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Network Working Group                                       R. DanielRequest for Comments: 2168             Los Alamos National LaboratoryCategory: Experimental                                    M. Mealling                                              Network Solutions, Inc.                                                            June 1997Resolution of Uniform Resource Identifiersusing the Domain Name SystemStatus 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.Abstract:=========   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.Introduction:=============   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 toDaniel & 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 ".urn.net", 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.Caveats=======   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.  WeDaniel & 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.Terminology===========   "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.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   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:                   urn:duns:002372413:annual-report-1997   The first step in the resolution process is to find out about the   DUNS namespace. The namespace identifier, "duns", is extracted from   the URN, prepended to urn.net, and the NAPTRs for duns.urn.net looked   up. It might return records of the form:duns.urn.net;;      order pref flags service          regexp        replacement IN NAPTR 100  10  "s" "dunslink+N2L+N2C" ""  dunslink.udp.isi.dandb.com IN NAPTR 100  20  "s" "rcds+N2C"         ""  rcds.udp.isi.dandb.com IN NAPTR 100  30  "s" "http+N2L+N2C+N2R" ""  http.tcp.isi.dandb.com   The order field contains equal values, indicating that no name   delegation order has to be followed. The preference field indicates   that the provider would like clients to use the special dunslink   protocol, followed by the RCDS protocol, and that HTTP is offered as   a last resort. All the records specify the "s" flag, which will be   explained momentarily.  The service fields say that if we speak   dunslink, we will be able to issue either the N2L or N2C requests to   obtain a URL or a URC (description) of the resource. The Resource   Cataloging and Distribution Service (RCDS)[7] could be used to get a   URC for the resource, while HTTP could be used to get a URL, URC, or   the resource itself.  All the records supply the next domain name to   query, none of them need to be rewritten with the aid of regular   expressions.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 rcds.udp.isi.dandb.com, which will   tell us hosts that can provide the necessary resolution service. That   lookup might return:    ;;                          Pref Weight Port Target    rcds.udp.isi.dandb.com IN SRV 0    0    1000 defduns.isi.dandb.com                           IN SRV 0    0    1000 dbmirror.com.au                           IN SRV 0    0    1000 ukmirror.com.uk   telling us three hosts that could actually do the resolution, and   giving us the port we should use to talk to their RCDS server.  (The   reader is referred to the SRV proposal [4] for the interpretation of   the fields above).   There is opportunity for significant optimization here. We can return   the SRV records as additional information for terminal NAPTRs (and   the A records as additional information for those SRVs). While this   recursive provision of additional information is not explicitly   blessed in the DNS specifications, it is not forbidden, and BIND does   take advantage of it [8]. This is a significant optimization. In   conjunction with a long TTL for *.urn.net records, the average number   of probes to DNS for resolving DUNS URNs would approach one.   Therefore, DNS server implementors SHOULD provide additional   information with NAPTR responses. The additional information will be   either SRV or A records.  If SRV records are available, their A   records should be provided as recursive additional information.   Note that the example NAPTR records above are intended to represent   the reply the client will see. They are not quite identical to what   the domain administrator would put into the zone files. For one   thing, the administrator should supply the trailing '.' character on   any FQDNs.Daniel & Mealling             Experimental                      [Page 7]

RFC 2168            Resolution of URIs Using the DNS           June 1997Example 2---------   Consider a URN namespace based on MIME Content-Ids. The URN might   look like this:                 urn:cid:199606121851.1@mordred.gatech.edu   (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 urn.net, and the NAPTR for cid.urn.net looked up. It   might return records of the form: cid.urn.net  ;;       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 "gatech.edu". 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, "gatech.edu").   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 "gatech.edu" might look like:gatech.edu IN NAPTR;;       order pref flags service           regexp  replacement  IN NAPTR 100  50  "s"  "z3950+N2L+N2C"     ""    z3950.tcp.gatech.edu  IN NAPTR 100  50  "s"  "rcds+N2C"          ""    rcds.udp.gatech.edu  IN NAPTR 100  50  "s"  "http+N2L+N2C+N2R"  ""    http.tcp.gatech.eduDaniel & 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   z3950.tcp.gatech.edu IN SRV 0    0      1000 z3950.gatech.edu                        IN SRV 0    0      1000 z3950.cc.gatech.edu                        IN SRV 0    0      1000 z3950.uga.edu   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 cid.urn.net 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,   etc.   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:http://www.foo.com/software/latest-beta.exeDaniel & Mealling             Experimental                      [Page 9]

RFC 2168            Resolution of URIs Using the DNS           June 1997   We extract the prefix, "http", and lookup NAPTR records for   http.urn.net. This might return a record of the form   http.urn.net 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 "www.foo.com". Looking up   NAPTR records for that might return:   www.foo.com   ;;       order pref flags   service  regexp     replacement    IN NAPTR 100  100  "s"   "http+L2R"   ""    http.tcp.foo.com    IN NAPTR 100  100  "s"   "ftp+L2R"    ""    ftp.tcp.foo.com   Looking up SRV records for http.tcp.foo.com would return information   on the hosts that foo.com has designated to be its mirror sites. The   client can then pick one for the user.NAPTR RR Format===============   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   where:   Domain          The domain name this resource record refers to.   TTL          Standard DNS Time To Live field   Class          Standard DNS meaningDaniel & Mealling             Experimental                     [Page 10]

RFC 2168            Resolution of URIs Using the DNS           June 1997   Order          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.   Preference          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.   Flags          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   Service          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 ofRFC 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                       multipart/alternative.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                       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.   Regexp          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.   Replacement          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  *flagsdelim-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],section2.8.4)repl         = dns_str /  backref / repl dns_str  / repl backrefdns_str      = 1*DNS_CHARbackref      = "\" 1POS_DIGITDaniel & Mealling             Experimental                     [Page 13]

RFC 2168            Resolution of URIs Using the DNS           June 1997flags        = "i"DNS_CHAR     = "-" / "0" / ... / "9" / "a" / ... / "z" / "A" / ... / "Z"POS_DIGIT    = "1" / "2" / ... / "9"  ; 0 is not an allowed backrefvalue domain name (seeRFC-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                      (A(B(C)DE)(F)G)   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 1997Advice 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 urn.net   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   The "a" flag allows the next lookup to be for A records rather than   SRV records. Since there is no place for a port specification in the   NAPTR record, when the "A" flag is used the specified protocol must   be running on its default port.   The URN Sytnax draft defines a canonical form for each URN, which   requires %encoding characters outside a limited repertoire. The   regular expressions MUST be written to operate on that canonical   form. Since international character sets will end up with extensive   use of %encoded characters, regular expressions operating on them   will be essentially impossible to read or write by hand.Usage=====   For the edification of implementers, pseudocode for a client routine   using NAPTRs is given below. This code is provided merely as a   convience, it does not have any weight as a standard way to process   NAPTR records. Also, as is the case with pseudocode, it has never   been executed and may contain logical errors. You have been warned.    //    // findResolver(URN)    // Given a URN, find a host that can resolve it.    //    findResolver(string URN) {      // prepend prefix to urn.net      sprintf(key, "%s.urn.net", extractNS(URN));      do {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             continue;          newkey = rewrite(URN, naptr[j].replacement, naptr[j].regexp);          if (!newkey) // Skip to next record if the rewrite didn't             match continue;          // We did do a rewrite, shrink max_order to current value          // so that delegation works properly          max_order = naptr[j].order;          // Will we know what to do with the protocol and services          // specified in the NAPTR? If not, try next record.          if(!isKnownProto(naptr[j].services)) {            continue;          }          if(!isKnownService(naptr[j].services)) {            continue;          }          // At this point we have a successful rewrite and we will          // know how to speak the protocol and request a known          // resolution service. Before we do the next lookup, check          // some optimization possibilities.          if (strcasecmp(flags, "S")           || strcasecmp(flags, "P"))           || strcasecmp(flags, "A")) {             terminal = true;             services = naptr[j].services;             addnl = any SRV and/or A records returned as additional                     info for naptr[j].          }          key = newkey;Daniel & Mealling             Experimental                     [Page 16]

RFC 2168            Resolution of URIs Using the DNS           June 1997          rewriteflag = true;          break;        }      } 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;    }Notes:======     -  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     -  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          or          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.Acknowledgments:=================   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.References:===========   [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,http://www.dlib.org/dlib/february96/02arms.html, 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        Specification,http://www.handle.net/docs/client_spec.html   [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 1997Security Considerations=======================   The use of "urn.net" 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   email:  rdaniel@lanl.gov   Michael Mealling   Network Solutions   505 Huntmar Park Drive   Herndon, VA  22070   voice: (703) 742-0400   fax: (703) 742-9552   email: michaelm@internic.net   URL:http://www.netsol.com/Daniel & Mealling             Experimental                     [Page 20]