Internet Engineering Task Force (IETF) W. Kumari
Request for Comments: 9476 Google
Category: Standards Track P. Hoffman
ISSN: 2070-1721 ICANN
September 2023
The .alt Special-Use Top-Level Domain
Abstract
This document reserves a Top-Level Domain (TLD) label "alt" to be
used in non-DNS contexts. It also provides advice and guidance to
developers creating alternative namespaces.
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9476.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Terminology
1.2. Requirements Terminology
2. The .alt Namespace
3. IANA Considerations
3.1. Special-Use Domain Name Registry
3.2. Domain Name Reservation Considerations
4. Privacy Considerations
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Many Internet protocols need to name entities. Names that look like
DNS names (a series of labels separated with dots) have become
common, even in systems that are not part of the global DNS
administered by IANA. This document reserves the top-level label
"alt" (short for "alternative") as a special-use domain name
[RFC6761]. This top-level label can be used as the final (rightmost)
label to signify that the name is not rooted in the global DNS and
that it should not be resolved using the DNS protocol.
Throughout the rest of this document, the top-level "alt" label is
shown as ".alt" to match the common presentation form of DNS names.
As detailed in Section 3.1, IANA has added the .alt name to the
"Special-Use Domain Name" registry. IANA sets aside names in that
registry, as described in <https://www.iana.org/domains/reserved>.
The techniques in this document are primarily intended to address
some of the issues discussed in [RFC8244], which contains additional
background on the issues with special-use domain names.
In this document, ".alt" was chosen for the special-use domain name
instead of something like "alt.arpa" so that systems that use the
name do not have to worry that a parent of their name would be
resolved if the name leaked to the Internet. Historically, some
systems that want to use non-DNS names wanted the entire name to be
not in the DNS, and reserving ".alt" fulfills that use case.
1.1. Terminology
This document assumes familiarity with DNS terms; please see
[RFC8499]. Terminology that is specific to this document is:
DNS name: Domain names that are intended to be used with DNS
resolution, either in the global DNS or in some other context.
DNS context: The namespace anchored at the globally unique DNS root
and administered by IANA. This is the namespace or context that
"normal" DNS uses.
non-DNS context: Any other (alternative) namespace.
pseudo-TLD: A label that appears in a fully qualified domain name in
the position of a TLD, which is not part of the global DNS. This
term is not intended to be pejorative.
TLD: See the definition in Section 2 of [RFC8499].
1.2. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. The .alt Namespace
This document reserves the .alt label for use as an unmanaged pseudo-
TLD namespace. The .alt label can be used in any domain name as a
pseudo-TLD to signify that this is an alternative (non-DNS) namespace
and should not be looked up in a DNS context.
This document uses ".alt" for the pseudo-TLD in the presentation
format for the DNS, corresponding to a 0x03616c7400 suffix in DNS
wire format. The on-the-wire formats for non-DNS protocols might be
different.
Because names beneath .alt are in an alternative namespace, they have
no significance in the regular DNS context. DNS stub and recursive
resolvers do not need to look them up in the DNS context.
DNS resolvers that serve the DNS protocol and non-DNS protocols at
the same time might consider .alt like a DNS entry in the "Transport-
Independent Locally-Served DNS Zone Registry" that is part of IANA's
"Locally-Served DNS Zones" registry, except that .alt is always used
to denote names that are to be resolved by non-DNS protocols. Note
that this document does not request adding .alt to these registries
because .alt, by this specification, is not a DNS name.
Note that using .alt as a pseudo-TLD does not mandate how the non-DNS
protocol will handle the name. To maximize compatibility with
existing applications, it is suggested, but not required, that non-
DNS protocols using names that end in .alt follow DNS name syntax.
If the non-DNS protocol has a wire format like the DNS wire format,
it might append the null label at the end of the name, but it also
might not. This document does not make any suggestion for how non-
DNS protocols deal with the wire format of their names.
Groups wishing to create new alternative namespaces may create their
alternative namespace under a label that names their namespace under
the .alt pseudo-TLD. This document defines neither a registry nor a
governance model for the .alt namespace, as it is not managed by the
IETF or IANA. There is no guarantee of unambiguous mappings from
names to name resolution mechanisms. Mitigation or resolution of
collisions that occur under .alt are outside the scope of this
document and outside the IETF's remit. Users are advised to consider
the associated risks when using names under .alt.
Regardless of the expectations above, names in the .alt pseudo-TLD
will leak outside the context in which they are valid. Decades of
experience show that such names will appear at recursive resolvers
and will thus also appear at the root servers for the global DNS.
Sending traffic to the root servers that is known to always elicit an
NXDOMAIN response, such as queries for names ending in .alt, wastes
resources on both the resolver and the root server. Caching
resolvers performing aggressive use of DNSSEC-validated caches
(described in [RFC8198]) may mitigate this by synthesizing negative
answers from cached NSEC records for names under .alt. Similarly,
caching resolvers using QNAME minimization (described in [RFC9156])
will cause less of this traffic to the root servers because the
negative responses will cover all names under .alt.
Currently deployed projects and protocols that are using pseudo-TLDs
are recommended to move under the .alt pseudo-TLD, but this is not a
requirement. Rather, the .alt pseudo-TLD is being reserved so that
current and future projects of a similar nature have a designated
place to create alternative resolution namespaces that will not
conflict with the regular DNS context.
3. IANA Considerations
3.1. Special-Use Domain Name Registry
The IANA has added the .alt name to the "Special-Use Domain Name"
registry [RFC6761] with a reference to this RFC.
3.2. Domain Name Reservation Considerations
This section exists to meet the requirements of [RFC6761]. The
questions posed in [RFC6761] were largely written assuming a DNS
resolution system, and so some of the questions are not especially
relevant or well suited.
1. Users might or might not recognize that names in the .alt pseudo-
TLD as special.
2. Application software that uses alternative namespaces in the .alt
pseudo-TLD are expected to have their own processing rules for
their own names, probably in specialized resolver APIs,
libraries, and/or application software. Application software
that is not specifically designed to use names in the .alt
pseudo-TLD are not expected to make their software recognize
these names as special.
3. Developers of name resolution APIs and libraries that are
specifically designed to implement resolution of an alternative
name resolution system are expected to recognize names in the
.alt pseudo-TLD as special and thus perform resolution of those
names. The exact mechanism used by the name resolution APIs and
libraries will obviously depend on the particular alternative
resolution system. Regular DNS resolution APIs and libraries are
not expected to recognize or treat names in the .alt pseudo-TLD
differently.
4. Caching DNS servers SHOULD NOT recognize names in the .alt
pseudo-TLD as special and SHOULD NOT perform any special handling
with them.
5. Authoritative DNS servers SHOULD NOT recognize names in the .alt
pseudo-TLD as special and SHOULD NOT perform any special handling
with them.
6. DNS server operators will treat names in the .alt pseudo-TLD as
they would names in any other TLD not in the global DNS. DNS
server operators may be aware that queries for names ending in
.alt are not DNS names and that queries for those names were
leaked into the DNS context. This information can be useful for
support or debugging purposes.
7. It is not possible for DNS registries/registrars to register DNS
names in the .alt pseudo-TLD as the .alt will not exist in the
global DNS root.
4. Privacy Considerations
This document reserves .alt to be used to indicate that a name is not
a DNS name. Unfortunately, these queries will undoubtedly leak into
the global DNS. This is a general problem with alternative
namespaces and not confined to names ending in .alt.
For example, a value such as "example.alt" could easily cause a
privacy issue for any names in that namespace that are leaked to the
Internet. In addition, if a name ending in .alt is sufficiently
unique, long-lasting, and frequently leaks into the global DNS, then
regardless of how the name is constructed, it can act similar to a
web cookie with all the associated downsides of identification or re-
identification.
5. Security Considerations
Because names in the .alt pseudo-TLD are explicitly outside of the
DNS context, it is impossible to rely on any DNS-related security
considerations. Care must be taken when mapping the pseudo-TLD into
its corresponding non-DNS name resolution system in order to get
whatever security is offered by that system.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<https://www.rfc-editor.org/info/rfc6761>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8244] Lemon, T., Droms, R., and W. Kumari, "Special-Use Domain
Names Problem Statement", RFC 8244, DOI 10.17487/RFC8244,
October 2017, <https://www.rfc-editor.org/info/rfc8244>.
6.2. Informative References
[RFC8198] Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
July 2017, <https://www.rfc-editor.org/info/rfc8198>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[RFC9156] Bortzmeyer, S., Dolmans, R., and P. Hoffman, "DNS Query
Name Minimisation to Improve Privacy", RFC 9156,
DOI 10.17487/RFC9156, November 2021,
<https://www.rfc-editor.org/info/rfc9156>.
Acknowledgements
We would like to thank Joe Abley, Mark Andrews, Erik Auerswald, Roy
Arends, Ray Bellis, Vittorio Bertola, Marc Blanchet, John Bond,
Stéphane Bortzmeyer, David Cake, Vint Cerf, David Conrad, Steve
Crocker, Vladimir Cunat, Brian Dickson, Ralph Droms, Robert Edmonds,
Patrik Fältström, Bernd Fix, Christian Grothoff, Olafur Gudmundsson,
Ted Hardie, Bob Harold, Wes Hardaker, Geoff Huston, Joel Jaeggli,
John C Klensin, Eliot Lear, Barry Leiba, Ted Lemon, Edward Lewis,
John Levine, George Michaelson, Ed Pascoe, Libor Peltan, Jim Reid,
Martin Schanzenbach, Ben Schwartz, Arturo Servin, Peter Thomassen,
Paul Vixie, Duane Wessels, Paul Wouters, and Suzanne Woolf for
feedback.
This document was many years in the making, and we would like to
sincerely apologize for anyone whom we forgot to credit.
We would also like to thank Rob Wilton for serving as Responsible AD
for this document.
In addition, Andrew Sullivan was an author from adoption (2015)
through version 14 (2021).
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: warren@kumari.net
Paul Hoffman
ICANN
Email: paul.hoffman@icann.org
