Network Working Group M. Mealling Request for Comments: 2915 Network Solutions, Inc. Updates: 2168 R. Daniel Category: Standards Track DATAFUSION, Inc. September 2000 The Naming Authority Pointer (NAPTR) DNS Resource Record Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved.
AbstractThis document describes a Domain Name System (DNS) resource record which specifies a regular expression based rewrite rule that, when applied to an existing string, will produce a new domain label or Uniform Resource Identifier (URI). Depending on the value of the flags field of the resource record, the resulting domain label or URI may be used in subsequent queries for the Naming Authority Pointer (NAPTR) resource records (to delegate the name lookup) or as the output of the entire process for which this system is used (a resolution server for URI resolution, a service URI for ENUM style e.164 number to URI mapping, etc). This allows the DNS to be used to lookup services for a wide variety of resource names (including URIs) which are not in domain name syntax. Reasons for doing this range from URN Resource Discovery Systems to moving out-of-date services to new domains. This document updates the portions of RFC 2168 specifically dealing with the definition of the NAPTR records and how other, non-URI specific applications, might use NAPTR.
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. NAPTR RR Format . . . . . . . . . . . . . . . . . . . . . . 3 3. Substitution Expression Grammar . . . . . . . . . . . . . . 7 4. The Basic NAPTR Algorithm . . . . . . . . . . . . . . . . . 8 5. Concerning How NAPTR Uses SRV Records . . . . . . . . . . . 9 6. Application Specifications . . . . . . . . . . . . . . . . . 10 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1 Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.2 Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . 12 7.3 Example 3 . . . . . . . . . . . . . . . . . . . . . . . . . 13 8. DNS Packet Format . . . . . . . . . . . . . . . . . . . . . 13 9. Master File Format . . . . . . . . . . . . . . . . . . . . . 14 10. Advice for DNS Administrators . . . . . . . . . . . . . . . 14 11. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 15 13. Security Considerations . . . . . . . . . . . . . . . . . . 15 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 16 References . . . . . . . . . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17 Full Copyright Statement . . . . . . . . . . . . . . . . . . 18 3] 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 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 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. 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 of the rewrite.
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. This document does not define applications that utilizes this rewrite functionality. Instead it specifies just the mechanics of how it is done. Why its done, what the rules concerning the inputs, and the types of rules used are reserved for other documents that fully specify a particular application. This separation is due to several different applications all wanting to take advantage of the rewrite rule lookup process. Each one has vastly different reasons for why and how it uses the service, thus requiring that the definition of the service be generic. 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. All references to Uniform Resource Identifiers in this document adhere to the 'absoluteURI' production of the "Collected ABNF" found in RFC 2396 . Specifically, the semantics of URI References do not apply since the concept of a Base makes no sense here. 1] for NAPTR is 35. Domain TTL Class Type Order Preference Flags Service Regexp Replacement Domain 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 (<something>.uri.arpa for example) or a new key that is the output of a replacement or regexp rewrite. Beyond this, it has the standard DNS requirements . TTL Standard DNS meaning . Class Standard DNS meaning . Type The Type Code  for NAPTR is 35.
Order 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). Preference 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. The important difference between Order and Preference is that once a match is found the client MUST NOT consider records with a different Order but they MAY process records with the same Order but different Preferences. I.e., Preference is used to give weight to rules that are considered the same from an authority standpoint but not from a simple load balancing standpoint. Flags A <character-string> 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. 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 records . See Section 5 for additional information on how NAPTR uses the SRV record type. "A" means that the next lookup should be for either an A, AAAA, or A6 record. The "U" flag means that the next step is not a DNS lookup but that the output of the Regexp field is an URI that adheres to the 'absoluteURI' production found in the ABNF of RFC 2396 . Since there may be applications that use NAPTR to also lookup aspects of URIs, implementors should be aware that this may cause loop conditions and should act accordingly.
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, 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). 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", "A", and "U" flags are called 'terminal' flags since they halt the looping rewrite algorithm. 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,AAAA, or A6 records in the additional information portion of the DNS packet. Clients are encouraged to check for additional information 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 ): 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. 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. 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  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. 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 substitution expression is given in the next section. 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 address records depending 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. 8], section 2.8.4. The result of applying the substitution expression to the original URI MUST result in either a string that obeys the syntax for DNS domain-names  or a URI  if the Flags field contains a 'u'. Since it is possible for the regexp field to be improperly specified, such that a non-conforming domain-name can be constructed, client software SHOULD verify that the result is a legal DNS domain-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.
o it has a Replacement field value instead of a Regexp field value. o or 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, 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 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.
characters. NAPTR applications that make use of SRV MUST NOT attempt to understand these domains or use them according to how the SRV specification structures its query domains. 3], 'cid', is extracted from the URN, prepended to urn.arpa. 'cid.urn.arpa' then becomes the first 'known' key in the NAPTR algorithm. The NAPTR records for cid.urn.arpa looked up and return a single record:
cid.urn.arpa. ;; order pref flags service regexp replacement IN NAPTR 100 10 "" "" "/urn:cid:.+@([^\.]+\.)(.*)$/\2/i" . 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. The record returned from the query on "gatech.edu" might look like: ;; order pref flags service regexp replacement IN NAPTR 100 50 "s" "z3950+I2L+I2C" "" _z3950._tcp.gatech.edu. IN NAPTR 100 50 "s" "rcds+I2C" "" _rcds._udp.gatech.edu. IN NAPTR 100 50 "s" "http+I2L+I2C+I2R" "" _http._tcp.gatech.edu. 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. 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 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.arpa record above, the regular expression entered into the master 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".
where: FLAGS A <character-string> which contains various flags. SERVICES A <character-string> which contains protocol and service identifiers. REGEXP A <character-string> which contains a regular expression. REPLACEMENT A <domain-name> which specifies the new value in the case where the regular expression is a simple replacement operation. <character-string> and <domain-name> as used here are defined in RFC1035 . RFC-1035 . 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 quoted <character-string>s. Since the Regexp field can contain numerous backslashes and thus should be treated with care. See Section 10 for how to correctly enter and escape the regular expression.
communication with a service. Those considerations are outside the scope of this document, and must be addressed by the specifications for particular communication protocols.  Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.  Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.  Moats, R., "URN Syntax", RFC 2141, May 1997.  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.  Crocker, D., "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997.  Daniel, R., "A Trivial Convention for using HTTP in URN Resolution", RFC 2169, June 1997.  Daniel, R. and M. Mealling, "Resolution of Uniform Resource Identifiers using the Domain Name System", RFC 2168, June 1997.  IEEE, "IEEE Standard for Information Technology - Portable Operating System Interface (POSIX) - Part 2: Shell and Utilities (Vol. 1)", IEEE Std 1003.2-1992, January 1993.  Berners-Lee, T., Fielding, R.T. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
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