[tor-dev] Proposal 208: IPv6 Exits Redux
nickm at torproject.org
Thu Oct 11 03:47:37 UTC 2012
Title: IPv6 Exits Redux
Author: Nick Mathewson
1. Obligatory Motivation Section
[Insert motivations for IPv6 here. Mention IPv4 address exhaustion.
Insert official timeline for official IPv6 adoption here.
Insert general desirability of being able to connect to whatever
address there is here.
Insert profession of firm conviction that eventually there will be
something somebody wants to connect to which requires the ability to
connect to an IPv6 address.]
Proposal 117 has been there since coderman wrote it in 2007, and it's
still mostly right. Rather than replicate it in full, I'll describe
this proposal as a patch to it.
2.1. Exit policies
Rather than specify IPv6 policies in full, we should move (as we have
been moving with IPv4 addresses) to summaries of which IPv6 ports
are generally permitted. So let's allow server descriptors to include
a list of accepted IPv6 ports, using the same format as the "p" line
in microdescriptors, using the "ipv6-policy" keyword.
"ipv6-policy" SP ("accept" / "reject") SP PortList NL
Exits should still, of course, be able to configure more complex
policies, but they should no longer need to tell the whole world
After this ipv6-policy line is validated, it should get copied into a
"p6" line in microdescriptors.
This change breaks the existing exit enclave idea for IPv6, but the
exiting exit enclave implementation never worked right in the first
place. If we can come up with a good way to support it, we can add
that back in.
2.2. Which addresses should we connect to?
One issue that's tripped us up a few times is how to decide whether
we can use IPv6 addresses. You can't use them with SOCKS4 or
SOCKS4a, IIUC. With SOCKS5, there's no way to indicate that you
prefer IPv4 or IPv6. It's possible that some SOCKS5 users won't
understand IPv6 addresses.
With this in mind, I'm going to suggest that with SOCKS4 or SOCKS4a,
clients should always require IPv4. With SOCKS5, clients should
If it proves necessary, we can also add per-SOCKSPort configuration
flags to override the above default behavior.
See also partitioning discussion in Security Notes below.
2.3. Extending BEGIN cells.
Prop117 (and the section above) says that clients should prefer one
address or another, but doesn't give them a means to tell the exit to
do so. Here's one.
We define an extension to the BEGIN cell as follows. After the
ADDRESS | ':' | PORT |  portion, the cell currently contains all
 bytes. We add a 32-bit flags field, stored as an unsigned 32
bit value, after the . All these flags default to 0, obviously.
We define the following flags:
1 -- IPv6 okay. We support learning about IPv6 addresses and
connecting to IPv6 addresses.
2 -- IPv4 not okay. We don't want to learn about IPv4 addresses
or connect to them.
3 -- IPv6 preferred. If there are both IPv4 and IPv6 addresses,
we want to connect to the IPv6 one. (By default, we connect
to the IPv4 address.)
4..32 -- Reserved.
As with so much else, clients should look at the platform version of
the exit they're using to see if it supports these flags before
2.4. Minor changes to proposal 117
GETINFO commands that return an address, and which should return two,
should not in fact begin returning two addresses separated by CRLF.
They should retain their current behavior, and there should be a new
"all my addresses" GETINFO target.
3. Security notes:
Letting clients signal that they want or will accept IPv6 addresses
creates two partitioning issues that didn't exist before. One is the
version partitioning issue: anybody who supports IPv6 addresses is
obviously running the new software. Another is option partitioning:
anybody who is using a SOCKS4a application will look different from
somebody who is using a SOCKS5 application.
We can't do much about version partitioning, I think. If we felt
especially clever, we could have a flag day. Is that necessary?
For option partitioning, are there many applications whose behavior
is indistinguishable except that they are sometimes configured to use
SOCKS4a and sometimes to use SOCKS5? If so, the answer may well be
to persuade as many users as possible to switch those to SOCKS5, so
that they get IPv6 support and have a large anonymity set.
IPv6 addresses are plentiful, which makes caching them dangerous
if you're hoping to avoid tracking over time. (With IPv4 addresses,
it's harder to give every user a different IPv4 address for a target
hostname with a long TTL, and then accept connections to those IPv4
addresses from different exits over time. With IPv6, it's easy.)
This makes proposal 205 especially necessary here.
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