Filename: 117-ipv6-exits.txt
Title: IPv6 exits
Author: coderman
Created: 10-Jul-2007
Status: Closed
Target: 0.2.4.x
Implemented-In: 0.2.4.7-alpha
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.
To 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