Re: [tor-dev] Pluggable Transports 2.0 Specification, Draft 2

Hi teor. Thank you for the detailed feedback on the Pluggable Transport 2.0, Draft 2 proposal.

Your email covers a number of topics, which I will attempt to address. I have grouped the issues you've raised into some different categories.

1. Issues inherited from the PT 1.0 specification - We tried not to make many large changes from PT 1.0 in order to preserve as much compatibility as possible. We would be happy to discuss making additional changes in the upcoming PT 2.1 specification process.

2. Feature requests to help serve the needs of Tor - Tor folks were certainly involved in the PT 2.0 specification process. Roger, Nick, and Yawning attended the meetings. However, I can see that there are some feature requests from you and Yawning that did not make it into the PT 2.0 specification. We would love to see these discussed as part of the PT 2.1 specification. I have already made a note to include them on the agenda for the Pluggable Transport Implementers Meeting, which is happening after TorDev. If you would like to participate in the specification process, that would be very helpful and ensure that these feature requests make it into the proposal and are specified in a way that meets your needs for Tor.

3. Requests for clarification - These have generally been incorporated into the latest draft, PT 2.0 draft 3. I will be sending this out shortly. I hope that you will find that the new draft addresses many of the areas that you found unclear in the previous draft.

4. Clarification of issues outside of the scope of the specification document - I will attempt to answer these below:

How will Tor implement the relevant interfaces?

These is discussed in this Trac issue from the last TorDev: https://trac.torproject.org/projects/tor/ticket/21816

The code changes to Tor are minimal. The authentication method uses to send per-connection arguments to transports has been changed in PT 2.0 to use a custom SOCKS authentication method instead of using the username and password authentication method. No other changes are required to Tor to use PT 2.0 transport providers. We have provided a patch to Tor to implement this functionality in a backwards-incompatible way, so both PT 1.0 and PT 2.0 providers can be used with the patch. We have also implemented a PT 2.0-compatible transport provider, which is available here:

https://github.com/OperatorFoundation/shapeshifter-dispatcher/

Can you please map each environmental variable to the Go interface?

There is not a one-to-one mapping between the environment variables and the Go interface. There are two parts to the PT implementation. There is the proxy and there are the transports. The environment variables are for configuring the proxy, which includes the information necessary to configure the transports. The Go interface is for using the transports directly in your code, without a proxy process. Therefore, only a subset of the information specified for the environment variables is required when using the Go interface, and so a one-to-one mapping is not possible.

What is the UDP equivalent to PT Client Per-Connection Arguments?

There is no equivalent to PT Client Per-Connection Arguments for UDP. PT 1.0 supported only one proxy mode, SOCKS5. PT 2.0 supports multiple proxy modes, but only the SOCKS5 proxy mode supports PT Client Per-Connection Arguments. This is because it is the SOCKS5 protocol that provides the channel to supply PT Client Per-Connection Arguments (through the custom authentication method). You may be wondering, how do you use a transport that requires PT Client Per-Connection Arguments in a proxy mode other than SOCKS5 (for instance, UDP). You can set them when you start the PT client using the -options command line flag. Of course, in this scenario, they will not be "per connection" as they will be the same for every connection. However, this should work fine with the current PTs. If you truly need per-connection arguments, then you are limited to SOCKS5.

I hope that I have answered some of your questions. I will be at TorDev all week, so I would love to discuss some of the finer details with you at your convenience.

Also, I wanted to clear up one thing that was said later in this thread. The Go transports API is not based on the obfs4proxy internal API. The very first draft was based on the obfs4proxy code and everyone agreed that this was not ideal. The current Go API specified in PT 2.0 draft 3 is based on the Go net.Conn API. following the general principle set forth in the specification document to make the transport library look as much as possible like the native socket implementation for the language.

On Tue, Jun 20, 2017 at 10:16 PM, teor <teor2345@xxxxxxxxx> wrote:

> On 21 Jun 2017, at 04:07, Brandon Wiley <brandon@xxxxxxxxx> wrote:
>
> If you have feedback on this draft, please send me your comments by July 20.

Thanks for putting this together.
It looks like it's taken a lot of work to do this and the implementation.

I seem to be missing some context:
What is the goal of writing this specification?

I'm going to assume you want to implement parts in tor and parts in Go.

Here is my feedback:

In general, have you searched tor trac for PT protocol bugs, and made
sure this spec doesn't suffer from the same issues?

In general, is there a separate document or proposal that describes
how Tor will implement the relevant interfaces? There doesn't seem
to be much on Tor-specific issues in this spec.

There is one "Tor" note in the spec, maybe it should be in that
separate document? Or maybe there should be more Tor notes in the
spec?

3.1. Pluggable Transport Naming

How are unique names coordinated?

When is a PT a different version of an older transport with the same
name? When does a PT need a new name?

3.2.1. Goals for interface design

Definitions:

The destination address (and many other terms) aren't defined
anywhere. Are they IPv4, IPv6, DNS, or protocol-specific?

If they are not just IPv4 addresses, please give examples with
other address types.

Maybe section 3.3.5.1 defines some of these terms?

3.3.1 Pluggable Transport Configuration Parameters

Optional Addresses:

Since the address is optional, what value is used when it is not
required by the transport?

It looks mandatory in the Go interface, but isn't mandatory in the
environmental variables. Can you please map each environmental
variable to the Go interface?

I ask because we have had some issues in Tor with PT 1.0,
because Tor Browser uses a fake IPv4 address for transports like meek.
This interacts really badly with ReachableAddresses and similar.

Any new Tor code will need to resolve this issue by using non-address
identifiers or a defined placeholder address.

We have also had bugs where tor connects to the actual bridge address
rather than a proxy. So using a placeholder address for all PTs might
be a good idea for security in tor.

Multiple Addresses:

If there are multiple addresses, are these separate instances of the
transport, or can one transport have multiple connections?
Does this differ between the client and the server?

For example, the same bridge can have an IPv4 and IPv6 address.
Or two different bridges can use the same PT.

How is this handled in tor and in Go?
(It's specified that each PT has zero or one addresses, but there
isn't anything explicit about using multiple addresses.)

Banned Addresses:

Tor users often configure ReachableAddresses (or similar) and expect
pluggable transports to respect them. Is there a standard way of
telling a transport which addresses it can't connect to?

Or, alternately, is there a standard way for a transport to tell tor
which addresses it is actually using *before* it connects to it?

Client Addresses:

The client protocol is missing a standard way to configure:
* a local bind address
* a remote server address
* other common PT info

Is this intentional?
If so, do we really want each transport defining its own slightly
different JSON keys for common items like addresses?
Even worse, what if they format addresses inconsistently?

This will be difficult to implement in applications if it is not
standardised.

I suggest we make the server address an environmental variable.

3.3.1.4 Command Line Flags

How is an environmental variable name turned into a command-line flag?
Or are the command-line flags different for each transport?
(Let's not do that, it would be annoying.)

You give examples, but some have typos:
obfs4proxy -state =/var/lib/tor/pt_state/
obfs4proxy - transports obfs3,scramblesuit
Obfs4proxy -options scramblesuit:key=banana;automata:rule=110;automata:depth=3

3.3.2. Pluggable Transport To Parent Process Communication

What is the correct IPv6 address quoting for CMETHOD and SMETHOD?
Please give an example in the text.

"Equal signs and commas MUST be escaped with a backslash."

This is unclear: equals signs and commas in Key and Value?
What about colons or backslashes?
(Otherwise, it is impossible to end a value with a backslash.)
Must Key be an identifier? What's the format?

Why not just use an existing escaping scheme?

"Tor: The ARGS are included in the transport line of the Bridge's extra-info document."

Really? This seems insecure. Do we publish bridge extra-infos anywhere?

3.3.5 UDP Support

The TOC breaks here.

3.3.5.1 Obfuscating Proxy Architecture

This section belongs at the top of the document.

3.3.5.2. Configuring the Transports

There are no details for how this works over UDP: what is the UDP
equivalent to Pluggable PT Client Per-Connection Arguments, and do
TCP implementations have to support that environmental variable as
well?

I suggest we standardise it as an environmental variable and
command-line flag, and make all transports support it.

3.3.5.5. Implementation of the PT Server

Is the text in this section two different sizes?

T
--
Tim Wilson-Brown (teor)

teor2345 at gmail dot com
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