Internet DRAFT - draft-ietf-urn-naptr


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INTERNET DRAFT                                                  Ron Daniel
draft-ietf-urn-naptr-04.txt                 Los Alamos National Laboratory
                                                          Michael Mealling
                                                   Network Solutions, Inc.
                                                            20 March, 1997

                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
    areas, and its working groups.  Note that other groups may also
    distribute working documents as Internet-Drafts.
    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other
    documents at any time.  It is inappropriate to use Internet-
    Drafts as reference material or to cite them other than as
    ``work in progress.''
    To learn the current status of any Internet-Draft, please check
    the ``1id-abstracts.txt'' listing contained in the Internet-
    Drafts Shadow Directories on (Africa), (Europe), (Pacific Rim), (US East Coast), or (US West Coast).

    This draft expires 26 Sept., 1997.

Uniform Resource Locators (URLs) are the foundation of the World Wide
Web, and are a vital Internet technology. However, they have proven to
be brittle in practice. The basic problem is that URLs typically
identify a particular path to a file on a particular host. There is no
graceful way of changing the path or host once the URL has been
assigned. Neither is there a graceful way of replicating the resource
located by the URL to achieve better network utilization and/or fault
tolerance. Uniform Resource Names (URNs) have been hypothesized as a
adjunct to URLs that would overcome such problems. URNs and URLs are
both instances of a broader class of identifiers known as Uniform
Resource Identifiers (URIs).

The requirements document for URN resolution systems[15] defines the concept
of a "resolver discovery service". This document describes the first,
experimental, RDS. It is implemented by a new DNS Resource Record, NAPTR
(Naming Authority PoinTeR), that provides rules for mapping parts of URIs
to domain names.  By changing the mapping rules, 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.

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 (Name to Location, Name to Description,
Name 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

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 provided 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.

Locating a resolver through the rewrite procedure may take multiple
steps, but the beginning is always the same. The start of the URI
is scanned to extract its colon-delimited prefix. (For URNs, the
prefix is always "urn:" and we extract the following colon-delimited
namespace identifier. [3]). NAPTR resolution begins by taking the
extracted string, appending the well-known suffix "", and
querying the DNS for NAPTR records at that domain name.  Based on the
results of this query, zero or more additional DNS queries may be
needed to locate resolvers for the URI. The details of the conversation
between the client and the resolver thus located are outside the bounds
of this draft. Three brief examples of this procedure are given in the
next section.

The NAPTR RR provides the level of indirection needed to keep the
naming system independent of the resolution system, its protocols, and
services.  Coupled with the new SRV resource record proposal[4] there
is also the potential for replicating the resolver on multiple hosts,
overcoming some of the most significant problems of URLs. This is an
important and subtle point. Not only do the NAPTR and SRV records allow
us to replicate the resource, we can replicate the resolvers that know
about the replicated resource. Preventing a single point of failure at
the resolver level is a significant benefit. Separating the resolution
procedure from the way names are constructed has additional benefits.
Different resolution procedures can be used over time, and resolution
procedures that are determined to be useful can be extended to deal
with additional namespaces.


The NAPTR proposal is the first resolution procedure to be considered
by the URN-WG. There are several concerns about the proposal which have
motivated the group to recommend it for publication as an Experimental
rather than a standards-track RFC.

First, URN resolution is new to the IETF and we wish to gain
operational experience before recommending any procedure for the
standards track. Second, the NAPTR proposal is based on DNS and
consequently inherits concerns about security and administration. The
recent advancement of the DNSSEC and secure update drafts to Proposed
Standard reduce these concerns, but we wish to experiment with those
new capabilities in the context of URN administration.  A third area of
concern is the potential for a noticeable impact on the DNS.  We
believe that the proposal makes appropriate use of caching and
additional information, but it is best to go slow where the potential
for impact on a core system like the DNS is concerned. Fourth, the
rewrite rules in the NAPTR proposal are based on regular expressions.
Since regular expressions are difficult for humans to construct
correctly, concerns exist about the usability and maintainability of
the rules. This is especially true where international character sets
are concerned. Finally, the URN-WG is developing a requirements document
for URN Resolution Services[15], but that document is not complete. That
document needs to precede any resolution service proposals on the standards


"Must" or "Shall" - Software that does not behave in the manner that this
           document says it must is not conformant to this document.
"Should" - Software that does not follow the behavior that this document
           says it should may still be conformant, but is probably broken
           in some fundamental way.
"May" -    Implementations may or may not provide the described behavior,
           while still remaining conformant to this document.

Brief overview and examples of the NAPTR RR:

A detailed description of the NAPTR RR will be given later, but to give
a flavor for the proposal we first give a simple description of the
record and three examples of its use.

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 N2R (URN to Resource), N2L (URN to URL), N2C (URN to URC), etc.
  These will be discussed in the URN 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.

* 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 fields.

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, "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+N2L+N2C"  ""
 IN NAPTR 100  20  "s"  "rcds+N2C"          ""
 IN NAPTR 100  30  "s"  "http+N2L+N2C+N2R"  ""

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

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, 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.

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 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, 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+N2L+N2C"     ""
  IN NAPTR 100  50  "s"  "rcds+N2C"          ""
  IN NAPTR 100  50  "s"  "http+N2L+N2C+N2R"  ""

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.

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:.+@([^\\.]+\\.)(*)$/\\2/i".
When the client code actually receives the record, the pattern will
have been converted to "/urn:cid:.+@([^\.]+\.)(*)$/\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,

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 "/http:\\/\\/([^\\/:]+)/\\1/i".).

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"   "http+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. 


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 


       The domain name this resource record refers to.
       Standard DNS Time To Live field
       Standard DNS meaning
       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.

       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. 

       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 likely to
       signal such an error than a client such as a browser). We
       anticipate that multiple flags will be allowed in the future, so
       implementers MUST NOT assume that the flags field can only
       contain 0 or 1 characters. Finally, if a client encounters a
       record with an unknown flag, it MUST ignore it and move to the
       next record. This test takes precedence even over the "order"
       field. Since flags can control the interpretation placed on
       fields, a novel flag might change the interpretation of the
       regexp and/or replacement fields such that it is impossible to
       determine if a record matched a URN.

       Specifies the resolution service(s) available down this rewrite
       path. It may also specify the particular protocol that is used
       to talk with a resolver. A protocol MUST be specified if the
       flags field states that the NAPTR is terminal. If a protocol is
       specified, but the flags field does not state that the NAPTR is
       terminal, the next lookup MUST be for a NAPTR. The client MAY
       choose not to perform the next lookup if the protocol is
       unknown, but that behavior MUST NOT be relied upon.

       The service field may take any of the values below (using the
       Augmented BNF of RFC 822[9]):

           service_field = [ [protocol] *("+" rs)]
           protocol      = ALPHA *31ALPHANUM
           rs            = ALPHA *31ALPHANUM
        // The protocol and rs fields are limited to 32
        // characters and must start with an alphabetic.
        // The current set of "known" strings are:
        // protocol      = "rcds" / "thttp" / "hdl" / "rwhois" / "z3950"
        // rs            = "N2L" / "N2Ls" / "N2R" / "N2Rs" / "N2C"
        //               / "N2Ns" / "L2R" / "L2Ns" / "L2Ls" / "L2C"

       i.e. an optional protocol specification followed by 0 or more
       resolution services. Each resolution service is indicated by
       an initial '+' character.

       Note that the empty string is also a valid service field. This
       will typically be seen at the top levels of a namespace, when it
       is impossible to know what services and protocols will be offered
       by a particular publisher within that name space.

       At this time the known protocols are rcds[7], hdl[10] (binary,
       UDP-based protocols),  thttp[5] (a textual, TCP-based protocol),
       rwhois[11] (textual, UDP or TCP based), and Z39.50[12] (binary,
       TCP-based). More will be allowed later.  The names of the
       protocols must be formed from the characters [a-Z0-9]. Case of
       the characters is not significant.

       The service requests currently allowed will be described in more
       detail in [6], but in brief they are:
             N2L  - Given a URN, return a URL
             N2Ls - Given a URN, return a set of URLs
             N2R  - Given a URN, return an instance of the resource.
             N2Rs - Given a URN, return multiple instances of the resouce,
                    typically encoded using multipart/alternative.
             N2C  - Given a URN, return a collection of meta-information
                    on the named resource. The format of this response is
                    the subject of another document.
             N2Ns - Given a URN, return all URNs that are also identifers
                    for the resource.
             L2R  - Given a URL, return the resource.
             L2Ns - Given a URL, return all the URNs that are identifiers
                    for the resource.
             L2Ls - Given a URL, return all the URLs for instances of
                    of the same resource.
             L2C  - Given a URL, return a description of the resource.

       The actual format of the service request and response will be
       determined by the resolution protocol, and is the subject for
       other documents (e.g. [5]). Protocols need not offer all
       services. The labels for service requests shall be formed from
       the set of characters [A-Z0-9]. The case of the alphabetic
       characters is not significant.

       A STRING containing a substitution expression that is applied to
       the original URI in order to construct the next domain name to
       lookup. The grammar of the substitution expression is given in
       the next section.

       The next NAME to query for NAPTR, SRV, or A records depending on
       the value of the flags field. As mentioned above, this may be

Substitution Expression Grammar:

The content of the regexp field is a substitution expression. True sed(1)
substitution expressions are not appropriate for use in this application for a
variety of reasons, therefore the contents of the regexp field MUST follow the
grammar below:

  subst_expr   = delim-char  ere  delim-char  repl  delim-char  *flags
  delim-char   = "/" / "!" / ... (Any non-digit or non-flag character other
                 than backslash '\'. All occurances of a delim_char in a
                 subst_expr must be the same character.)
  ere          = POSIX Extended Regular Expression (see [13], section 2.8.4)
  repl         = dns_str /  backref / repl dns_str  / repl backref
  dns_str      = 1*DNS_CHAR 
  backref      = "\" 1POS_DIGIT
  flags        = "i" 
  DNS_CHAR     = "-" / "0" / ... / "9" / "a" / ... / "z" / "A" / ... / "Z"
  POS_DIGIT    = "1" / "2" / ... / "9"  ; 0 is not an allowed backref value
domain name (see RFC-1123 [14]).

The result of applying the substitution expression to the original URI MUST
result in a string that obeys the syntax for DNS host names [14]. Since it
is possible for the regexp field to be improperly specified, such that a
non-conforming host name can be constructed, client software SHOULD verify
that the result is a legal host name before making queries on it. 

Backref expressions in the repl portion of the substitution expression
are replaced by the (possibly empty) string of characters enclosed by '('
and ')' in the ERE portion of the substitution expression. N is a single
digit from 1 through 9, inclusive. It specifies the N'th backref expression,
the one that begins with the N'th '(' and continues to the matching ')'.
For example, the ERE
has backref expressions:
                    \1  = ABCDEFG
                    \2  = BCDE
                    \3  = C
                    \4  = F
                \5..\9  = error - no matching subexpression
The "i" flag indicates that the ERE matching SHALL be performed in a
case-insensitive fashion. Furthermore, any backref replacements MAY be
normalized to lower case when the "i" flag is given.

The first character in the substitution expression shall be used as the
character that delimits the components of the substitution expression.
There must be exactly three non-escaped occurrences of the delimiter
character in a substitution expression. Since escaped occurrences of
the delimiter character will be interpreted as occurrences of that
character, digits MUST NOT be used as delimiters. Backrefs would be
confused with literal digits were this allowed. Similarly, if flags are
specified in the substitution expression, the delimiter character must not
also be a flag character. 

Advice to domain administrators:

Beware of regular expressions. Not only are they a pain to get
correct on their own, but there is the previously mentioned interaction
with DNS. Any backslashes in a regexp must be entered twice in a zone
file in order to appear once in a query response. More seriously, the
need for double backslashes has probably not been tested by all
implementors of DNS servers. We anticipate that will be the
heaviest user of regexps. Only when delegating portions 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
'*', '(', etc.

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.


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
          // 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;

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

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

[3]  Ryan Moats, "URN Syntax", draft-ietf-urn-syntax-02.txt, Feb. 1997.

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

[5]  RFC-xxxx, "A Trivial Convention for using HTTP in URN Resolution",
     Ron Daniel Jr., currently available as draft-ietf-urn-http-conv-01.txt,
     Feb. 1997.

[6]  RFC-xxxx, "URN Resolution Services", ???, draft-ietf-urn-???
     (This document is on the URN-WG's list of documents to prepare, but
     has not yet been written. It will get its start from the treatment of
     resolution services in [4]).

[7]  Keith Moore, 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]  RFC-822, "Standard for the Format of ARPA Internet Text Messages",
     Dave H. Crocker, August 1982.  

[10] Charles Orth, Bill Arms; Handle Resolution Protocol Specification,

[11] RFC-1714, "Referral Whois Protocol (RWhois)", S. Williamson and
     M. Kosters, November 1994.

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

[13] 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

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

[15] RFC-xxxx, "Requirements and a Framework for URN Resolution Systems",
     Karen Sollins, draft-ietf-urn-req-frame-00.txt, November 1996.

Security Considerations

  The use of "" as the registry for URN 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 problem.

  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.

Author Contact Information:

Ron Daniel
Los Alamos National Laboratory
MS B287
Los Alamos, NM, USA, 87545
voice:  +1 505 665 0597
fax:    +1 505 665 4939

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

    This draft expires 26 Sept., 1997.