Improving the robustness of Tor Check

[Steven J. Murdoch]

The default home page for the Tor Browser Bundle is
http://check.torproject.org/ which tells the user whether they are
using Tor. This catches quite a few misconfiguration errors so is very
valuable for usability and security.

However, there are a few cases of where it will get it wrong. This in
itself is undesirable and it also means that the message shown is
qualified as "probably", which is disconcerting.

These errors (false positives and false negatives) are inherent to the
approach taken by Tor Check (and the other services). To resolve them
a new design is needed. The proposal below suggests how this could be
implemented.

Any comments would be appreciated. I'd be especially interested in
spotting any security issues, and how to more easily implement it.

Steven.

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Filename: xxx-verify-tor-usage.txt
Title: Help users to verify they are using Tor
Version: $Revision: 13937 $
Last-Modified: $Date: 2008-03-10 11:08:31 +0000 (Mon, 10 Mar 2008) $
Author: Steven J. Murdoch
Created: 2008-01-25
Status: Needs-Revision

Overview:

  Websites for checking whether a user is accessing them via Tor are a
  very helpful aid to configuring web browsers correctly. Existing
  solutions have both false positives and false negatives when
  checking if Tor is being used. This proposal will discuss how to
  modify Tor so as to make testing more reliable.

Motivation:

  Currently deployed websites for detecting Tor use work by comparing
  the client IP address for a request with a list of known Tor nodes.
  This approach is generally effective, but suffers from both false
  positives and false negatives. 

  If a user has a Tor exit node installed, or just happens to have
  been allocated an IP address previously used by a Tor exit node, any
  web requests will be incorrectly flagged as coming from Tor. If any
  customer of an ISP which implements a transparent proxy runs an exit
  node, all other users of the ISP will be flagged as Tor users.

  Conversely, if the exit node chosen by a Tor user has not yet been
  recorded by the Tor checking website, requests will be incorrectly
  flagged as not coming via Tor.
  
  The only reliable way to tell whether Tor is being used or not is for
  the Tor client to flag this to the browser.

Proposal:

  A DNS name should be registered and point to an IP address 
  controlled by the Tor project and likely to remain so for the
  useful lifetime of a Tor client. A web server should be placed
  at this IP address.
  
  Tor should be modified to treat requests to port 80, at the
  specified DNS name or IP address specially. Instead of opening a
  circuit, it should respond to a HTTP request with a helpful web
  page:

  - If the request to open a connection was to the domain name, the web
    page should state that Tor is working properly.
  - If the request was to the IP address, the web page should state
    that there is a DNS-leakage vulnerability.

  If the request goes through to the real web server, the page
  should state that Tor has not been set up properly.

Extensions:

  Identifying proxy server:

  If needed, other applications between the web browser and Tor (e.g.
  Polipo and Privoxy) could piggyback on the same mechanism to flag
  whether they are in use. All three possible web pages should include
  a machine-readable placeholder, into which another program could
  insert their own message.

  For example, the webpage returned by Tor to indicate a successful
  configuration could include the following HTML:
   <h2>Connection chain</h2>
   <ul>
   <li>Tor 0.1.2.14-alpha
   <!-- Tor Connectivity Check: success -->
   </ul>

  When the proxy server observes this string, in response to a request
  for the Tor connectivity check web page, it would prepend it's own
  message, resulting in the following being returned to the web
  browser:
   <h2>Connection chain
   <ul>
   <li>Tor 0.1.2.14-alpha
   <li>Polipo version 1.0.4
   <!-- Tor Connectivity Check: success -->
   </ul>

  Checking external connectivity:

  If Tor intercepts a request, and returns a response itself, the user
  will not actually confirm whether Tor is able to build a successful
  circuit. It may then be advantageous to include an image in the web
  page which is loaded from a different domain. If this is able to be
  loaded then the user will know that external connectivity through
  Tor works.

Security and resiliency implications:

  What attacks are possible?

  If the IP addressed used for this feature moves there will be two
  consequences:
   - A new website at this IP address will remain inaccessible over
     Tor
   - Tor users who are leaking DNS will be informed that Tor is not
     working, rather than that it is active but leaking DNS
  We should thus attempt to find an IP address which we reasonably
  belive can remain static.

Open issues:

  If a Tor version which does not support this extra feature is used,
  the webpage returned will indicate that Tor is not being used. Can
  this be safely fixed?

Related work:

  The proposed mechanism is very similar to config.privoxy.org. The
  most significant difference is that if the web browser is
  misconfigured, Tor will only get an IP address. Even in this case,
  Tor should be able to respond with a webpage to notify the user of how
  to fix the problem. This also implies that Tor must be told of the
  special IP address, and so must be effectively permanent.
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w: http://www.cl.cam.ac.uk/users/sjm217/