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Internet Engineering Task Force (IETF)                       S. CheshireRequest for Comments: 6761                                   M. KrochmalUpdates:1918,2606                                           Apple Inc.Category: Standards Track                                  February 2013ISSN: 2070-1721Special-Use Domain NamesAbstract   This document describes what it means to say that a Domain Name (DNS   name) is reserved for special use, when reserving such a name is   appropriate, and the procedure for doing so.  It establishes an IANA   registry for such domain names, and seeds it with entries for some of   the already established special domain names.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6761.Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Cheshire & Krochmal          Standards Track                    [Page 1]

RFC 6761                Special-Use Domain Names           February 20131.  Introduction   Certain individual IP addresses and IP address ranges are treated   specially by network implementations and, consequently, are not   suitable for use as unicast addresses.  For example, IPv4 addresses   224.0.0.0 to 239.255.255.255 are multicast addresses [RFC5735], with   224.0.0.1 being the "all hosts" multicast address [RFC1112]   [RFC5771].  Another example is 127.0.0.1, the IPv4 "local host"   address [RFC5735].   Analogous to Special-Use IPv4 Addresses [RFC5735], the Domain Name   System (DNS) [RFC1034][RFC1035] has its own concept of reserved   names, such as "example.com.", "example.net.", and "example.org.", or   any name falling under the top-level pseudo-domain "invalid."   [RFC2606].  However, "Reserved Top Level DNS Names" [RFC2606] does   not state whether implementations are expected to treat such names   differently, and if so, in what way.   This document specifies under what circumstances special treatment is   appropriate, and in what ways.2.  Terminology   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 "Key words for use in   RFCs to Indicate Requirement Levels" [RFC2119].3.  Applicability   When IP multicast was created [RFC1112], implementations had to be   updated to understand what an IP multicast address means and what to   do with it.  Adding IP multicast to a networking stack entailed more   than merely adding the right routing table entries for those   addresses.  Moreover, supporting IP multicast entails some level of   commonality that is consistent across all conformant hosts,   independent of what networks those hosts may be connected to.  While   it is possible to build a private isolated network using whatever   valid unicast IP addresses and routing topology one chooses   (regardless of whether those unicast IP addresses are already in use   by other hosts on the public Internet), the IPv4 multicast address   224.0.0.1 is always the "all hosts" multicast address, and that's not   a local decision.   Similarly, if a domain name has special properties that affect the   way hardware and software implementations handle the name, that apply   universally regardless of what network the implementation may be   connected to, then that domain name may be a candidate for having theCheshire & Krochmal          Standards Track                    [Page 2]

RFC 6761                Special-Use Domain Names           February 2013   IETF declare it to be a Special-Use Domain Name and specify what   special treatment implementations should give to that name.  On the   other hand, if declaring a given name to be special would result in   no change to any implementations, then that suggests that the name   may not be special in any material way, and it may be more   appropriate to use the existing DNS mechanisms [RFC1034] to provide   the desired delegation, data, or lack-of-data, for the name in   question.  Where the desired behaviour can be achieved via the   existing domain name registration processes, that process should be   used.  Reservation of a Special-Use Domain Name is not a mechanism   for circumventing normal domain name registration processes.   As an example, suppose there were to be an IETF document specifying   that a particular name (or set of names) is guaranteed to produce an   NXDOMAIN ("Name Error" [RFC1035]) result.  Such a document falls   within the responsibilities of the IETF.  The IETF is responsible for   protocol rules.  The IETF defines name character set, length limits,   syntax, the fact that in DNS "A" is equivalent to "a", etc.   [RFC1034] [RFC1035].  Portions of the namespace created by those   rules are given to ICANN to manage, but, due to established DNS   protocol rules, ICANN is not free to allocate "COM" and "com" to two   different name servers.  The IETF has responsibility for specifying   how the DNS protocol works, and ICANN is responsible for allocating   the names made possible by that DNS protocol.  Now, suppose a   developer were to use this special "guaranteed nonexistent" name,   "knowing" that it's guaranteed to return NXDOMAIN, and suppose also   that the user's DNS server fails to return NXDOMAIN for this name.   The developer's software then fails.  Who do the user and/or   developer complain to?  ICANN?  The IETF?  The DNS server operator?   If the developer can't depend on the special "guaranteed nonexistent"   name to always return NXDOMAIN, then the special name is worthless,   because it can't be relied on to do what it is supposed to.  For this   special "guaranteed nonexistent" name to have any use, it has to be   defined to return NXDOMAIN, BY PROTOCOL, for all installations, not   just by ICANN allocation on the public Internet.  ICANN has no   jurisdiction over how users choose to configure their own private DNS   servers on their own private networks, but developers need a protocol   specification that states that returning positive answers for the   special "guaranteed nonexistent" name is a protocol violation on   *all* networks, not just the public Internet.  Hence, the act of   defining such a special name creates a higher-level protocol rule,   above ICANN's management of allocable names on the public Internet.Cheshire & Krochmal          Standards Track                    [Page 3]

RFC 6761                Special-Use Domain Names           February 20134.  Procedure   If it is determined that special handling of a name is required in   order to implement some desired new functionality, then an IETF   "Standards Action" or "IESG Approval" specification [RFC5226] MUST be   published describing the new functionality.   The specification MUST state how implementations determine that the   special handling is required for any given name.  This is typically   done by stating that any fully qualified domain name ending in a   certain suffix (i.e., falling within a specified parent pseudo-   domain) will receive the special behaviour.  In effect, this carves   off a sub-tree of the DNS namespace in which the modified name   treatment rules apply, analogous to how IP multicast [RFC1112] or IP   link-local addresses [RFC3927] [RFC4862] carve off chunks of the IP   address space in which their respective modified address treatment   rules apply.   The specification also MUST state, in each of the seven "Domain Name   Reservation Considerations" categories below, what special treatment,   if any, is to be applied.  If in all seven categories the answer is   "none", then possibly no special treatment is required and requesting   reservation of a Special-Use Domain Name may not be appropriate.5.  Domain Name Reservation Considerations   An IETF "Standards Action" or "IESG Approval" document specifying   some new naming behaviour, which requires a Special-Use Domain Name   be reserved to implement this desired new behaviour, needs to contain   a subsection of the "IANA Considerations" section titled "Domain Name   Reservation Considerations" giving answers in the seven categories   listed below.  In the case of algorithmically generated DNS names,   the specifying document needs to clearly identify the set of names   generated by the algorithm that would require the proposed special   treatment.   1.  Users:       Are human users expected to recognize these names as special and       use them differently?  In what way?   2.  Application Software:       Are writers of application software expected to make their       software recognize these names as special and treat them       differently?  In what way?  (For example, if a human user enters       such a name, should the application software reject it with an       error message?)Cheshire & Krochmal          Standards Track                    [Page 4]

RFC 6761                Special-Use Domain Names           February 2013   3.  Name Resolution APIs and Libraries:       Are writers of name resolution APIs and libraries expected to       make their software recognize these names as special and treat       them differently?  If so, how?   4.  Caching DNS Servers:       Are developers of caching domain name servers expected to make       their implementations recognize these names as special and treat       them differently?  If so, how?   5.  Authoritative DNS Servers:       Are developers of authoritative domain name servers expected to       make their implementations recognize these names as special and       treat them differently?  If so, how?   6.  DNS Server Operators:       Does this reserved Special-Use Domain Name have any potential       impact on DNS server operators?  If they try to configure their       authoritative DNS server as authoritative for this reserved name,       will compliant name server software reject it as invalid?  Do DNS       server operators need to know about that and understand why?       Even if the name server software doesn't prevent them from using       this reserved name, are there other ways that it may not work as       expected, of which the DNS server operator should be aware?   7.  DNS Registries/Registrars:       How should DNS Registries/Registrars treat requests to register       this reserved domain name?  Should such requests be denied?       Should such requests be allowed, but only to a specially-       designated entity?  (For example, the name "www.example.org" is       reserved for documentation examples and is not available for       registration; however, the name is in fact registered; and there       is even a web site at that name, which states circularly that the       name is reserved for use in documentation and cannot be       registered!)Cheshire & Krochmal          Standards Track                    [Page 5]

RFC 6761                Special-Use Domain Names           February 20136.  Initial Registry   The initial IANA "Special-Use Domain Names" registry shall contain   entries for the private-address [RFC1918] reverse-mapping domains and   for the existing Reserved Top Level DNS Names [RFC2606].6.1.  Domain Name Reservation Considerations for Private Addresses   The private-address [RFC1918] reverse-mapping domains listed below,   and any names falling within those domains, are Special-Use Domain   Names:     10.in-addr.arpa.      21.172.in-addr.arpa.  26.172.in-addr.arpa.     16.172.in-addr.arpa.  22.172.in-addr.arpa.  27.172.in-addr.arpa.     17.172.in-addr.arpa.  30.172.in-addr.arpa.  28.172.in-addr.arpa.     18.172.in-addr.arpa.  23.172.in-addr.arpa.  29.172.in-addr.arpa.     19.172.in-addr.arpa.  24.172.in-addr.arpa.  31.172.in-addr.arpa.     20.172.in-addr.arpa.  25.172.in-addr.arpa.  168.192.in-addr.arpa.   These domains, and any names falling within these domains, are   special in the following ways:   1.  Users are free to use these names as they would any other       reverse-mapping names.  However, since there is no central       authority responsible for use of private addresses, users SHOULD       be aware that these names are likely to yield different results       on different networks.   2.  Application software SHOULD NOT recognize these names as special,       and SHOULD use these names as they would other reverse-mapping       names.   3.  Name resolution APIs and libraries SHOULD NOT recognize these       names as special and SHOULD NOT treat them differently.  Name       resolution APIs SHOULD send queries for these names to their       configured caching DNS server(s).   4.  Caching DNS servers SHOULD recognize these names as special and       SHOULD NOT, by default, attempt to look up NS records for them,       or otherwise query authoritative DNS servers in an attempt to       resolve these names.  Instead, caching DNS servers SHOULD, by       default, generate immediate (positive or negative) responses for       all such queries.  This is to avoid unnecessary load on the root       name servers and other name servers.  Caching DNS servers SHOULD       offer a configuration option (disabled by default) to enable       upstream resolution of such names, for use in private networks       where private-address reverse-mapping names are known to be       handled by an authoritative DNS server in said private network.Cheshire & Krochmal          Standards Track                    [Page 6]

RFC 6761                Special-Use Domain Names           February 2013   5.  Authoritative DNS servers SHOULD recognize these names as special       and SHOULD, by default, generate immediate negative responses for       all such queries, unless explicitly configured by the       administrator to give positive answers for private-address       reverse-mapping names.   6.  DNS server operators SHOULD, if they are using private addresses,       configure their authoritative DNS servers to act as authoritative       for these names.   7.  DNS Registries/Registrars MUST NOT grant requests to register any       of these names in the normal way to any person or entity.  These       names are reserved for use in private networks, and fall outside       the set of names available for allocation by registries/       registrars.  Attempting to allocate one of these names as if it       were a normal DNS domain name will probably not work as desired,       for reasons 4, 5 and 6 above.6.2.  Domain Name Reservation Considerations for "test."   The domain "test.", and any names falling within ".test.", are   special in the following ways:   1.  Users are free to use these test names as they would any other       domain names.  However, since there is no central authority       responsible for use of test names, users SHOULD be aware that       these names are likely to yield different results on different       networks.   2.  Application software SHOULD NOT recognize test names as special,       and SHOULD use test names as they would other domain names.   3.  Name resolution APIs and libraries SHOULD NOT recognize test       names as special and SHOULD NOT treat them differently.  Name       resolution APIs SHOULD send queries for test names to their       configured caching DNS server(s).   4.  Caching DNS servers SHOULD recognize test names as special and       SHOULD NOT, by default, attempt to look up NS records for them,       or otherwise query authoritative DNS servers in an attempt to       resolve test names.  Instead, caching DNS servers SHOULD, by       default, generate immediate negative responses for all such       queries.  This is to avoid unnecessary load on the root name       servers and other name servers.  Caching DNS servers SHOULD offer       a configuration option (disabled by default) to enable upstream       resolving of test names, for use in networks where test names are       known to be handled by an authoritative DNS server in said       private network.Cheshire & Krochmal          Standards Track                    [Page 7]

RFC 6761                Special-Use Domain Names           February 2013   5.  Authoritative DNS servers SHOULD recognize test names as special       and SHOULD, by default, generate immediate negative responses for       all such queries, unless explicitly configured by the       administrator to give positive answers for test names.   6.  DNS server operators SHOULD, if they are using test names,       configure their authoritative DNS servers to act as authoritative       for test names.   7.  DNS Registries/Registrars MUST NOT grant requests to register       test names in the normal way to any person or entity.  Test names       are reserved for use in private networks and fall outside the set       of names available for allocation by registries/registrars.       Attempting to allocate a test name as if it were a normal DNS       domain name will probably not work as desired, for reasons 4, 5,       and 6 above.6.3.  Domain Name Reservation Considerations for "localhost."   The domain "localhost." and any names falling within ".localhost."   are special in the following ways:   1.  Users are free to use localhost names as they would any other       domain names.  Users may assume that IPv4 and IPv6 address       queries for localhost names will always resolve to the respective       IP loopback address.   2.  Application software MAY recognize localhost names as special, or       MAY pass them to name resolution APIs as they would for other       domain names.   3.  Name resolution APIs and libraries SHOULD recognize localhost       names as special and SHOULD always return the IP loopback address       for address queries and negative responses for all other query       types.  Name resolution APIs SHOULD NOT send queries for       localhost names to their configured caching DNS server(s).   4.  Caching DNS servers SHOULD recognize localhost names as special       and SHOULD NOT attempt to look up NS records for them, or       otherwise query authoritative DNS servers in an attempt to       resolve localhost names.  Instead, caching DNS servers SHOULD,       for all such address queries, generate an immediate positive       response giving the IP loopback address, and for all other query       types, generate an immediate negative response.  This is to avoid       unnecessary load on the root name servers and other name servers.Cheshire & Krochmal          Standards Track                    [Page 8]

RFC 6761                Special-Use Domain Names           February 2013   5.  Authoritative DNS servers SHOULD recognize localhost names as       special and handle them as described above for caching DNS       servers.   6.  DNS server operators SHOULD be aware that the effective RDATA for       localhost names is defined by protocol specification and cannot       be modified by local configuration.   7.  DNS Registries/Registrars MUST NOT grant requests to register       localhost names in the normal way to any person or entity.       Localhost names are defined by protocol specification and fall       outside the set of names available for allocation by registries/       registrars.  Attempting to allocate a localhost name as if it       were a normal DNS domain name will probably not work as desired,       for reasons 2, 3, 4, and 5 above.6.4.  Domain Name Reservation Considerations for "invalid."   The domain "invalid." and any names falling within ".invalid." are   special in the ways listed below.  In the text below, the term   "invalid" is used in quotes to signify such names, as opposed to   names that may be invalid for other reasons (e.g., being too long).   1.  Users are free to use "invalid" names as they would any other       domain names.  Users MAY assume that queries for "invalid" names       will always return NXDOMAIN responses.   2.  Application software MAY recognize "invalid" names as special or       MAY pass them to name resolution APIs as they would for other       domain names.   3.  Name resolution APIs and libraries SHOULD recognize "invalid"       names as special and SHOULD always return immediate negative       responses.  Name resolution APIs SHOULD NOT send queries for       "invalid" names to their configured caching DNS server(s).   4.  Caching DNS servers SHOULD recognize "invalid" names as special       and SHOULD NOT attempt to look up NS records for them, or       otherwise query authoritative DNS servers in an attempt to       resolve "invalid" names.  Instead, caching DNS servers SHOULD       generate immediate NXDOMAIN responses for all such queries.  This       is to avoid unnecessary load on the root name servers and other       name servers.   5.  Authoritative DNS servers SHOULD recognize "invalid" names as       special and handle them as described above for caching DNS       servers.Cheshire & Krochmal          Standards Track                    [Page 9]

RFC 6761                Special-Use Domain Names           February 2013   6.  DNS server operators SHOULD be aware that the effective RDATA for       "invalid" names is defined by protocol specification to be       nonexistent and cannot be modified by local configuration.   7.  DNS Registries/Registrars MUST NOT grant requests to register       "invalid" names in the normal way to any person or entity.  These       "invalid" names are defined by protocol specification to be       nonexistent, and they fall outside the set of names available for       allocation by registries/registrars.  Attempting to allocate a       "invalid" name as if it were a normal DNS domain name will       probably not work as desired, for reasons 2, 3, 4, and 5 above.6.5.  Domain Name Reservation Considerations for Example Domains   The domains "example.", "example.com.", "example.net.",   "example.org.", and any names falling within those domains, are   special in the following ways:   1.  Users SHOULD understand that example names are reserved for use       in documentation.   2.  Application software SHOULD NOT recognize example names as       special and SHOULD use example names as they would other domain       names.   3.  Name resolution APIs and libraries SHOULD NOT recognize example       names as special and SHOULD NOT treat them differently.  Name       resolution APIs SHOULD send queries for example names to their       configured caching DNS server(s).   4.  Caching DNS servers SHOULD NOT recognize example names as special       and SHOULD resolve them normally.   5.  Authoritative DNS servers SHOULD NOT recognize example names as       special.   6.  DNS server operators SHOULD be aware that example names are       reserved for use in documentation.   7.  DNS Registries/Registrars MUST NOT grant requests to register       example names in the normal way to any person or entity.  All       example names are registered in perpetuity to IANA:Cheshire & Krochmal          Standards Track                   [Page 10]

RFC 6761                Special-Use Domain Names           February 2013        Domain Name: EXAMPLE.COM        Registrar: RESERVED-INTERNET ASSIGNED NUMBERS AUTHORITY        Whois Server: whois.iana.org        Referral URL:http://res-dom.iana.org        Name Server: A.IANA-SERVERS.NET        Name Server: B.IANA-SERVERS.NET        Status: clientDeleteProhibited        Status: clientTransferProhibited        Status: clientUpdateProhibited        Updated Date: 26-mar-2004        Creation Date: 14-aug-1995        Expiration Date: 13-aug-2011   IANA currently maintains a web server providing a web page explaining   the purpose of example domains.7.  Security Considerations   This document outlines the circumstances in which reserving a domain   name for special use is appropriate, and the procedure for having   that Special-Use Domain Name recorded by IANA.  Any document   requesting such a Special-Use Domain Name needs to contain an   appropriate "Security Considerations" section which describes any   security issues relevant to that special use.8.  IANA Considerations   IANA has created a new registry of Special-Use Domain Names,   initially populated with the private-address reverse-mapping domains   and the Reserved Top Level DNS Names outlined above inSection 6.   When IANA receives a request to record a new "Special-Use Domain   Name", it should verify, in consultation with the IESG, that the IETF   "Standards Action" or "IESG Approval" document [RFC5226] includes the   required "Domain Name Reservation Considerations" section stating how   the special meaning of this name affects the behavior of hardware,   software, and humans in the seven categories.  If IANA and the IESG   determine that special handling of this "Special-Use Domain Name" is   appropriate, IANA should record the Special-Use Domain Name, and a   reference to the specification that documents it, in the registry.Cheshire & Krochmal          Standards Track                   [Page 11]

RFC 6761                Special-Use Domain Names           February 20139.  References9.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",              STD 13,RFC 1034, November 1987.   [RFC1035]  Mockapetris, P., "Domain names - implementation and              specification", STD 13,RFC 1035, November 1987.   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an              IANA Considerations Section in RFCs",BCP 26,RFC 5226,              May 2008.9.2.  Informative References   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,RFC 1112, August 1989.   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and              E. Lear, "Address Allocation for Private Internets",BCP 5,RFC 1918, February 1996.   [RFC2606]  Eastlake, D. and A. Panitz, "Reserved Top Level DNS              Names",BCP 32,RFC 2606, June 1999.   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic              Configuration of IPv4 Link-Local Addresses",RFC 3927,              May 2005.   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless              Address Autoconfiguration",RFC 4862, September 2007.   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",BCP 153,RFC 5735, January 2010.   [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for              IPv4 Multicast Address Assignments",BCP 51,RFC 5771,              March 2010.Cheshire & Krochmal          Standards Track                   [Page 12]

RFC 6761                Special-Use Domain Names           February 2013Authors' Addresses   Stuart Cheshire   Apple Inc.   1 Infinite Loop   Cupertino, CA  95014   USA   Phone: +1 408 974 3207   EMail: cheshire@apple.com   Marc Krochmal   Apple Inc.   1 Infinite Loop   Cupertino, CA  95014   USA   Phone: +1 408 974 4368   EMail: marc@apple.comCheshire & Krochmal          Standards Track                   [Page 13]