[tor-dev] Pluggable Transport through SOCKS proxy

Arturo Filastò hellais at torproject.org
Wed Feb 29 00:12:53 UTC 2012

Following up on the discussion at the Tor dev meeting with Nick this
is the proposal for the Pluggable Transport through SOCKS proxies.

You can also find it committed on github:

Filename: xxx-pluggable-transports-through-proxy.txt
Title: Pluggable Transport through SOCKS proxy
Author: Arturo Filastò
Created: 28 February 2012
Status: Draft


  Tor introduced Pluggable Transports in proposal 180 Pluggable Transports
  for circumvention.

  The problem is that Tor currently cannot use a Pluggable Transport proxy
  and a SOCKS proxy at the same time. This has been noticed by users in
  #5195, where Tor would be failing saying "Unacceptable option value:
  You have configured more than one proxy type".


  This comes from a discussion that came up with Nick and I promised
  to write a proposal for it if I wanted to hear what he had to say.
  Nick spoke and I am writing this proposal.


  Most of the credit goes to Nick Mathewson for the main idea and
  the rest of it goes to George Kadianakis for helping me out in writing


  After looking at some options we decided to go for this solution solution
  since it guarantees backwards compatibility and is not particularly
  costly to implement.

Design overview

  When Tor is configured to use both a Pluggable Transport proxy and SOCKS
  proxy it should delegate the proxying to the pluggable transport proxy.

  This can be achieved by setting the environment variables for the SOCKS
  proxy to that specified inside of the torrc.

  When the pluggable transport proxy starts it will first read the environment
  variables and if it detects that it should be using a SOCKS proxy make
  all it's traffic go through it. Once the pluggable transport proxy has successfully
  established a connection to the SOCKS proxy it should notify Tor of it's
  success or failure.
  When both the SOCKS and the PluggableTransport directives are set Tor
  should set the environemnt variable start the pluggabletransport proxy and wait
  for it to report back on the SOCKS proxy status. If the pluggable transport
  reports back a failure or it does not report back at all (maybe because
  it is an outdated version), Tor should notify the user of the failure
  and exit with an error.

  The environment variables can also contain the credentials for accessing
  the proxy.

Specifications: Tor Pluggable Transport communication

  When Tor detects a SOCKS proxy directive and a Pluggable Transport
  proxy directive it sets the environment variable:

    "TOR_PT_PROXY" -- This is the address of the proxy to be used by
    the pluggable transport proxy. It is in the format:
    ex. socks5://tor:test1234 at

  If the pluggable transport proxy detects that the TOR_PT_PROXY environment
  variable is set it attempts connecting to it. On successs it will
  write to stdout (as specified in 180-pluggable-transport.txt)
  PROXY true. On failure it should write PROXY-ERROR <errormessage>.

  If Tor does not read any PROXY line or it reads a PROXY-ERROR line
  and it is configured to use both SOCKS and PluggableTransport it should
  exit with error.

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