dnsop                                                       J. Appelbaum
Internet-Draft                                      The Tor Project, Inc
Intended status: Standards Track                              A. Muffett
Expires: December 21, 2015                                      Facebook
                                                           June 19, 2015

                   The .onion Special-Use Domain Name


   This document registers the ".onion" Special-Use Domain Name.

Status of This Memo

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   This Internet-Draft will expire on December 21, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   2
   2.  The ".onion" Special-Use Domain Name  . . . . . . . . . . . .   2
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The Tor network [Dingledine2004] has the ability to host network
   services using the ".onion" Special-Use Top-Level Domain.  Such
   addresses 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.

   In this way, .onion names are "special" in the sense defined by
   [RFC6761] Section 3; 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 addresses 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].

   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]):

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   1.  Users: human users are expected to recognize .onion names as
       having different security properties, and also being only
       available through software that is aware of onion addresses.

   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 either
       respond to requests for .onion names by resolving them according
       to [tor-rendezvous] or by responding with NXDOMAIN.

   4.  Caching DNS Servers: Caching servers 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.

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

   .onion names are often 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.

   These properties can be compromised if, for example:

   o  The server "leaks" its identity in another way (e.g., in an
      application-level message), or

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   o  The access protocol is implemented or deployed incorrectly, or

   o  The access protocol itself is found to have a flaw.

   .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 which corresponds to
   the algorithmic derivation of the name.

   Users must take special precautions to ensure that the .onion name
   they are communicating with is correct, as attackers may be able to
   find keys which produce service names that are visually or
   semantically similar to the desired service.

   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.

   If client software attempts to resolve a .onion name, it can leak the
   identity of the service that the user is attempting to access to DNS
   resolvers, authoritative DNS servers, and observers on the
   intervening network.  This can be mitigated by following the
   recommendations in Section 2.

5.  References

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5.1.  Normative References

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

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

5.2.  Informative References

              Dingledine, R., Mathewson, N., and P. Syverson, "Tor: the
              second-generation onion router", 2004, <https://www.onion-

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

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

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

              Mathewson, N. and R. Dingledine, "Special Hostnames in
              Tor", September 2001,

              Mathewson, N. and R. Dingledine, "Tor Rendezvous
              Specification", April 2014,

Appendix A.  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.

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Authors' Addresses

   Jacob Appelbaum
   The Tor Project, Inc

   Email: jacob@appelbaum.net

   Alec Muffett

   Email: alecm@fb.com

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