Filename: 117-ipv6-exits.txt
Title: IPv6 exits
Author: coderman
Created: 10-Jul-2007
Status: Accepted
Target: 0.2.1.x

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 IPv6 address 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 IPv6
   address is available Tor should generate a warning and not publish the
   IPv6 policies 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.

   [NOTE: In a future version, when we extend the behavior of RESOLVE to
    encapsulate more of real DNS, it will make sense to allow more
    flexibility here. -nickm]

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.

   [NOTE: This means that SocksListenAddress and DNSListenAddress should
    support IPv6 addresses.  Perhaps there should also be a general option
    to have listeners that default to 127.0.0.1 and 0.0.0.0 listen
    additionally or instead on ::1 and :: -nickm]

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."

   [NOTE: the EXITPOLICY end-cell reason says that it can hold an ipv4 or an
    ipv6 address, but doesn't say how.  We may want a separate EXITPOLICY2
    type that can hold an ipv6 address, since the way we encode ipv6
    addresses elsewhere ("0.0.0.0 indicates that the next 16 bytes are ipv6")
    is a bit dumb. -nickm]
   [Actually, the length field lets us distinguish EXITPOLICY. -nickm]

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?

   [IMO not till anybody needs it. -nickm]

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?

   Many applications support a inet6-only or prefer-family type option
   that provides the user manual control over address preference.  This
   could be provided as a Tor configuration option.

   An explicit preference is still possible 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 transparent proxy clients

   It may be useful to support IPv6 only transparent proxy clients using
   IPv4 mapped IPv6 like addresses.  This would require transparent DNS
   proxy using IPv6 transport and the ability to map A record responses
   into IPv4 mapped IPv6 like addresses in the manner described in the
   "NAT-PT" RFC for a traditional Basic-NAT-PT with DNS-ALG.  The
   transparent TCP proxy would thus need to detect these mapped addresses
   and connect to the desired IPv4 host.

   The IPv6 prefix used for this purpose must not be the actual IPv4
   mapped IPv6 address prefix, though the manner in which IPv4 addresses
   are embedded in IPv6 addresses would be the same.

   The lack of any IPv6 only hosts which would use this transparent proxy
   method makes this a lot of work for very little gain.  Is there a
   compelling reason to support this NAT-PT like 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.

   There was also concern expressed about the ability of existing clients
   to cope with new RELAY_RESOLVE responses that contain IPv6 addresses.
   If this breaks backward compatibility, a new request type may be
   necessary, like RELAY_RESOLVE6, or some other mechanism of indicating
   the ability to parse IPv6 responses when making the request.

3.5. IPv4 and IPv6 bindings in MAPADDRESS

   It may be troublesome to try and support two distinct address mappings
   for the same name in the existing MAPADDRESS implementation.  If this
   cannot be accommodated then the behavior should replace existing
   mappings with the new address regardless of family.  A warning when
   this occurs would be useful to assist clients who encounter problems
   when both an IPv4 and IPv6 application are using MAPADDRESS for the
   same names concurrently, causing lost connections for one of them.

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

   'Network Address Translation - Protocol Translation (NAT-PT)'
   http://www.ietf.org/rfc/rfc2766.txt
