[tor-dev] (revised) Proposal 180: Pluggable Transports for Circumvention

[Nick Mathewson]

We sent out an earlier version of this, well, earlier.  Here it is
more complete and more revised.

I've tried to incorporate people's comments.  There's some worthwhile
stuff it doesn't do yet, like provide automatic and easy ways to
compose plugins.  I think it should work well enough, but more comment
is always worthwhile.

yrs, -- Nick

=====
Filename: 180-pluggable-transport.txt
Title: Pluggable transports for circumvention
Author: Jacob Appelbaum, Nick Mathewson
Created: 15-Oct-2010
Status: Open

Overview

  This proposal describes a way to decouple protocol-level obfuscation
  from the core Tor protocol in order to better resist client-bridge
  censorship.  Our approach is to specify a means to add pluggable
  transport implementations to Tor clients and bridges so that they can
  negotiate a superencipherment for the Tor protocol.

Scope

  This is a document about transport plugins; it does not cover
  discovery improvements, or bridgedb improvements.  While these
  requirements might be solved by a program that also functions as a
  transport plugin, this proposal only covers the requirements and
  operation of transport plugins.

Motivation

  Frequently, people want to try a novel circumvention method to help
  users connect to Tor bridges.  Some of these methods are already
  pretty easy to deploy: if the user knows an unblocked VPN or open
  SOCKS proxy, they can just use that with the Tor client today.

  Less easy to deploy are methods that require participation by both the
  client and the bridge.  In order of increasing sophistication, we
  might want to support:

  1. A protocol obfuscation tool that transforms the output of a TLS
     connection into something that looks like HTTP as it leaves the
     client, and back to TLS as it arrives at the bridge.
  2. An additional authentication step that a client would need to
     perform for a given bridge before being allowed to connect.
  3. An information passing system that uses a side-channel in some
     existing protocol to convey traffic between a client and a bridge
     without the two of them ever communicating directly.
  4. A set of clients to tunnel client->bridge traffic over an existing
     large p2p network, such that the bridge is known by an identifier
     in that network rather than by an IP address.

  We could in theory support these almost fine with Tor as it stands
  today: every Tor client can take a SOCKS proxy to use for its outgoing
  traffic, so a suitable client proxy could handle the client's traffic
  and connections on its behalf, while a corresponding program on the
  bridge side could handle the bridge's side of the protocol
  transformation.  Nevertheless, there are some reasons to add support
  for transportation plugins to Tor itself:

  1. It would be good for bridges to have a standard way to advertise
     which transports they support, so that clients can have multiple
     local transport proxies, and automatically use the right one for
     the right bridge.

  2. There are some changes to our architecture that we'll need for a
     system like this to work.  For testing purposes, if a bridge blocks
     off its regular ORPort and instead has an obfuscated ORPort, the
     bridge authority has no way to test it.  Also, unless the bridge
     has some way to tell that the bridge-side proxy at 127.0.0.1 is not
     the origin of all the connections it is relaying, it might decide
     that there are too many connections from 127.0.0.1, and start
     paring them down to avoid a DoS.

  3. Censorship and anticensorship techniques often evolve faster than
     the typical Tor release cycle.  As such, it's a good idea to
     provide ways to test out new anticensorship mechanisms on a more
     rapid basis.

  4. Transport obfuscation is a relatively distinct problem
     from the other privacy problems that Tor tries to solve, and it
     requires a fairly distinct skill-set from hacking the rest of Tor.
     By decoupling transport obfuscation from the Tor core, we hope to
     encourage people working on transport obfuscation who would
     otherwise not be interested in hacking Tor.

  5. Finally, we hope that defining a generic transport obfuscation plugin
     mechanism will be useful to other anticensorship projects.

Non-Goals

  We're not going to talk about automatic verification of plugin
  correctness and safety via sandboxing, proof-carrying code, or
  whatever.

  We need to do more with discovery and distribution, but that's not
  what this proposal is about.  We're pretty convinced that the problems
  are sufficiently orthogonal that we should be fine so long as we don't
  preclude a single program from implementing both transport and
  discovery extensions.

  This proposal is not about what transport plugins are the best ones
  for people to write.  We do, however, make some general
  recommendations for plugin authors in an appendix.

  We've considered issues involved with completely replacing Tor's TLS
  with another encryption layer, rather than layering it inside the
  obfuscation layer.  We describe how to do this in an appendix to the
  current proposal, though we are not currently sure whether it's a good
  idea to implement.

  We deliberately reject any design that would involve linking more code
  into Tor's process space.

Design overview

  To write a new transport protocol, an implementer must provide two
  pieces: a "Client Proxy" to run at the initiator side, and a "Server
  Proxy" to run at the server side.  These two pieces may or may not be
  implemented by the same program.

  Each client may run any number of Client Proxies.  Each one acts like
  a SOCKS proxy that accepts connections on localhost.  Each one
  runs on a different port, and implements one or more transport
  methods.  If the protocol has any parameters, they are passed from Tor
  inside the regular username/password parts of the SOCKS protocol.

  Bridges (and maybe relays) may run any number of Server Proxies: these
  programs provide an interface like stunnel: they get connections from the
  network (typically by listening for connections on the network) and relay
  them to the Bridge's real ORPort.

  To configure one of these programs, it should be sufficient simply to
  list it in your torrc.  The program tells Tor which transports it
  provides.  The Tor consensus should carry a new approved version number that
  is specific for pluggable transport; this will allow Tor to know when a
  particular transport is known to be unsafe safe or non-functional.

  Bridges (and maybe relays) report in their descriptors which transport
  protocols they support.  This information can be copied into bridge
  lines.  Bridges using a transport protocol may have multiple bridge
  lines.

  Any methods that are wildly successful, we can bake into Tor.

Specifications: Client behavior

  We extend the bridge line format to allow you to say which method
  to use to connect to a bridge.

  The new format is:
     "bridge method address:port [[keyid=]id-fingerprint] [k=v] [k=v] [k=v]"

  To connect to such a bridge, the Tor program needs to know which
  local SOCKS proxy will support the transport called "method".  It
  then connects to this proxy, and asks it to connect to
  address:port.  If [id-fingerprint] is provided, Tor should expect
  the public identity key on the TLS connection to match the digest
  provided in [id-fingerprint].  If any [k=v] items are provided,
  they are configuration parameters for the proxy: Tor should
  separate them with semicolons and put them in the user and
  password fields of the request, splitting them across the fields
  as necessary.  If a key or value value must contain a semicolon or
  a backslash, it is escaped with a backslash.

  The "id-fingerprint" field is always provided in a field named
  "keyid", if it was given.  Method names must be C identifiers.

  Example: if the bridge line is "bridge trebuchet www.example.com:3333
     rocks=20 height=5.6m" AND if the Tor client knows that the
     'trebuchet' method is provided by a SOCKS5 proxy on
     127.0.0.1:19999, the client should connect to that proxy, ask it to
     connect to www.example.com, and provide the string
     "rocks=20;height=5.6m" as the username, the password, or split
     across the username and password.

  There are two ways to tell Tor clients about protocol proxies:
  external proxies and managed proxies.  An external proxy is configured
  with
     ClientTransportPlugin method socks4 address:port [auth=X]
  or
     ClientTransportPlugin method socks5 address:port [username=X] [password=Y]
  as in
     "ClientTransportPlugin trebuchet socks5 127.0.0.1:9999".
  This example tells Tor that another program is already running to handle
  'trubuchet' connections, and Tor doesn't need to worry about it.

  A managed proxy is configured with
     ClientTransportPlugin <method> exec <path> [options]
  as in
    "ClientTransportPlugin trebuchet exec /usr/libexec/trebuchet --managed"
  This example tells Tor to launch an external program to provide a
  socks proxy for 'trebuchet' connections. The Tor client only
  launches one instance of each external program with a given set of
  options, even if the same executable and options are listed for
  more than one method.

  If instead of a transport name, the torrc lists "*" for a managed proxy,
  tor uses that proxy for all transports that it supports.  So
  "ClientTransportPlugin * exec /usr/libexec/tor/foobar" tells Tor
  that it should use the foobar plugin for everything that it supports.

  If two proxies support the same method, Tor should use whichever
  one is listed first.

  The same program can implement a managed or an external proxy: it just
  needs to take an argument saying which one to be.

  See "Managed proxy behavior" for more information on the managed
  proxy interface.

Server behavior

  Server proxies are configured similarly to client proxies.  When
  launching a proxy, the server must tell it what ORPort it has
  configured, and what address (if any) it can listen on.  The
  server must tell the proxy which (if any) methods it should
  provide if it can; the proxy needs to tell the server which
  methods it is actually providing, and on what ports.

  When a client connects to the proxy, the proxy may need a way to
  tell the server some identifier for the client address.  It does
  this in-band.

  As before, the server lists proxies in its torrc.  These can be
  external proxies that run on their own, or managedproxies that Tor
  launches.

  An external server proxy is configured as
     ServerTransportPlugin method proxy address:port param=val..
  as in
     ServerTransportPlugin trebuchet proxy 127.0.0.1:999 rocks=heavy
  The param=val pairs and the address are used to make the bridge
  configuration information that we'll tell users.

  A managed proxy is configured as
      ServerTransportPlugin method exec /path/to/binary [options]
  or
      ServerTransportPlugin * exec /path/to/binary [options]

  When possible, Tor should launch only one binary of each binary/option
  pair configured.  So if the torrc contains

     ClientTransportPlugin foo exec /usr/bin/megaproxy --foo
     ClientTransportPlugin bar exec /usr/bin/megaproxy --bar
     ServerTransportPlugin * exec /usr/bin/megaproxy --foo

  then Tor will launch the megaproxy binary twice: once with the option
  --foo and once with the option --bar.

Managed proxy interface

   When the Tor client launches a client proxy from the command
   line, it communicates via environment variables.  At a minimum,
   it sets:

      {Client and server}
      HOME, PATH -- as you'd expect.

      "STATE_LOCATION" -- a directory where the proxy should store
       state if it wants to.  This directory is not required to
       exist, but the proxy SHOULD be able to create it if it
       doesn't.  The proxy SHOULD NOT store state elsewhere.

      "MANAGED_TRANSPORT_VER=1" -- To tell the proxy which versions
       of this configuration protocol Tor supports.  Future versions
       will give a comma-separated list.  Clients MUST accept
       comma-separated lists containing any version that they
       recognize, and MUST work correctly even if some of the
       versions they don't recognize are non-numeric.

      {Client only}

      "CLIENT_TRANSPORTS" -- a comma-separated list of which methods
        this client should enable, or * if all methods should be
        enabled.  The proxy SHOULD ignore methods that it doesn't
        recognize.

      {Server only}

      "EXT_SERVER_PORT=addr:portnum" -- A port (probably on localhost) that
        speaks the extended server protocol.

      "ORPORT=addr:portnum" -- Our regular ORPort in a form suitable
        for local connections.

      "BINDADDR=addr" -- An address on which to listen for local
         connections.  This might be the advertised address, or might
         be a local address that Tor will forward ports to.  It MUST
         be an address that will work with bind().

      "SERVER_TRANSPORTS=..." -- A comma-separated list of server
          methods that the proxy should support, or *

  The transport proxy replies by writing NL-terminated lines to
  stdout.  The metaformat is

      Keyword OptArgs NL
      OptArgs = Args |
      Args = SP ArgChar | Args ArgChar
      ArgChar = Any character but NUL or NL
      Keyword = KeywordChar | Keyword KeywordChar
      KeyWordChar = All alphanumeric characters, dash, and underscore.

  Tor MUST ignore lines with keywords that it doesn't recognize.

  First, the proxy writes "VERSION 1" to say that it supports this
  protocol. It must either pick a version that Tor told it about, or
  pick no version at all, and say "ERROR no-version\n" and exit.

  The proxy should then open its ports.  If running as a client
  proxy, it should not use fixed ports; instead it should autoselect
  ports to avoid conflicts.  A client proxy should by default only
  listen on localhost for connections.

  A server proxy SHOULD try listen at a consistent port, though it
  SHOULD pick a different one if the port it last used is now allocated.

  A client or server proxy then should tell which methods it has
  made available and how.  It does this by printing zero or more
  CMETHOD and SMETHOD lines to its stdout.  These lines look like:

   CMETHOD methodname SOCKS4/SOCKS5 address:port [ARGS=arglist] \
        [OPT-ARGS=arglist]

  as in

   CMETHOD trebuchet SOCKS5 127.0.0.1:19999 ARGS=rocks,height \
              OPT-ARGS=tensile-strength

  The ARGS field lists mandatory parameters that must appear in
  every bridge line for this method. The OPT-ARGS field lists
  optional parameters.  If no ARGS or OPT-ARGS field is provided,
  Tor should not check the parameters in bridge lines for this
  method.

  The proxy should print a single "CMETHODS DONE" line after it is
  finished telling Tor about the client methods it provides.  If it
  tries to supply a client method but can't for some reason, it
  should say:
    CMETHOD-ERROR methodname "Message"

  A proxy should tell Tor about the server methods it is providing
  by printing zero or more SMETHOD lines.  These lines look like:

    SMETHOD methodname address:port  [Options]

  If there's an error setting up a configured server method, the
  proxy should say:
    SMETHOD-ERROR methodname "message"

  The 'address:port' part of an SMETHOD line is the address to put
  in the bridge line.  The ARGS: part is a list of key-value pairs
  that the client needs to know.  The Options part is a list of
  space-separated K:V flags that Tor should know about.  Recognized
  options are:

      - FORWARD:1

        If this option is set, and address:port is not a publicly
        accessible address, then the bridge needs to forward some
        other address:port to address:port via upnp-helper.

      - ARGS:k=v,k=v,k=v

        If this option is set, the K=V arguments are added to the
        extrainfo document.

      - DECLARE:K=V,...

        If this option is set, all the K=V options should be
        added as extension entries to the router descriptor.  (See
        below)

      - USE-EXTPORT:1

        If this option is set, the server plugin is using the
        extended server port.

  SMETHOD and CMETHOD lines may be interspersed.  After the list
  SMETHOD line, the proxy says "SMETHODS DONE"

  The proxy SHOULD NOT tell Tor about a server or client method
  unless it is actually open and ready to use.

  Tor clients SHOULD NOT use any method from a client proxy or
  advertise any method from a server proxy UNLESS it is listed as a
  possible method for that proxy in torrc, and it is listed by the
  proxy as a method it supports.

  Proxies should respond to a single INT signal by closing their
  listener ports and not accepting any new connections, but keeping
  all connections open, then terminating when connections are all
  closed.  Proxies should respond to a second INT signal by shutting
  down cleanly.

The extended ORPort protocol.

  Server transports may need to connect to the bridge and pass
  additional information about client connections that the bridge
  would ordinarily receive from the kernel's TCP stack.  To to this,
  they connect to the "extended server port" as given in
  SERVER_PORT, sent a short amount of information, wait for a
  response, and then send the user traffic on that port.

  The extended server port protocol is as follows:

     COMMAND [2 bytes, big-endian]
     BODYLEN [2 bytes, big-endian]
     BODY [Bodylen bytes]

     Commands sent from the transport to the server are:

     [0x0000] DONE: There is no more information to give. (body ignored)

     [0x0001] USERADDR: an address:port string that represents the user's
       address.  If the transport doesn't actually do addresses,
       this shouldn't be sent.

     Replies sent from tor to the proxy are:

     [0x1001] OKAY: Send the user's traffic. (body ignored)

     [0x1002] DENY: Tor would prefer not to get more traffic from
       this address for a while. (body ignored)

  [We could also use an out-of-band signalling method to tell Tor
  about client addresses, but that's a historically error-prone way
  to go about annotating connections.]

Advertising bridge methods:

  Bridges put the 'method' lines in their extra-info documents.

     method SP methodname SP address:port SP arglist NL

  The address:port parse are as returned from an SMETHOD line.  The
  arglist is a K=V,... list as retuned in the ARGS part of the
  SMETHOD line.

  If the SMETHOD line includes a DECLARE: part, the routerinfo gets
  a new line:

     method-info SP methodname SP arglist NL

Bridge authority behavior

  We need to specify a way to test different transport methods that
  bridges claim to support.  We should test as many as possible.  We
  should NOT require that we have a way to tra

Bridgedb behavior:

  Bridgedb can, given a set of router descriptors and their
  corresponding extrainfo documents, generate a set of bridge lines
  for each descriptor.  Bridgedb may want to avoid handing out
  methods that seem to get bridges blocked quickly.

Implementation plan

  First, we should implement per-bridge socks settings (as
  described above in "manually configuring a client proxy for a
  bridge") and the extended-server-port mechanism.  This will let
  bridges run transport proxies such that they can hand-generate
  bridge lines to give to clients for testing.

  Once that's done, we can improve usability a little bit by
  implementing external proxies.  Once that's done, we can see if we
  need any managed proxies, or if the whole idea there is silly.

  If we do, the next most important part seems to be getting
  the client-side automatic part written.  And once that's done, we
  can evaluate how much of the server side is easy for people to do
  and how much is hard.

  The "obfsproxy" obfuscating proxy is a likely candidate for an
  initial transport, as is Steven Murdoch's http thing or something
  similar.

Notes on plugins to write:

   We should ship a couple of null plugin implementations in one or two
   popular, portable languages so that people get an idea of how to
   write the stuff.

   1. We should have one that's just a proof of concept that does
      nothing but transfer bytes back and forth.

   1. We should not do a rot13 one.

   2. We should implement a basic proxy that does not transform the bytes at all

   1. We should implement DNS or HTTP using other software (as goodesll
      did years ago with DNS) as an example of wrapping existing code into
      our plugin model.

   2. The obfuscated-ssh superencipherment is pretty trivial and pretty
   useful.  It makes the protocol stringwise unfingerprintable.

      1. Nick needs to be told firmly not to bikeshed the obfuscated-ssh
        superencipherment too badly

         1. Go ahead, bikeshed my day

   1. If we do a raw-traffic proxy, openssh tunnels would be the logical choice.

Appendix: recommendations for transports

  Be free/open-source software.  Also, if you think your code might
  someday do so well at circumvention that it should be implemented
  inside Tor, it should use the same license as Tor.

  Use libraries that Tor already requires. (You can rely on openssl and
  libevent being present if current Tor is present.)

  Be portable: most Tor users are on Windows, and most Tor developers
  are not, so designing your code for just one of these platforms will
  make it either get a small userbase, or poor auditing.

  Think secure: if your code is in a C-like language, and it's hard to
  read it and become convinced it's safe, then it's probably not safe.

  Think small: we want to minimize the bytes that a Windows user needs
  to download for a transport client.

  Avoid security-through-obscurity if possible.  Specify.

  Resist trivial fingerprinting: There should be no good string or regex
  to search for to distinguish your protocol from protocols permitted by
  censors.

  Imitate a real profile: There are many ways to implement most
  protocols -- and in many cases, most possible variants of a given
  protocol won't actually exist in the wild.