time periods, and
   forms the foundation for a new proposal for Uniform Resource Names.

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   In addition to locating resolvers, the NAPTR provides for other
   naming systems to be grandfathered into the URN world, provides
   independence between the name assignment system and the resolution
   protocol system, and allows multiple services (Identifier to
   Location, Identifier to Description, Identifier to Resource, ...)
   to be offered.  In conjunction with the SRV RR, the NAPTR record
   allows those services to be replicated for the purposes of fault
   tolerance and load balancing.


   Uniform Resource Locators have been a significant advance in
   retrieving Internet-accessible resources. However, their  brittle
   nature over time has been recognized for several years. The Uniform
   Resource Identifier working group proposed the development of Uniform
   Resource Names to serve as persistent, location-independent
   identifiers for Internet resources in order to overcome most of the
   problems with URLs. RFC-1737 [1] sets forth requirements on URNs.

   During the lifetime of the URI-WG, a number of URN proposals were
   generated. The developers of several of those proposals met in a
   series of meetings, resulting in a compromise known as the Knoxville
   framework.  The major principle behind the Knoxville framework is
   that the resolution system must be separate from the way names are
   assigned. This is in marked contrast to most URLs, which identify the
   host to contact and the protocol to use. Readers are referred to [2]
   for background on the Knoxville framework and for additional
   information on the context and purpose of this proposal.

   Separating the way names are resolved from the way they are
   constructed provides several benefits. It allows multiple naming
   approaches and resolution approaches to compete, as it allows
   different protocols and resolvers to be used. There is just one
   problem with such a separation - how do we resolve a name when it
   can't give us directions to its resolver?

   For the short term, DNS is the obvious candidate for the resolution
   framework, since it is widely deployed and understood. However, it is
   not appropriate to use DNS to maintain information on a per-resource
   basis. First of all, DNS was never intended to handle that many
   records. Second, the limited record size is inappropriate for catalog
   information. Third, domain names are not appropriate as URNs.

   Therefore our approach is to use DNS to locate "resolvers" that can
   provide information on individual resources, potentially including
   the resource itself. To accomplish this, we "rewrite" the URI into a
   domain name following the rules found in NAPTR records. Rewrite
   rules provide considerable power, which is important when trying to
   meet the goals listed above. However, collections of rules can become
   difficult to understand. To lessen this problem, the NAPTR rules are
   *always* applied to the original URI, *never* to the output of
   previous rules.

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      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
      "OPTIONAL" in this document are to be interpreted as described in
      RFC 2119.

Overview of the NAPTR Record

   The NAPTR record is defined in RFCXXXX [16]. To summarize, the key
   fields in the NAPTR RR are Order, Preference, Service, Flags, Regexp,
   and Replacement:

   * The order field specifies the order in which records MUST be
     processed when multiple NAPTR records are returned in response to a
     single query.  A naming authority may have delegated a portion of
     its namespace to another agency. Evaluating the NAPTR records in
     the correct order is necessary for delegation to work properly.

   * The preference field specifies the order in which records SHOULD be
     processed when multiple NAPTR records have the same value of
     "order".  This field lets a service provider specify the order in
     which resolvers are contacted, so that more capable machines are
     contacted in preference to less capable ones.

   * The service field specifies the resolution protocol and resolution
     service(s) that will be available if the rewrite specified by the
     regexp or replacement fields is applied. Resolution protocols are
     the protocols used to talk with a resolver. They will be specified
     in other documents, such as [5]. Resolution services are operations
     such as I2R (URI to Resource), I2L (URI to URL), I2C (URI to URC),
     etc.  These are specified in the URI Resolution Services
     document[6], and their behavior in a particular resolution protocol
     will be given in the specification for that protocol (see [5] for a
     concrete example).

   * The flags field contains modifiers that affect what happens in the
     next DNS lookup, typically for optimizing the process. Flags may
     also affect the interpretation of the other fields in the record,
     therefore, clients MUST skip NAPTR records which contain an unknown
     flag value.

   * The regexp field is one of two fields used for the rewrite rules,
     and is the core concept of the NAPTR record. The regexp field is a
     String containing a sed-like substitution expression. (The actual
     grammar for the substitution expressions is given later in this
     draft). The substitution expression is applied to the original URN
     to determine the next domain name to be queried. The regexp field
     should be used when the domain name to be generated is conditional
     on information in the URI. If the next domain name is always known,
     which is anticipated to be a common occurrence, the replacement
     field should be used instead.

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   * The replacement field is the other field that may be used for the
     rewrite rule. It is an optimization of the rewrite process for the
     case where the next domain name is fixed instead of being
     conditional on the content of the URI. The replacement field is a
     domain name (subject to compression if a DNS sender knows that a
     given recipient is able to decompress names in this RR type's RDATA
     field). If the rewrite is more complex than a simple substitution
     of a domain name, the replacement field should be set to . and the
     regexp field used.

   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

The Distinction between URNs and URLs

  From the point of view of this system, there is no theoretical
  difference between resolving URIs in the general case and URNs
  in the specific case. Operationally however, there is a difference
  that stems from the unknown case of URI resolution not becoming
  widespread. If URN resolution is collapsed into generic URI
  resolution, URNs may suffer by the lack of adoption of URI resolution.
  The technically correct solution however should discourage
  such a case.

  The solution is to allow for shortcutting for URN resolution. In
  the following specification generic URI resolution starts by
  inserting rules for known URI shemes into the '' registry.
  For URN resolution one of the rules would be for the 'urn' URI
  scheme. This rule would simply delegate to the '' zone
  for additional NAPTRS based on the URN namespace.

  Since this rule is the basis for the entire URN RDS, it can be
  shortcutted by simply starting URN resolution at the ''
  registry. This the distinction between the '' and ''
  well known keys seen below.

The DNS RDS Algorithm

  Since the general RDS framework was the basis for the original NAPTR
  algorithm, the two match very well. The only pieces missing from the
  general NAPTR specification are the original key, protocols and

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The First Known Key

  In the generic URI case, the first known key is created by taking
  the URI scheme and appending '' to the end. In the specific,
  shortcutted URN case, the first known key is created by taking
  the Namespace Identifier and appending '' to the end.

  URI Example
  ----------- would have a first known key of ''.

  URN Example
  urn:foo:12345 would have a first known key of ''.


  The services that make sense for URI resolution are generic for
  both URI and URN resolution since the input value types itself
  based on the URI scheme. Some valid services are defined in
  RFCXXXX (draft-ietf-urn-resolution-services-07.txt).

  Examples of some of these services are:

  I2L: given a URI return one URL that identifies a location where the
       original URI can be found

  I2Ls: given a URI return one or more URLs that identify multiple
       locations where the original URI can be found

  I2R: given a URI return one instance of the resource identified by
       that URI.

  I2Rs: given a URI return one or more instances of the resources
       identified by that URI.

  I2C: given a URI return one instance of a description of that

  I2N: given a URI return one URN that names the resource
      (Caution: equality with respect to URNs is non-trivial. See
      [15] for examples of why.)


  The protocols used in the Services field are currently limited to
  THTTP [5]. Simply specifying any protocol in the services field
  is insufficient since there are additional semantics surrounding URI
  resolution that are not specified within the protocols.

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  For example, if Z39.50 were to be specified as a valid protocol it
  would have to define how it would encode requests for specific
  services, how the URI is encoded, and what information is returned.

  Thus, for this document the only valid value used in the examples is


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:


   The first step in the resolution process is to find out about the
   DUNS namespace. The namespace identifier [3], "duns", is extracted
   from the URN, prepended to, and the NAPTRs for
   looked up. It might return records of the form:
;;      order pref flags service          regexp        replacement
 IN NAPTR 100  10  "s" "dunslink+I2L+I2C" ""
 IN NAPTR 100  20  "s" "rcds+I2C"         ""
 IN NAPTR 100  30  "s" "thttp+I2L+I2C+I2R" ""

   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 I2L or I2C 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

   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.

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   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, which will
   tell us hosts that can provide the necessary resolution service. That
   lookup might return:

    ;;                          Pref Weight Port Target IN SRV 0    0    1000
                           IN SRV 0    0    1000
                           IN SRV 0    0    1000

   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 * 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.

   Also note that there could have been an additional first step where
   the URN was resolved as a generic URI by looking up
   The resulting rule would have specified that the NID be extracted
   from the URN and '' appended to it resulting in the
   new key '' which is the first step from above.

Example 2

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


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

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   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, and the NAPTR for looked up. It
   might return records of the form:
  ;;       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 "". 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, "").

   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 "" might look like: IN NAPTR
  ;;       order pref flags service           regexp  replacement
  IN NAPTR 100  50  "s"  "z3950+I2L+I2C"     ""
  IN NAPTR 100  50  "s"  "rcds+I2C"          ""
  IN NAPTR 100  50  "s"  "thttp+I2L+I2C+I2R"  ""

   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 IN SRV 0    0      1000
                        IN SRV 0    0      1000
                        IN SRV 0    0      1000

   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.

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   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 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

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,

   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:

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

   Applying this pattern to the URL extracts "". Looking up
   NAPTR records for that might return:
   ;;       order pref flags   service  regexp     replacement
    IN NAPTR 100  100  "s"   "thttp+L2R"   ""
    IN NAPTR 100  100  "s"   "ftp+L2R"    ""

   Looking up SRV records for would return information
   on the hosts that has designated to be its mirror sites. The
   client can then pick one for the user.

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     -  Registration procedures for the and DNS zones
        is specified in "Assignment Procedures for the URI Resolution
        using DNS (RFC2168)" [17]

     -  A client MUST process multiple NAPTR records in the order
        specified by the "order" field, it MUST NOT simply use the first
        record that provides a known protocol and service combination.

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     -  If a record at a particular order matches the URI, but the
        client doesn't know the specified protocol and service, the
        client SHOULD continue to examine records that have the same
        order. The client MUST NOT consider records with a higher value
        of order. This is necessary to make delegation of portions of
        the namespace work.  The order field is what lets site
        administrators say "all requests for URIs matching pattern x go
        to server 1, all others go to server 2".
        (A match is defined as:
          1)  The NAPTR provides a replacement domain name
          2) The regular expression matches the URN

     -  When multiple RRs have the same "order", the client should use
        the value of the preference field to select the next NAPTR to
        consider. However, because of preferred protocols or services,
        estimates of network distance and bandwidth, etc. clients may
        use different criteria to sort the records.
     -  If the lookup after a rewrite fails, clients are strongly
        encouraged to report a failure, rather than backing up to pursue
        other rewrite paths.
     -  When a namespace is to be delegated among a set of resolvers,
        regexps must be used. Each regexp appears in a separate NAPTR
        RR.  Administrators should do as little delegation as possible,
        because of limitations on the size of DNS responses.
     -  Note that SRV RRs impose additional requirements on clients.


   The editors would like to thank Keith Moore for all his consultations
   during the development of this draft. We would also like to thank
   Paul Vixie for his assistance in debugging our implementation, and
   his answers on our questions. Finally, we would like to acknowledge
   our enormous intellectual debt to the participants in the Knoxville
   series of meetings, as well as to the participants in the URI and URN
   working groups.


   [1]  Sollins, Karen and Larry Masinter, "Functional Requirements
        for Uniform Resource Names", RFC-1737, Dec. 1994.

   [2]  The URN Implementors, Uniform Resource Names: A Progress Report,, D-Lib Magazine,
        February 1996.

   [3]  Moats, Ryan, "URN Syntax", RFC-2141, May 1997.

   [4]  Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying
        the location of services (DNS SRV)", RFC-2052, October 1996.

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RFC nnnn            Resolution of URIs Using the DNS       November 1998

   [5]  Daniel, Jr., Ron, "A Trivial Convention for using HTTP in URN
        Resolution", RFC-2169, June 1997.

   [6]  URN-WG, "URN Resolution Services", Work in Progress. RFCXXXX

   [7]  Moore, Keith,  Shirley Browne, Jason Cox, and Jonathan Gettler,
        Resource Cataloging and Distribution System, Technical Report
        CS-97-346, University of Tennessee, Knoxville, December 1996

   [8]  Paul Vixie, personal communication.

   [9] Orth, Charles and Bill Arms; Handle Resolution Protocol

   [10] Williamson, S., M. Kosters, D. Blacka, J. Singh, K. Zeilstra,
        "Referral Whois Protocol (RWhois)", RFC-2167, June 1997.

   [11] Information Retrieval (Z39.50): Application Service Definition
        and Protocol Specification, ANSI/NISO Z39.50-1995, July 1995.

   [12] IEEE Standard for Information Technology - Portable Operating
        System Interface (POSIX) - Part 2: Shell and Utilities (Vol. 1);
        IEEE Std 1003.2-1992; The Institute of Electrical and
        Electronics Engineers; New York; 1993. ISBN:1-55937-255-9

   [13] Braden, R., "Requirements for Internet Hosts - Application and
        and Support", RFC-1123, Oct. 1989.

   [14] Sollins, Karen, "Architectural Principles of Uniform Resource
        Name Resolution", RFC2276. January 1998.

   [15] Mealling, Michael, Daniel, Jr., Ron. "The Naming Authority
        Pointer (NAPTR) DNS Resource Record",
        (draft-urn-naptr-rr-00.txt), November 1998.

   [16] Mealling, Michael. "Assignment Procedures for the URI Resolution
        using DNS (RFC2168)", (,
        November 1998.

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IANA Considerations

   The use of the "" and "" zones requires registration
   policies and procedures to be followed and for the operation of
   those DNS zones to be maintained. These policies and procedures
   are spelled out in a "Assignment Procedures for the URI Resolution
   using DNS (RFC2168)" [17]. The operation of those zones imposes
   operational and adminstrative responsibilities on the IANA.

   The registration methods used for specifying values for the Services
   (both protocols and services) and Flags fields that are specific to
   URI resolution is for a specification to be published as an RFC
   and approved by the IESG.

   The registration policies for URLs and URNs are also specified
   elsewhere and thus those impacts on the IANA are spelled out there.

Security Considerations

   The use of "" and "" as the registry for namespaces
   is subject to denial of service attacks, as well as other DNS
   spoofing attacks. The interactions with DNSSEC are currently being
   studied. It is expected that NAPTR records will be signed with
   SIG records once the DNSSEC work is deployed.

   The rewrite rules make identifiers from other namespaces subject to
   the same attacks as normal domain names. Since they have not been
   easily resolvable before, this may or may not be considered a

   Regular expressions should be checked for sanity, not blindly passed
   to something like PERL.

   This document has discussed a way of locating a resolver, but has not
   discussed any detail of how the communication with the resolver takes
   place. There are significant security considerations attached to the
   communication with a resolver. Those considerations are outside the
   scope of this document, and must be addressed by the specifications
   for particular resolver communication protocols.

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Author Contact Information:

   Michael Mealling
   Network Solutions
   505 Huntmar Park Drive
   Herndon, VA  22070
   voice: (703) 742-0400
   fax: (703) 742-9552

   Ron Daniel Jr.
   139 Townsend Street, Ste. 100
   San Francisco, CA  94107
   415.222.0100 fax 415.222.0150

Appendix A -- Psuedo Code

   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
      sprintf(key, "", extractNS(URN));
      do {
        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
          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)) {
          if(!isKnownService(naptr[j].services)) {

          // 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;
          rewriteflag = true;
      } while (rewriteflag && !terminal);

      // Did we not find our way to a resolver?
      if (!rewrite_flag) {
         report an error
         return NULL;

      // Leave rest to another protocol?
      if (strcasecmp(flags, "P")) {
         return key as host to talk to;

      // If not, keep plugging
      if (!addnl) { // No SRVs came in as additional info, look them up
        srvs = lookup(type=SRV, key);

      sort SRV records by preference, weight, ...
      foreach (SRV record) { // in order of preference
        try contacting srv[j].target using the protocol and one of the
            resolution service requests from the "services" field of the
            last NAPTR record.
        if (successful)
          return (target, protocol, service);
          // Actually we would probably return a result, but this
          // code was supposed to just tell us a good host to talk to.
      die with an "unable to find a host" error;

Mealling & Daniel                                              [Page 15]