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IETF RFC 6761
Last modified on Wednesday, February 20th, 2013
 Permanent link to RFC 6761
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Internet Engineering Task Force (IETF) S. Cheshire
Request for Comments: 6761 M. Krochmal
Updates: 1918, 2606 Apple Inc.
Category: Standards Track February 2013
ISSN: 2070-1721
Special-Use Domain Names
 Abstract
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 in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6761.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 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 2013
1. 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 [RFC 5735], with
224.0.0.1 being the "all hosts" multicast address [RFC 1112]
[RFC 5771]. Another example is 127.0.0.1, the IPv4 "local host"
address [RFC 5735].
Analogous to Special-Use IPv4 Addresses [RFC 5735], the Domain Name
System (DNS) [RFC 1034][RFC 1035] 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."
[RFC 2606]. However, "Reserved Top Level DNS Names" [RFC 2606] 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" [RFC 2119].
3. Applicability
When IP multicast was created [RFC 1112], 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 the
Cheshire & 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 [RFC 1034] 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" [RFC 1035]) 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.
[RFC 1034] [RFC 1035]. 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 2013
4. 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 [RFC 5226] 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 [RFC 1112] or IP
link-local addresses [RFC 3927] [RFC 4862] 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?)
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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 2013
6. Initial Registry
The initial IANA "Special-Use Domain Names" registry shall contain
entries for the private-address [RFC 1918] reverse-mapping domains and
for the existing Reserved Top Level DNS Names [RFC 2606].
6.1. Domain Name Reservation Considerations for Private Addresses
The private-address [RFC 1918] 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 in Section 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 [RFC 5226] 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 2013
9. References
9.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
9.2. Informative References
[RFC 1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[RFC 1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC 2606] Eastlake, D. and A. Panitz, "Reserved Top Level DNS
Names", BCP 32, RFC 2606, June 1999.
[RFC 3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
May 2005.
[RFC 4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC 5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[RFC 5771] 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 2013
Authors' 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.com
Cheshire & Krochmal Standards Track PAGE 13
RFC TOTAL SIZE: 28862 bytes
PUBLICATION DATE: Wednesday, February 20th, 2013
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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