IPv6 exit proposal
coderman at gmail.com
Sat Jul 7 17:54:50 UTC 2007
apologies for formatting; available at
http://peertech.org/pub/tor-ipv6-exit-proposal.txt if this is
Proposal : IPv6 exit
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.
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.
1.1. 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
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
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.
184.108.40.206. 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
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
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
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 '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
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
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.
4.1. Sample IPv6 default exit policy
# accept6 [2000::]/3:* is implied
4.2. Additional resources
'DNS Extensions to Support IP Version 6'
'DNS Extensions to Support IPv6 Address Aggregation and Renumbering'
'SOCKS Protocol Version 5'
'Unique Local IPv6 Unicast Addresses'
'INTERNET PROTOCOL VERSION 6 ADDRESS SPACE'
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