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IETF RFC 7686
Last modified on Saturday, October 24th, 2015
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Internet Engineering Task Force (IETF) J. Appelbaum
Request for Comments: 7686 The Tor Project, Inc.
Category: Standards Track A. Muffett
ISSN: 2070-1721 Facebook
October 2015
The ".onion" Special-Use Domain Name
 Abstract
This document registers the ".onion" Special-Use Domain Name.
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 7686.
Copyright Notice
Copyright (c) 2015 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.
Appelbaum & Muffett Standards Track PAGE 1 
RFC 7686 .onion October 2015
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The ".onion" Special-Use Domain Name . . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The Tor network [Dingledine2004] has the ability to host network
services using the ".onion" Special-Use Top-Level Domain Name. Such
names can be used as other domain names would be (e.g., in URLs
[RFC 3986]), but instead of using the DNS infrastructure, .onion names
functionally correspond to the identity of a given service, thereby
combining location and authentication.
.onion names are used to provide access to end to end encrypted,
secure, anonymized services; that is, the identity and location of
the server is obscured from the client. The location of the client
is obscured from the server. The identity of the client may or may
not be disclosed through an optional cryptographic authentication
process.
.onion names are self-authenticating, in that they are derived from
the cryptographic keys used by the server in a client-verifiable
manner during connection establishment. As a result, the
cryptographic label component of a .onion name is not intended to be
human-meaningful.
The Tor network is designed to not be subject to any central
controlling authorities with regards to routing and service
publication, so .onion names cannot be registered, assigned,
transferred or revoked. "Ownership" of a .onion name is derived
solely from control of a public/private key pair that corresponds to
the algorithmic derivation of the name.
In this way, .onion names are "special" in the sense defined by
Section 3 of [RFC 6761]; they require hardware and software
implementations to change their handling in order to achieve the
desired properties of the name (see Section 4). These differences
are listed in Section 2.
Appelbaum & Muffett Standards Track PAGE 2
RFC 7686 .onion October 2015
Like Top-Level Domain Names, .onion names can have an arbitrary
number of subdomain components. This information is not meaningful
to the Tor protocol, but can be used in application protocols like
HTTP [RFC 7230].
Note that .onion names are required to conform with DNS name syntax
(as defined in Section 3.5 of [RFC 1034] and Section 2.1 of
[RFC 1123]), as they will still be exposed to DNS implementations.
See [tor-address] and [tor-rendezvous] for the details of the
creation and use of .onion names.
1.1. Notational Conventions
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].
2. The ".onion" Special-Use Domain Name
These properties have the following effects upon parties using or
processing .onion names (as per [RFC 6761]):
1. Users: Human users are expected to recognize .onion names as
having different security properties (see Section 1) and also as
being only available through software that is aware of .onion
names.
2. Application Software: Applications (including proxies) that
implement the Tor protocol MUST recognize .onion names as special
by either accessing them directly or using a proxy (e.g., SOCKS
[RFC 1928]) to do so. Applications that do not implement the Tor
protocol SHOULD generate an error upon the use of .onion and
SHOULD NOT perform a DNS lookup.
3. Name Resolution APIs and Libraries: Resolvers MUST either respond
to requests for .onion names by resolving them according to
[tor-rendezvous] or by responding with NXDOMAIN [RFC 1035].
4. Caching DNS Servers: Caching servers, where not explicitly
adapted to interoperate with Tor, SHOULD NOT attempt to look up
records for .onion names. They MUST generate NXDOMAIN for all
such queries.
5. Authoritative DNS Servers: Authoritative servers MUST respond to
queries for .onion with NXDOMAIN.
Appelbaum & Muffett Standards Track PAGE 3
RFC 7686 .onion October 2015
6. DNS Server Operators: Operators MUST NOT configure an
authoritative DNS server to answer queries for .onion. If they
do so, client software is likely to ignore any results (see
above).
7. DNS Registries/Registrars: Registrars MUST NOT register .onion
names; all such requests MUST be denied.
Note that the restriction upon the registration of .onion names does
not prohibit IANA from inserting a record into the root zone database
to reserve the name.
Likewise, it does not prevent non-DNS service providers (such as
trust providers) from supporting .onion names in their applications.
3. IANA Considerations
This document registers ".onion" in the registry of Special-Use
Domain Names [RFC 6761]. See Section 2 for the registration template.
4. Security Considerations
The security properties of .onion names can be compromised if, for
example:
o The server "leaks" its identity in another way (e.g., in an
application-level message), or
o The access protocol is implemented or deployed incorrectly, or
o The access protocol itself is found to have a flaw.
Users must take special precautions to ensure that the .onion name
they are communicating with is the intended one, as attackers may be
able to find keys that produce service names that are visually or
semantically similar to the desired service. This risk is magnified
because .onion names are typically not human-meaningful. It can be
mitigated by generating human-meaningful .onion names (at
considerable computing expense) or through users using bookmarks and
other trusted stores when following links.
Also, users need to understand the difference between a .onion name
used and accessed directly via Tor-capable software, versus .onion
subdomains of other top-level domain names and providers (e.g., the
difference between example.onion and example.onion.tld).
Appelbaum & Muffett Standards Track PAGE 4
RFC 7686 .onion October 2015
The cryptographic label for a .onion name is constructed by applying
a function to the public key of the server, the output of which is
rendered as a string and concatenated with the string .onion.
Dependent upon the specifics of the function used, an attacker may be
able to find a key that produces a collision with the same .onion
name with substantially less work than a cryptographic attack on the
full strength key. If this is possible the attacker may be able to
impersonate the service on the network.
A legacy client may inadvertently attempt to resolve a .onion name
through the DNS. This causes a disclosure that the client is
attempting to use Tor to reach a specific service. Malicious
resolvers could be engineered to capture and record such leaks, which
might have very adverse consequences for the well-being of the user.
This issue is mitigated if the client's software is updated to not
leak such queries or updated to support [tor-rendezvous], or if the
client's DNS software is updated to drop any request to the .onion
special-use domain name.
5. References
5.1. Normative References
[Dingledine2004]
Dingledine, R., Mathewson, N., and P. Syverson, "Tor: The
Second-Generation Onion Router", August 2004,
<https://svn.torproject.org/svn/projects/design-paper/
tor-design.html>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC 6761, February 2013,
<http://www.rfc-editor.org/info/RFC 6761>.
[tor-address]
Mathewson, N. and The Tor Project, "Special Hostnames in
Tor", 2006, <https://spec.torproject.org/address-spec>.
[tor-rendezvous]
The Tor Project, "Tor Rendezvous Specification", April
2014, <https://spec.torproject.org/rend-spec>.
Appelbaum & Muffett Standards Track PAGE 5
RFC 7686 .onion October 2015
5.2. Informative References
[RFC 1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC 1034, November 1987,
<http://www.rfc-editor.org/info/RFC 1034>.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC 1035,
November 1987, <http://www.rfc-editor.org/info/RFC 1035>.
[RFC 1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC 1123, October 1989,
<http://www.rfc-editor.org/info/RFC 1123>.
[RFC 1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928,
DOI 10.17487/RFC 1928, March 1996,
<http://www.rfc-editor.org/info/RFC 1928>.
[RFC 3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC 3986, January 2005,
<http://www.rfc-editor.org/info/RFC 3986>.
[RFC 7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC 7230, June 2014,
<http://www.rfc-editor.org/info/RFC 7230>.
Acknowledgements
Thanks to Roger Dingledine, Linus Nordberg, and Seth David Schoen for
their input and review.
This specification builds upon previous work by Christian Grothoff,
Matthias Wachs, Hellekin O. Wolf, Jacob Appelbaum, and Leif Ryge to
register .onion in conjunction with other, similar Special-Use Top-
Level Domain Names.
Appelbaum & Muffett Standards Track PAGE 6
RFC 7686 .onion October 2015
Authors' Addresses
Jacob Appelbaum
The Tor Project, Inc. & Technische Universiteit Eindhoven
Email: jacob@appelbaum.net
Alec Muffett
Facebook
Email: alecm@fb.com
Appelbaum & Muffett Standards Track PAGE 7
RFC TOTAL SIZE: 13475 bytes
PUBLICATION DATE: Saturday, October 24th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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