tor-parallel-relay-conn/doc/spec/proposals/117-ipv6-exits.txt

Proposal : IPv6 exit

Overview

   Extend Tor for TCP exit via IPv6 transport and DNS resolution of IPv6
   addresses.  This proposal does not imply any IPv6 support for OR traffic,
   only exit and name resolution.


Contents

0. Motivation

   As the IPv4 address space becomes more scarce there is increasing effort to
   provide Internet services via the IPv6 protocol.  Many hosts are available
   at IPv6 endpoints which are currently inaccessible for Tor users.

   Extending Tor to support IPv6 exit streams and IPv6 DNS name resolution will
   allow users of the Tor network to access these hosts.  This capability would
   be present for those who do not currently have IPv6 access, thus increasing
   the utility of Tor and furthering adoption of IPv6.


1. Design

1.1. General design overview

   There are three main components to this proposal.  The first is a method for
   routers to advertise their ability to exit IPv6 traffic.  The second is the
   manner in which routers resolve names to IPv6 addresses.  Last but not least
   is the method in which clients communicate with Tor to resolve and connect
   to IPv6 endpoints anonymously.

1.2. Router IPv6 exit support

   In order to specify exit policies and IPv6 capability new directives in the
   Tor configuration will be needed.  If a router advertises IPv6 exit policies
   in its descriptor this will signal the ability to provide IPv6 exit.  There
   are a number of additional default deny rules associated with this new
   address space which are detailed in the addendum.

   When Tor is started on a host it should check for the presence of a global
   unicast address, [2000::]/3, and if present include the default IPv6 exit
   policies and any user specified IPv6 exit policies.

   If a user provides IPv6 exit policies but no global unicast address is
   available Tor should generate a warning and not publish the IPv6 policy in
   the router descriptor.

   It should be noted that IPv4 mapped IPv6 addresses are not valid exit
   destinations.  This mechanism is mainly used to interoperate with both IPv4
   and IPv6 clients on the same socket.  Any attempts to use an IPv4 mapped
   IPv6 address, perhaps to circumvent exit policy for IPv4, must be refused.

1.3. DNS name resolution of IPv6 addresses (AAAA records)

   In addition to exit support for IPv6 TCP connections, a method to resolve
   domain names to their respective IPv6 addresses is also needed.  This is
   accomplished in the existing DNS system via AAAA records.  Routers will
   perform both A and AAAA requests when resolving a name so that the client can
   utilize an IPv6 endpoint when available or preferred.

   To avoid potential problems with caching DNS servers that behave poorly all
   NXDOMAIN responses to AAAA requests should be ignored if a successful
   response is received for an A request.  This implies that both AAAA and A
   requests will always be performed for each name resolution.

   For reverse lookups on IPv6 addresses, like that used for RESOLVE_PTR, Tor
   will perform the necessary PTR requests via IP6.ARPA.

   All routers which perform DNS resolution on behalf of clients (RELAY_RESOLVE)
   should perform and respond with both A and AAAA resources.

1.4. Client interaction with IPv6 exit capability

1.4.1. Usability goals

   There are a number of behaviors which Tor can provide when interacting with
   clients that will improve the usability of IPv6 exit capability.  These
   behaviors are designed to make it simple for clients to express a preference
   for IPv6 transport and utilize IPv6 host services.

1.4.2. SOCKSv5 IPv6 client behavior

   The SOCKS version 5 protocol supports IPv6 connections.  When using SOCKSv5
   with hostnames it is difficult to determine if a client wishes to use an IPv4
   or IPv6 address to connect to the desired host if it resolves to both address
   types.

   In order to make this more intuitive the SOCKSv5 protocol can be supported on
   a local IPv6 endpoint, [::1] port 9050 for example.  When a client requests
   a connection to the desired host via an IPv6 SOCKS connection Tor will prefer
   IPv6 addresses when resolving the host name and connecting to the host.

   Likewise, RESOLVE and RESOLVE_PTR requests from an IPv6 SOCKS connection will
   return IPv6 addresses when available, and fall back to IPv4 addresses if not.

1.4.3. MAPADDRESS behavior

   The MAPADDRESS capability supports clients that may not be able to use the
   SOCKSv4a or SOCKSv5 hostname support to resolve names via Tor.  This ability
   should be extended to IPv6 addresses in SOCKSv5 as well.

   When a client requests an address mapping from the wildcard IPv6 address,
   [::0], the server will respond with a unique local IPv6 address on success.
   It is important to note that there may be two mappings for the same name
   if both an IPv4 and IPv6 address are associated with the host.  In this case
   a CONNECT to a mapped IPv6 address should prefer IPv6 for the connection to
   the host, if available, while CONNECT to a mapped IPv4 address will prefer
   IPv4.

   It should be noted that IPv6 does not provide the concept of a host local
   subnet, like 127.0.0.0/8 in IPv4.  For this reason integration of Tor with
   IPv6 clients should consider a firewall or filter rule to drop unique
   local addresses to or from the network when possible.  These packets should
   not be routed, however, keeping them off the subnet entirely is worthwhile.

1.4.3.1. Generating unique local IPv6 addresses

   The usual manner of generating a unique local IPv6 address is to select a
   Global ID part randomly, along with a Subnet ID, and sharing this prefix
   among the communicating parties who each have their own distinct Interface
   ID.  In this style a given Tor instance might select a random Global and
   Subnet ID and provide MAPADDRESS assignments with a random Interface ID as
   needed.  This has the potential to associate unique Global/Subnet identifiers
   with a given Tor instance and may expose attacks against the anonymity of Tor
   users.

   Tor avoid this potential problem entirely MAPADDRESS must always generate the
   Global, Subnet, and Interface IDs randomly for each request.  It is also
   highly suggested that explicitly specifying an IPv6 source address instead of
   the wildcard address not be supported to ensure that a good random address is
   used.

1.4.4. DNSProxy IPv6 client behavior

   A new capability in recent Tor versions is the transparent DNS proxy.  This
   feature will need to return both A and AAAA resource records when responding
   to client name resolution requests.

   The transparent DNS proxy should also support reverse lookups for IPv6
   addresses.  It is suggested that any such requests to the deprecated IP6.INT
   domain should be translated to IP6.ARPA instead.  This translation is not
   likely to be used and is of low priority.

   It would be nice to support DNS over IPv6 transport as well, however, this
   is not likely to be used and is of low priority.

1.4.5. TransPort IPv6 client behavior

   Tor also provides transparent TCP proxy support via the Trans* directives in
   the configuration.  The TransListenAddress directive should accept an IPv6
   address in addition to IPv4 so that IPv6 TCP connections can be transparently
   proxied.

1.5. Additional changes

   The RedirectExit option should be deprecated rather than extending this
   feature to IPv6.


2. Spec changes

2.1. Tor specification

   In '6.2. Opening streams and transferring data' the following should be
   changed to indicate IPv6 exit capability:

      "No version of Tor currently generates the IPv6 format."

   In '6.4. Remote hostname lookup' the following should be updated to reflect
   use of ip6.arpa in addition to in-addr.arpa.

      "For a reverse lookup, the OP sends a RELAY_RESOLVE cell containing an
       in-addr.arpa address."

   In 'A.1. Differences between spec and implementation' the following should
   be updated to indicate IPv6 exit capability:

      "The current codebase has no IPv6 support at all."

2.2. Directory specification

   In '2.1. Router descriptor format' a new set of directives is needed for
   IPv6 exit policy.  The existing accept/reject directives should be
   clarified to indicate IPv4 or wildcard address relevance.  The new IPv6
   directives will be in the form of:

      "accept6" exitpattern NL
      "reject6" exitpattern NL

   The section describing accept6/reject6 should explain that the presence
   of accept6 or reject6 exit policies in a router descriptor signals the
   ability of that router to exit IPv6 traffic (according to IPv6 exit
   policies).

   The "[::]/0" notation is used to represent "all IPv6 addresses".  "[::0]/0"
   may also be used for this representation.

   If a user specifies a 'reject6 [::]/0:*' policy in the Tor configuration this
   will be interpreted as forcing no IPv6 exit support and no accept6/reject6
   policies will be included in the published descriptor.  This will prevent
   IPv6 exit if the router host has a global unicast IPv6 address present.

   It is important to note that a wildcard address in an accept or reject policy
   applies to both IPv4 and IPv6 addresses.

2.3. Control specification

   In '3.8. MAPADDRESS' the potential to have to addresses for a given name
   should be explained.  The method for generating unique local addresses
   for IPv6 mappings needs explanation as described above.

   When IPv6 addresses are used in this document they should include the
   brackets for consistency.  For example, the null IPv6 address should be
   written as "[::0]" and not "::0".  The control commands will expect the
   same syntax as well.

   In '3.9. GETINFO' the "address" command should return both public IPv4 and
   IPv6 addresses if present.  These addresses should be separated via \r\n.


2.4. Tor SOCKS extensions

   In '2. Name lookup' a description of IPv6 address resolution is needed for
   SOCKSv5 as described above.  IPv6 addresses should be supported in both the
   RESOLVE and RESOLVE_PTR extensions.

   A new section describing the ability to accept SOCKSv5 clients on a local
   IPv6 address to indicate a preference for IPv6 transport as described above
   is also needed.  The behavior of Tor SOCKSv5 proxy with an IPv6 preference
   should be explained, for example, preferring IPv6 transport to a named host
   with both IPv4 and IPv6 addresses available (A and AAAA records).


3. Questions and concerns

3.1. DNS A6 records

   A6 is explicitly avoided in this document.  There are potential reasons for
   implementing this, however, the inherent complexity of the protocol and
   resolvers make this unappealing.  Is there a compelling reason to consider
   A6 as part of IPv6 exit support?

3.2. IPv4 and IPv6 preference

   The design above tries to infer a preference for IPv4 or IPv6 transport
   based on client interactions with Tor.  It might be useful to provide
   more explicit control over this preference.  For example, an IPv4 SOCKSv5
   client may want to use IPv6 transport to named hosts in CONNECT requests
   while the current implementation would assume an IPv4 preference.  Should
   more explicit control be available, through either configuration directives
   or control commands?

   This can be worked around by resolving names and then CONNECTing to an IPv4
   or IPv6 address as desired, however, not all client applications may have
   this option available.

3.3. Support for IPv6 only clients

   It may be useful to support IPv6 only clients using IPv4 mapped IPv6
   addresses.  This would require transparent DNS proxy using IPv6 
   transport and the ability to map A record responses into IPv4 mapped
   IPv6 addresses.  The transparent TCP proxy would thus need to detect these
   mapped addresses and connect to the desired IPv4 host.

   The relative lack of any IPv6 only hosts or applications makes this a lot of
   work for very little gain.  Is there a compelling reason to support this
   capability?

3.4. IPv6 DNS and older Tor routers

   It is expected that many routers will continue to run with older versions of
   Tor when the IPv6 exit capability is released.  Clients who wish to use IPv6
   will need to route RELAY_RESOLVE requests to the newer routers which will
   respond with both A and AAAA resource records when possible.

   One way to do this is to route RELAY_RESOLVE requests to routers with IPv6
   exit policies published, however, this would not utilize current routers
   that can resolve IPv6 addresses even if they can't exit such traffic.


4. Addendum

4.1. Sample IPv6 default exit policy

   reject 0.0.0.0/8
   reject 169.254.0.0/16
   reject 127.0.0.0/8
   reject 192.168.0.0/16
   reject 10.0.0.0/8
   reject 172.16.0.0/12
   reject6 [0000::]/8
   reject6 [0100::]/8
   reject6 [0200::]/7
   reject6 [0400::]/6
   reject6 [0800::]/5
   reject6 [1000::]/4
   reject6 [4000::]/3
   reject6 [6000::]/3
   reject6 [8000::]/3
   reject6 [A000::]/3
   reject6 [C000::]/3
   reject6 [E000::]/4
   reject6 [F000::]/5
   reject6 [F800::]/6
   reject6 [FC00::]/7
   reject6 [FE00::]/9
   reject6 [FE80::]/10
   reject6 [FEC0::]/10
   reject6 [FF00::]/8
   reject *:25
   reject *:119
   reject *:135-139
   reject *:445
   reject *:1214
   reject *:4661-4666
   reject *:6346-6429
   reject *:6699
   reject *:6881-6999
   accept *:*
   # accept6 [2000::]/3:* is implied

4.2. Additional resources

   'DNS Extensions to Support IP Version 6'
   http://www.ietf.org/rfc/rfc3596.txt

   'DNS Extensions to Support IPv6 Address Aggregation and Renumbering'
   http://www.ietf.org/rfc/rfc2874.txt

   'SOCKS Protocol Version 5'
   http://www.ietf.org/rfc/rfc1928.txt

   'Unique Local IPv6 Unicast Addresses'
   http://www.ietf.org/rfc/rfc4193.txt

   'INTERNET PROTOCOL VERSION 6 ADDRESS SPACE'
   http://www.iana.org/assignments/ipv6-address-space