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RFC 7686 - The ".onion" Special-Use Domain Name

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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


   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

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.

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
   [RFC3986]), 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

   .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

   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 [RFC6761]; 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.

   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 [RFC7230].

   Note that .onion names are required to conform with DNS name syntax
   (as defined in Section 3.5 of [RFC1034] and Section 2.1 of
   [RFC1123]), 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",
   document are to be interpreted as described in [RFC2119].

2.  The ".onion" Special-Use Domain Name

   These properties have the following effects upon parties using or
   processing .onion names (as per [RFC6761]):

   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

   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
       [RFC1928]) 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 [RFC1035].

   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.

   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

   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 [RFC6761].  See Section 2 for the registration template.

4.  Security Considerations

   The security properties of .onion names can be compromised if, for

   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).

   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

              Dingledine, R., Mathewson, N., and P. Syverson, "Tor: The
              Second-Generation Onion Router", August 2004,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,

              Mathewson, N. and The Tor Project, "Special Hostnames in
              Tor", 2006, <https://spec.torproject.org/address-spec>.

              The Tor Project, "Tor Rendezvous Specification", April
              2014, <https://spec.torproject.org/rend-spec>.

5.2.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,

   [RFC1928]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
              L. Jones, "SOCKS Protocol Version 5", RFC 1928,
              DOI 10.17487/RFC1928, March 1996,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,


   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.

Authors' Addresses

   Jacob Appelbaum
   The Tor Project, Inc. & Technische Universiteit Eindhoven

   Email: jacob@appelbaum.net

   Alec Muffett

   Email: alecm@fb.com


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