Internet DRAFT - draft-grothoff-iesg-special-use-p2p-exit
draft-grothoff-iesg-special-use-p2p-exit
Internet Engineering Task Force C. Grothoff
Internet-Draft INRIA
Intended status: Informational M. Wachs
Expires: December 24, 2015 Technische Universitaet Muenchen
H. Wolf, Ed.
GNU consensus
J. Appelbaum
L. Ryge
Tor Project Inc.
June 30, 2015
The .exit Special-Use Domain Name of Tor
draft-grothoff-iesg-special-use-p2p-exit-00
Abstract
This document registers a Special-Use Domain Name for use with the
Tor Project, as per RFC6761.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 24, 2015.
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
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Terminology and Conventions Used in This Document . . . . . . 3
4. The "EXIT" Client Source Routing pTLD . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The Domain Name System (DNS) is primarily used to map human-memorable
names to IP addresses, which are used for routing but generally not
meaningful for humans.
The Tor project supports the use of names to specify where the user
wishes to exit the P2P overlay.
As compatibility with applications using domain names is desired,
this mechanism requires an exclusive alternative Top-Level Domains to
avoid conflict between the Tor namespace and the DNS hierarchy.
In order to avoid interoperability issues with DNS as well as to
address security and privacy concerns, this document registers the
"EXIT" Special-Use Domain Names for use within the Tor network, as
per [RFC6761].
The Tor network uses this pTLD to control overlay routing and to
securely specify path selection choices [TOR-PATH].
2. Applicability
[RFC6761] Section 3 states:
"[I]f a domain name has special properties that affect the way
hardware and software implementations handle the name, that apply
universally regardless of what network the implementation may be
connected to, then that domain name may be a candidate for having
the IETF declare it to be a Special-Use Domain Name and specify
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what special treatment implementations should give to that name.
On the other hand, if declaring a given name to be special would
result in no change to any implementations, then that suggests
that the name may not be special in any material way, and it may
be more appropriate to use the existing DNS mechanisms [RFC1034]
to provide the desired delegation, data, or lack-of-data, for the
name in question. Where the desired behaviour can be achieved via
the existing domain name registration processes, that process
should be used. Reservation of a Special-Use Domain Name is not a
mechanism for circumventing normal domain name registration
processes."
The set "EXIT" pTLD reserved by this document meets this requirement,
as it has the following specificities:
o "EXIT" resolution does not depend on the DNS context: The name
specifies a Tor exit node, and thus the response is not even
really DNS-compatible; Tor uses its own P2P protocols for
resolving the destination specified in an .exit name.
o When Tor is properly implemented, the implementation MUST
intercept queries for the "EXIT" to ensure that these Tor-specific
names cannot leak into the DNS.
o Finally, in order for Tor to properly interoperate with DNS and to
provide security and privacy features matching user expectations,
this document specifies desirable changes in existing DNS software
and DNS operations.
3. Terminology and Conventions Used in This Document
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 [RFC2119].
The word "peer" is used in the meaning of a individual system on the
network.
The abbreviation "pTLD" is used in this document to mean a pseudo
Top-Level Domain, i.e., a Special-Use Domain Name per [RFC6761]
reserved to P2P Systems in this document. A pTLD is mentioned in
capitals, and within double quotes to mark the difference with a
regular DNS gTLD.
In this document, ".tld" (lowercase, with quotes) means: any domain
or hostname within the scope of a given pTLD, while .tld (lowercase,
without quotes) refers to an adjective form.
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The word "NXDOMAIN" refers to an alternate expression for the "Name
Error" RCODE as described in section 4.1.1 of [RFC1035]. When
referring to "NXDOMAIN" and negative caching [RFC2308] response, this
document means an authoritative (AA=1) name error (RCODE=3) response
exclusively.
The Tor-related names such as 'circuit', 'exit', 'node', 'relay',
'stream', and related Tor terms are described in [Dingledine2004] and
the Tor protocol specification [TOR-PROTOCOL].
4. The "EXIT" Client Source Routing pTLD
The .exit suffix is used as an in-band source routing control
channel, usually for selection of a specific Tor relay during path
creation as the last node in the Tor circuit.
It may be used to access a DNS host via specific Torservers, in the
form "hostname.nickname-or-fingerprint.exit", where the "hostname" is
a valid hostname, and the "nickname-or-fingerprint" is either the
nickname of a Tor relay in the Tor network consensus, or the hex-
encoded SHA1 digest of the given node's public key (fingerprint).
For example, "gnu.org.noisetor.exit" will route the client to
"gnu.org" via the Tor node nicknamed "noisetor". Using the
fingerprint instead of the nickname ensures that the path selection
uses a specific Tor exit node, and is harder to remember: e.g.,
"gnu.org.f97f3b153fed6604230cd497a3d1e9815b007637.exit".
When Tor sees an address in this format, it uses the specified
"nickname-or-fingerprint" as the exit node. If no "hostname"
component is given, Tor defaults to the published IPv4 address of the
Tor exit node [TOR-EXTSOCKS].
Because "hostname" is allegedly valid, the total length of a .exit
construct may exceed the maximum length allowed for domain names.
Moreover, the resolution of "hostname" happens at the exit node.
Trying to resolve such invalid domain names, including chaining .exit
names will likely return a DNS lookup failure at the first exit node.
The "EXIT" domain is special in the following ways:
1. Users can use these names as they would other domain names,
entering them anywhere that they would otherwise enter a
conventional DNS domain name.
Since .exit names correspond to a Tor-specific routing construct
to reach target hosts via chosen Tor exit nodes, users need to be
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aware that they do not belong to regular DNS and that the actual
target precedes the second-level domain name.
2. Application software MAY recognize that .exit domains are special
and when they do SHOULD NOT pass requests for these domains to
DNS resolvers and libraries.
As mentioned in items 4 and 5 below, regular DNS resolution is
expected to respond with NXDOMAIN. Therefore, if it can
differentiate between DNS and P2P name resolution, application
software:
* MUST expect NXDOMAIN as the only valid DNS response, and
* SHOULD treat other answers from DNS as errors.
Tor-aware applications MAY also use Tor resolvers directly.
3. Name resolution APIs and libraries SHOULD either respond to
requests for .exit names by resolving them via the Tor protocol,
or respond with NXDOMAIN.
4. Caching DNS servers SHOULD recognize .exit names as special and
SHOULD NOT, by default, attempt to look up NS records for them,
or otherwise query authoritative DNS servers in an attempt to
resolve .exit names. Instead, caching DNS servers SHOULD, by
default, generate immediate negative responses for all such
queries.
5. Authoritative DNS servers are not expected to treat .exit domain
requests specially. In practice, they MUST answer with NXDOMAIN,
as "EXIT" is not available via global DNS resolution, and not
doing so MAY put users' privacy at risk (see item 6).
6. DNS server operators SHOULD be aware that .exit names are
reserved for use with Tor, and MUST NOT override their resolution
(e.g., to redirect users to another service or error
information).
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7. DNS registries/registrars MUST NOT grant any request to register
.exit names. This helps avoid conflicts [SAC45]. These names
are defined by the Tor address specification, and they fall
outside the set of names available for allocation by registries/
registrars.
5. Security Considerations
Specific software performs the resolution of the six Special-Use
Domain Names presented in this document; this resolution process
happens outside of the scope of DNS. Leakage of requests to such
domains to the global operational DNS can cause interception of
traffic that might be misused to monitor, censor, or abuse the user's
trust, and lead to privacy issues with potentially tragic
consequences for the user.
This document reserves these Top-Level Domain names to minimize the
possibility of confusion, conflict, and especially privacy risks for
users.
In the introduction of this document, there's a requirement that DNS
operators do not override resolution of the "EXIT" Names. This is a
regulatory measure and cannot prevent such malicious abuse in
practice. Its purpose is to limit any information leak that would
result from incorrectly configured systems, and to avoid that
resolvers make unnecessary contact to the DNS Root Zone for such
domains. Verisign, Inc., as well as several Internet service
providers (ISPs) have notoriously abused their position to override
NXDOMAIN responses to their customers in the past
[SSAC-NXDOMAIN-Abuse]. For example, if a DNS operator would decide
to override NXDOMAIN and send advertising to leaked .onion sites, the
information leak to the DNS would extend to the advertising server,
with unpredictable consequences. Thus, implementors should be aware
that any positive response coming from DNS must be considered with
extra care, as it suggests a leak to DNS has been made, contrary to
user's privacy expectations.
The reality of X.509 Certificate Authorities (CAs) creating
misleading certificates for these pTLDs due to ignorance stresses the
need to document their special use. Certificate Authorities MUST NOT
create certificates for "EXIT" Top-level domains. Nevertheless,
clients SHOULD accept certificates for these Top-Level domains as
they may be created legitimately by local proxies on the fly.
Finally, legacy applications that do not explicitly support the pTLD
significantly increase the risk of pTLD queries escaping to DNS, as
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they are entirely dependent on the correct configuration on the
operating system.
6. IANA Considerations
The Internet Assigned Numbers Authority (IANA) reserved the following
entries in the Special-Use Domain Names registry [RFC6761]:
.exit
[TO REMOVE: the assignement URL is https://www.iana.org/assignments/
special-use-domain-names/ ]
7. Acknowledgements
The authors thank the I2P and Namecoin developers for their
constructive feedback, as well as Mark Nottingham for his proof-
reading and valuable feedback. The authors also thank the members of
DNSOP WG for their critiques and suggestions.
8. References
8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, February 2013.
8.2. Informative References
[Dingledine2004]
Dingledine, R., Mathewson, N., and P. Syverson, "Tor: the
second-generation onion router", 2004, <https://www.onion-
router.net/Publications/tor-design.pdf>.
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[SAC45] ICANN Security and Stability Advisory Committee, "Invalid
Top Level Domain Queries at the Root Level of the Domain
Name System", November 2010,
<http://www.icann.org/en/groups/ssac/documents/
sac-045-en.pdf>.
[SSAC-NXDOMAIN-Abuse]
ICANN Security and Stability Advisory Committee,
"Redirection in the COM and NET Domains", July 2004,
<http://www.icann.org/committees/security/
ssac-report-09jul04.pdf>.
[TOR-EXTSOCKS]
Mathewson, N. and R. Dingledine, "Tor's extensions to the
SOCKS protocol", February 2014,
<https://gitweb.torproject.org/torspec.git/plain/socks-
extensions.txt>.
[TOR-PATH]
Mathewson, N. and R. Dingledine, "Tor Path Specification",
November 2014,
<https://gitweb.torproject.org/torspec.git/plain/path-
spec.txt>.
[TOR-PROTOCOL]
Dingledine, R. and N. Mathewson, "Tor Protocol
Specification", August 2014,
<https://gitweb.torproject.org/torspec.git/plain/tor-
spec.txt>.
Authors' Addresses
Christian Grothoff
INRIA
Equipe Decentralisee
INRIA Rennes Bretagne Atlantique
263 avenue du General Leclerc
Campus Universitaire de Beaulieu
Rennes, Bretagne F-35042
FR
Email: christian@grothoff.org
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Matthias Wachs
Technische Universitaet Muenchen
Free Secure Network Systems Group
Lehrstuhl fuer Netzarchitekturen und Netzdienste
Boltzmannstrasse 3
Technische Universitaet Muenchen
Garching bei Muenchen, Bayern D-85748
DE
Email: wachs@net.in.tum.de
Hellekin O. Wolf (editor)
GNU consensus
Email: hellekin@gnu.org
Jacob Appelbaum
Tor Project Inc.
Email: jacob@appelbaum.net
Leif Ryge
Tor Project Inc.
Email: leif@synthesize.us
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