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The .onion Special-Use Domain Name


dnsop                                                       J. Appelbaum
Internet-Draft                                          Tor Project Inc.
Intended status: Standards Track                              A. Muffett
Expires: September 6, 2015                                      Facebook
                                                           March 5, 2015

                   The .onion Special-Use Domain Name


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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 6, 2015.

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   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   2
   2.  The ".onion" Special-Use TLD  . . . . . . . . . . . . . . . .   2
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   The Tor network [Dingledine2004] has the ability to host network
   services using the ".onion" 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 are hashes that
   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 other TLDs, .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

   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 TLD

   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, and also being only
       available through software that is aware of onion addresses.

   2.  Application Software: Applications 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 that implement the
       Tor protocol MUST either respond to requests for .onion names by
       resolving them (see [tor-rendezvous]) or by responding with
       NXDOMAIN.  Other resolvers SHOULD respond with NXDOMAIN.

   4.  Caching DNS Servers: Caching servers SHOULD NOT attempt to look
       up records for .onion names.  They SHOULD generate NXDOMAIN for
       all such queries.

   5.  Authoritative DNS Servers: Authoritative servers SHOULD respond
       to queries for .onion with NXDOMAIN.

   6.  DNS Server Operators: Operators SHOULD 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 the "onion" TLD in the registry of Special-
   Use Domain Names [RFC6761].  See Section 2 for the registration

4.  Security Considerations

   .onion names are often used 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

   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

   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 apparently
   semantically similar to the desired service.

   Also, users need be aware of the difference between a .onion name
   used and accessed directly via Tor-capable software, versus .onion
   subdomains of other TLDs and providers (e.g., the difference between
   example.onion and example.onion.tld).

   The cryptographic label for an .onion name is constructed by hashing
   the public key of the service with SHA1, truncating the output of the
   hash to 80 bits in length and the resulting hash output is
   concatenated with the string ".onion".  As the number of output bits
   in generating the .onion name is less than the full size of the
   corresponding public key, an attacker may also 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

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,

Authors' Addresses

   Jacob Appelbaum
   Tor Project Inc.

   Alec Muffett