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Network Working Group R. Daniel Request for Comments: 2168 Los Alamos National Laboratory Category: Experimental M. Mealling

                                            Network Solutions, Inc.
                                                          June 1997
             Resolution of Uniform Resource Identifiers
                    using the Domain Name System

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  This memo does not specify an Internet standard of any
 kind.  Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.


 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.

Daniel & Mealling Experimental [Page 1] RFC 2168 Resolution of URIs Using the DNS June 1997

 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 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 provided in NAPTR records. Rewrite
 rules provide considerable power, which is important when trying to

Daniel & Mealling Experimental [Page 2] RFC 2168 Resolution of URIs Using the DNS June 1997

 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

Daniel & Mealling Experimental [Page 3] RFC 2168 Resolution of URIs Using the DNS June 1997

 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 track.


 "Must" or "Shall" - Software that does not behave in the manner that
            this document says it must is not conformant to this
 "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

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.

Daniel & Mealling Experimental [Page 4] RFC 2168 Resolution of URIs Using the DNS June 1997

  • 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.

Daniel & Mealling Experimental [Page 5] RFC 2168 Resolution of URIs Using the DNS June 1997

 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

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

Daniel & Mealling Experimental [Page 6] 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, 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.

Daniel & Mealling Experimental [Page 7] RFC 2168 Resolution of URIs Using the DNS June 1997

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"  ""

Daniel & Mealling Experimental [Page 8] RFC 2168 Resolution of URIs Using the DNS June 1997

 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

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:

Daniel & Mealling Experimental [Page 9] RFC 2168 Resolution of URIs Using the DNS June 1997

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

Daniel & Mealling Experimental [Page 10] RFC 2168 Resolution of URIs Using the DNS June 1997

        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.

Daniel & Mealling Experimental [Page 11] RFC 2168 Resolution of URIs Using the DNS June 1997

        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
        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
                     resource, typically encoded using

Daniel & Mealling Experimental [Page 12] RFC 2168 Resolution of URIs Using the DNS June 1997

              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
        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 compressed.

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


repl = dns_str / backref / repl dns_str / repl backref dns_str = 1*DNS_CHAR backref = "\" 1POS_DIGIT

Daniel & Mealling Experimental [Page 13] RFC 2168 Resolution of URIs Using the DNS June 1997

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.

Daniel & Mealling Experimental [Page 14] RFC 2168 Resolution of URIs Using the DNS June 1997

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
 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.


 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 {

Daniel & Mealling Experimental [Page 15] 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
        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;

Daniel & Mealling Experimental [Page 16] RFC 2168 Resolution of URIs Using the DNS June 1997

        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;


  1. 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.

Daniel & Mealling Experimental [Page 17] RFC 2168 Resolution of URIs Using the DNS June 1997

  1. 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
  1. 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.

Daniel & Mealling Experimental [Page 18] RFC 2168 Resolution of URIs Using the DNS June 1997

 [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.
 [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.
 [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]  Crocker, Dave H. "Standard for the Format of ARPA Internet Text
      Messages", RFC-822, August 1982.
 [10] Orth, Charles and Bill Arms; Handle Resolution Protocol
 [11] Williamson, S., M. Kosters, D. Blacka, J. Singh, K. Zeilstra,
      "Referral Whois Protocol (RWhois)", RFC-2167, June 1997.
 [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] Braden, R., "Requirements for Internet Hosts - Application and
      and Support", RFC-1123, Oct. 1989.
 [15] Sollins, Karen, "Requirements and a Framework for URN Resolution
      Systems", November 1996, Work in Progress.

Daniel & Mealling Experimental [Page 19] RFC 2168 Resolution of URIs Using the DNS June 1997

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
 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

Daniel & Mealling Experimental [Page 20]

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