--- 1/draft-ietf-urn-naptr-rr-00.txt 2007-12-18 19:09:49.000000000 +0100 +++ 2/draft-ietf-urn-naptr-rr-01.txt 2007-12-18 19:09:49.000000000 +0100 @@ -1,235 +1,244 @@ Network Working Group M. Mealling -draft-ietf-urn-naptr-rr-00.txt Network Solutions, Inc. +draft-ietf-urn-naptr-rr-01.txt Network Solutions, Inc. Category: Standards Track R. Daniel -Expires: May, 1999 DATAFUSION, Inc. +Expires: August, 1999 DATAFUSION, Inc. The Naming Authority Pointer (NAPTR) DNS Resource Record 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. + This document is an Internet-Draft and is in full conformance + with all provisions of Section 10 of RFC2026. + + 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 view the entire list of current Internet-Drafts, please check - the "1id-abstracts.txt" listing contained in the Internet-Drafts - Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net - (Northern Europe), ftp.nis.garr.it (Southern Europe), - munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or - ftp.isi.edu (US West Coast). + The list of current Internet-Drafts can be accessed at + http://www.ietf.org/ietf/1id-abstracts.txt + + The list of Internet-Draft Shadow Directories can be accessed at + http://www.ietf.org/shadow.html. Abstract: ========= - This document describes a DNS Resource Record which specifies - a rewrite rule that, when applied to an existing string + This document describes a DNS Resource Record (RR) which specifies + a rewrite rule that, when applied to an existing string, will produce a new domain. Reasons for rewriting a domain - vary from URN Resource Discovery Systems to moving out of date + vary from URN Resource Discovery Systems to moving out-of-date services to new domains. - This document updates the portions of RFC2168 specifically + This document updates those portions of RFC2168 specifically dealing with the definition of the NAPTR record. Introduction: ============= This RR was originally produced by the URN [3] Working Group as a way to encode rule-sets in DNS so that the delegated sections of a URI could be decomposed in such a way that they could be changed and re-delegated over time. The result was - a Resource Record that included a regular expression which would - be used by a client program to rewrite a string into a domain-name. + a Resource Record that included a regular expression that would + be used by a client program to rewrite a string into a domain name. Regular expressions were chosen for their compactness to expressivity ratio allowing for a great deal of information to be encoded in a rather small DNS packet. - The function of rewriting a string according to the rules in a the - record has usefullness in several different applications. This - document defines the basic assumptions that all of those applications - must adhere to. It does not define the reasons for why the rewrite - is used or what the expected outcomes are or what they are used - for. Those are specified by applications that define how they use - the NAPTR record and algorithms within their contexts. +Mealling & Daniel [Page 1] + The function of rewriting a string according to the rules in a + record has usefulness in several different applications. This + document defines the basic assumptions to which all of those + applications must adhere to. It does not define the reasons the + rewrite is used, what the expected outcomes are, or what they are + used for. Those are specified by applications that define how they + use the NAPTR record and algorithms within their contexts. -Mealling & Daniel [Page X1] Flags and other fields are also specified in the RR to control the rewrite procedure in various ways or to provide information on how - to communicate with the host at the domain-name that was the result + to communicate with the host at the domain name that was the result of the rewrite. - The final result is a RR that has several fields which interact + The final result is a RR that has several fields that interact in a non-trivial but implementable way. This document specifies those fields and their values. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. NAPTR RR Format =============== The format of the NAPTR RR is given below. The DNS type code for NAPTR is 35. [1] [2] - Domain TTL Class Order Preference Flags Service Regexp - Replacement + Domain TTL Class Order Preference Flags Service Regexp Replacement Domain - The domain name this resource record refers to. This is the - 'key' for this entry in the rule database. This value will - either be the first well known key (.uri.net for - example) or a new key that is the output of a replacement or - regexp rewrite. Beyond this it has the standard DNS + The domain name to which this resource record refers. This + is the 'key' for this entry in the rule database. This value + will either be the first well known key (.uri.net + for example) or a new key that is the output of a replacement + or regexp rewrite. Beyond this, it has the standard DNS requirements. [1] TTL Standard DNS meaning. [1] Class Standard DNS meaning [1] Order - A 16-bit integer specifying the order in which the NAPTR - records MUST be processed to ensure the correct ordering of - rules. Low numbers are processed before high numbers, and - once a NAPTR is found whose rule "matches" the target, - the client MUST NOT consider any NAPTRs with a higher - value for order (except as noted below for the Flags field). + A 16-bit unsigned integer specifying the order in which + the NAPTR records MUST be processed to ensure the correct + ordering of rules. Low numbers are processed before high + numbers, and once a NAPTR is found whose rule "matches" + the target, the client MUST NOT consider any NAPTRs with + a higher value for order (except as noted below for the + Flags field). +Mealling & Daniel [Page 2] 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. A client MAY look at records with - higher preference values if it has a good reason to do so - such as no understanding the preferred protocol or service. + A 16-bit unsigned integer that 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 client MAY + look at records with higher preference values if it has a + good reason to do so such as not understanding the preferred + protocol or service. 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. + A containing 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. -Mealling & Daniel [Page X2] - At this time only three flags, "S", "A", and "P", are defined. - "S" means that the next lookup should be for SRV [4] 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 application side algorithm shall be carried out in - a Protocol-specific fashion. The new set of rules is - identified by the Protocol specified in the Services field. - The record that contains the 'P' flag is the last record - that is interpreted by the rules specified in this document. - The new rules are dependent on the application for which - they are being used and the protocol specified. For example, - if the application is a URI RDS and the protocol is WIRE - then the new set of rules are governed by the algorithms - surrounding the WIRE HTTP specification and not this - document [17]. + At this time only four flags, "S", "A", "U", and "P", are + defined. The "S", "A" and "U" flags denote a terminal lookup. + This means that this NAPTR record is the last one and that the + flag determines what the next stage should be. The "S" flag + means that the next lookup should be for SRV [4] records. + "A" means that the next lookup should be for A records. + The "U" flag means that the next step is not a DNS lookup + but that the output of the Regexp field is a URL [10]. + + The "P" flag says that the remainder of the application side + algorithm shall be carried out in a Protocol-specific + fashion. The new set of rules is identified by the Protocol + specified in the Services field. The record that contains + the 'P' flag is the last record that is interpreted by the + rules specified in this document. The new rules are + dependent on the application for which they are being used + and the protocol specified. For example, if the application + is a URI RDS and the protocol is WIRE then the new set of + rules are governed by the algorithms surrounding the WIRE + HTTP specification and not this document. The remaining alphabetic flags are reserved for future versions of the NAPTR specification. The numeric flags - may be used for local experimentation. The S, A, and P flags + may be used for local experimentation. The S, A, U 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 - given target. + browser). It is anticipated 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 given target. - The "S" and "A" flags are called 'terminal' flags since they - halt any looping rewrite algorithms. If those flags are - not present then clients may assume that another NAPTR RR - exists at the domain-name produced by the current rewrite - rule. Since the "P" flag specifies a new algorithm, it may - or may not be terminal, thus the client cannot assume that - another NAPTR exists since this case is determined elsewhere. +Mealling & Daniel [Page 3] + The "S", "A", and "U" flags are called 'terminal' flags + since they halt any looping rewrite algorithms. If those + flags are not present, clients may assume that another + NAPTR RR exists at the domain name produced by the current + rewrite rule. Since the "P" flag specifies a new algorithm, + it may or may not be 'terminal'. Thus, the client cannot + assume that another NAPTR exists since this case is + determined elsewhere. DNS servers MAY interpret these flags and values and use that information to include appropriate SRV and A records in the additional information portion of the DNS packet. Clients are encouraged to check for additional information - but are no required to do so. + but are not required to do so. Service Specifies the service(s) available down this rewrite path. It may also specify the particular protocol that is used to talk with a service. 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 2234[5]): -Mealling & Daniel [Page X3] 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 protocol and rs fields are limited to 32 + ; characters and must start with an alphabetic. - i.e. an optional protocol specification followed by 0 or more - resolution services. Each resolution service is indicated by - an initial '+' character. + For example, 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 beginning of a series of rules, when it is impossible to know what services and protocols will be offered by a particular service. The actual format of the service request and response will be determined by the resolution protocol, and is the subject for other documents. 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. +Mealling & Daniel [Page 4] The list of "valid" protocols for any given NAPTR record is any protocol that implements some or all of the services defined for a NAPTR application. Currently, THTTP [6] is the only protocol that is known to make that claim at the time of publication. Any other protocol that is to be used must have documentation specifying: * how it implements the services of the application * how it is to appear in the NAPTR record (i.e., the string id of the protocol) The list of valid Resolution Services is defined by the documents that specify individual NAPTR based applications. - One example is RFCXXXX, "Resolution of Uniform Resource - Identifiers using the Domain Name System" [7], from which - this document was extracted. + One example is RFC-XXXX, "Resolution of Uniform Resource + Identifiers using the Domain Name System" [7]. It is worth noting that the interpretation of this field is subject to being changed by new flags, and that the current specification is oriented towards telling clients how to talk with a URN resolver. Regexp A STRING containing a substitution expression that is applied to the original string held by the client in order to construct the next domain name to lookup. The grammar of the @@ -238,49 +247,49 @@ The regular expressions MUST NOT be used in a cumulative fashion, that is, they should only be applied to the original string held by the client, never to the domain name produced by a previous NAPTR rewrite. The latter is tempting in some applications but experience has shown such use to be extremely fault sensitive, very error prone, and extremely difficult to debug. Replacement The next NAME to query for NAPTR, SRV, or A records depending - on the value of the flags field. This MUST be DNS compressed. - -Mealling & Daniel [Page X4] + on the value of the flags field. This MUST be a fully qualified + domain-name. Unless and until permitted by future standards + action, name compression is not to be used for this field. 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: +Mealling & Daniel [Page 5] + subst_expr = delim-char ere delim-char repl delim-char *flags -delim-char = "/" / "!" / ... (Any non-digit or non-flag character +delim-char = "/" / "!" / ... +ere = POSIX Extended Regular Expression (see [8], section 2.8.4) -repl = dns_str / backref / repl dns_str / repl backref -dns_str = 1*DNS_CHAR +repl = 1 * ( OCTET / backref ) 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 [7]). +POS_DIGIT = %x31-39 ; 0 is not an allowed backref The result of applying the substitution expression to the original - URI MUST result in a string that obeys the syntax for DNS host names - [7]. Since it is possible for the regexp field to be improperly + URI MUST result in either a string that obeys the syntax for DNS + host names [1] or a URI [10] if the Flags field contains a 'U'. + 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 @@ -289,31 +301,32 @@ \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. -Mealling & Daniel [Page X5] 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. +Mealling & Daniel [Page 6] + The Basic NAPTR Algorithm ============================================ The behavior and meaning of the flags and services assume an algorithm where the output of one rewrite is a new key that points to another rule. This looping algorithm allows NAPTR records to incrementally specify a complete rule. These incremental rules can be delegated which allows other entities to specify rules so that one entity does not need to understand _all_ rules. @@ -327,59 +340,59 @@ is found. A record is considered a match iff: 1) it has a Replacement field value instead of a Regexp field value. or 2) the Regexp field matches the string held by the client. The first match MUST be the match that is used. Once a match is found, the Services field is examined for whether or not this rule - advances toward the desired result. If so, then the rule is + advances toward the desired result. If so, the rule is applied to the target string. If not, the process halts. The domain that results from the regular expression is then used as the domain of the next loop through the NAPTR algorithm. Note that the same target string is used throughout the algorithm. This looping is extremely important since it is the method by which complex rules are broken down into manageable delegated chunks. The flags fields simply determine at which point the looping should stop (or other specialized behavior). Since flags are valid at any level of the algorithm, the degenerative - case is to never loop but to lookup the NATPR and then stop. In + case is to never loop but to look up the NAPTR and then stop. In many specialized cases this is all that is needed. Implementors should be aware that the degenerative case should not become the common case. +Mealling & Daniel [Page 7] + Application Specifications ========================== It should be noted that the NAPTR algorithm is the basic assumption about how NAPTR works. The reasons for the rewrite and the expected output and its use are specified by documents that define what - applicatiions the NAPTR record and algorithm are used for. Any + applications the NAPTR record and algorithm are used for. Any document that defines such an application must define the following: * The first known key or how to build it * The valid Services and Protocols * What the expected use is for the output of the last rewrite * The validity and/or behavior of any 'P' flag protocols. * The general semantics surrounding why and how NAPTR and its algorithm are being used. - Currently the only example of such a document is RFCXXXX, + Currently the only example of such a document is RFC-XXXX, "Resolution of Uniform Resource Identifiers using the Domain Name System" [7]. -Mealling & Daniel [Page X6] - Examples ============================================ NOTE: These are examples only. They are taken from ongoing work and may not represent the end result of that work. They are here for pedagogical reasons only. Example 1 --------- @@ -399,47 +412,47 @@ The first step in the resolution process is to find out about the CID namespace. The namespace identifier [3], cid, is extracted from the URN, prepended to urn.net. 'cid.urn.net' then becomes the first 'known' key in the NAPTR algorithm. the NAPTR for cid.urn.net looked up and returns a record: 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 +Mealling & Daniel [Page 8] + There is only one NAPTR response, so ordering the responses is not a + problem. The replacement field is empty, so the pattern provided + in the regexp field is used . 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 where the new domain is 'gatech.edu' and the string is the same string as before. 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. -Mealling & Daniel [Page X7] 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.edu - Continuing with our example, we note that the values of the order and + Continuing with the example, 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 @@ -452,20 +465,22 @@ Recall that the regular expression used \2 to extract a domain name from the CID, and \. for matching the literal '.' characters separating 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". +Mealling & Daniel [Page 9] + Example 2 --------- 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 that URNs be able to grandfather in names from other naming systems, such as ISO Formal Public Identifiers, Library of Congress Call Numbers, ISBNs, ISSNs, etc. @@ -473,29 +488,28 @@ 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.exe We extract the prefix, "http", and lookup NAPTR records for http.uri.net. This might return a record of the form -Mealling & Daniel [Page X8] http.uri.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 + 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 @@ -514,57 +528,102 @@ 0.0.0.4.6.2.6.5.8.6.4.e164.int. IN NAPTR 100 10 "s" "h323call+N2R" "" tele2.se. IN NAPTR 102 10 "s" "potscall+N2R" "" tele2.se. IN NAPTR 102 10 "s" "smtp+N2R" "" tele2.se. In these examples the domain is an encoded E164 telephone number. The services field specifies that, for this particular telephone number, the services that are available are h323call, potscall and smtp; and that "tele2.se" is the target that provides those - services. Since the flag is "s" then the next step should be a + services. Since the flag is "s", the next step should be a query for an SRV record which will contain specific information about the "tele2.se" domain. -Advice to domain administrators: -================================ +DNS Packet Format +================= - Beware of regular expressions. Not only are they a pain to get + The packet format for the NAPTR record is as follows + 1 1 1 1 1 1 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + | ORDER | + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + | PREFERENCE | + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + / FLAGS / + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + / SERVICES / + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + / REGEXP / + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + / REPLACEMENT / + / / + +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ + + where: + + FLAGS A which contains various flags. + + SERVICES A which contains protocol + and service identifiers. + + REGEXP A which contains a regular + expression. + + REPLACEMENT A which specifies the new value in + the case where the regular expression is a simple + replacement operation. + +Master File Format +================== + + The master file format follows the standard rules in RFC-1035 [1]. + Order and preference, being 16-bit unsigned integers, shall be + an integer between 0 and 65535. The Flags and Services and Regexp + fields are all s that cannot contain spaces and + thus can be included in their above specified form. While the + Regexp field is also a it can contain + numerous backslashes and thus should be treated with care. + +Advice to domain administrators +=============================== + + Beware of regular expressions. Not only are they difficult 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. -Mealling & Daniel [Page X9] 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 Syntax 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. 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. - 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 + consider. However, because of preferred protocols or services as + well as estimates of network distance and bandwidth, 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. - Note that SRV RRs impose additional requirements on clients. Acknowledgments: ================= The editors would like to thank Keith Moore for all his consultations @@ -589,30 +648,34 @@ [4] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the location of services (DNS SRV)", RFC-2052, October 1996. [5] Crocker, D., Overell, P. "Augmented BNF for Syntax Specifications: ABNF", RFC-2234, November 1997. [6] Daniel R. "A Trivial Convention for using HTTP in URN Resolution". RFC2169. June 1997. [7] Mealling, M., Daniel, R., "Resolution of Uniform Resource - Identifiers using the Domain Name System". RFCXXXX. November 1998. + Identifiers using the Domain Name System", RFC-XXXX, + November 1998. [8] 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 [9] Braden, R., "Requirements for Internet Hosts - Application and and Support", RFC-1123, Oct. 1989. + [10] Berners-Lee, T., R. Fielding, L. Masinter. "Uniform Resource + Identifiers (URI): Generic Syntax", RFC-2396, August 1998. + IANA Considerations =================== The only registration function that impacts the IANA is for the values that are standardized for the Services and Flags fields. To extend the valid values of the Flags field beyond what is specified in this document requires a published specification that is approved by the IESG. The values for the Services field will be determined by the