r18859@catbus: nickm | 2008-03-16 22:52:52 -0400

 Add proposal 132 from Robert Hogan.


svn:r14061

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+Filename: 132-browser-check-tor-service.txt
+Title: A Tor Web Service For Verifying Correct Browser Configuration
+Version: $Revision$
+Last-Modified: $Date$
+Author: Robert Hogan
+Created: 2008-03-08
+Status: Draft
+
+Overview:
+
+  Tor should operate a primitive web service on the loopback network device
+  that tests the operation of user's browser, privacy proxy and Tor client.
+  The tests are performed by serving unique, randomly generated elements in
+  image URLs embedded in static HTML. The images are only displayed if the DNS
+  and HTTP requests for them are routed through Tor, otherwise the 'alt' text
+  may be displayed. The proposal assumes that 'alt' text is not displayed on
+  all browsers so suggests that text and links should accompany each image
+  advising the user on next steps in case the test fails.
+
+  The service is primarily for the use of controllers, since presumably users
+  aren't going to want to edit text files and then type something exotic like
+  127.0.0.1:9999 into their address bar. In the main use case the controller
+  will have configured the actual port for the webservice so will know where
+  to direct the request. It would also be the responsibility of the controller
+  to ensure the webservice is available, and tor is running, before allowing
+  the user to access the page through their browser.
+
+Motivation:
+
+  This is a complementary approach to proposal 131. It overcomes some of the
+  limitations of the approach described in proposal 131: reliance
+  on a permanent, real IP address and compatibility with older versions of
+  Tor. Unlike 131, it is not as useful to Tor users who are not running a
+  controller.
+
+Objective:
+
+  Provide a reliable means of helping users to determine if their Tor
+  installation, privacy proxy and browser are properly configured for
+  anonymous browsing.
+
+Proposal:
+
+  When configured to do so, Tor should run a basic web service available
+  on a configured port on 127.0.0.1. The purpose of this web service is to
+  serve a number of basic test images that will allow the user to determine
+  if their browser is properly configured and that Tor is working normally.
+
+  The service can consist of a single web page with two columns. The left
+  column contains images, the right column contains advice on what the
+  display/non-display of the column means.
+
+  The rest of this proposal assumes that the service is running on port
+  9999. The port should be configurable, and configuring the port enables the
+  service. The service must run on 127.0.0.1.
+
+  In all the examples below [uniquesessionid] refers to a random, base64
+  encoded string that is unique to the URL it is contained in. Tor only ever
+  stores the most recently generated [uniquesessionid] for each URL, storing 3
+  in total. Tor should generate a [uniquesessionid] for each of the test URLs
+  below every time a HTTP GET is received at 127.0.0.1:9999 for index.htm.
+
+  The most suitable image for each test case is an implementation decision.
+  Tor will need to store and serve images for the first and second test
+  images, and possibly the third (see 'Open Issues').
+
+  1. DNS Request Test Image
+  
+  This is a HTML element embedded in the page served by Tor at
+  http://127.0.0.1:9999:
+
+  <IMG src="http://[uniquesessionid]:9999/torlogo.jpg" alt="If you can see
+  this text, your browser's DNS requests are not being routed through Tor."
+  width="200" height="200" align="middle" border="2">
+
+  If the browser's DNS request for [uniquesessionid] is routed through Tor,
+  Tor will intercept the request and return 127.0.0.1 as the resolved IP
+  address. This will shortly be followed by a HTTP request from the browser
+  for http://127.0.0.1:9999/torlogo.jpg. This request should be served with
+  the appropriate image.
+
+  If the browser's DNS request for [uniquesessionid] is not routed through Tor
+  the browser may display the 'alt' text specified in the html element. The
+  HTML served by Tor should also contain text accompanying the image to advise
+  users what it means if they do not see an image. It should also provide a
+  link to click that provides information on how to remedy the problem. This
+  behaviour also applies to the images described in 2. and 3. below, so should
+  be assumed there as well.
+
+
+  2. Proxy Configuration Test Image
+
+  This is a HTML element embedded in the page served by Tor at
+  http://127.0.0.1:9999:
+
+  <IMG src="http://torproject.org/[uniquesessionid].jpg" alt="If you can see
+  this text, your browser is not configured to work with Tor." width="200"
+  height="200" align="middle" border="2">
+
+  If the HTTP request for the resource [uniquesessionid].jpg is received by
+  Tor it will serve the appropriate image in response. It should serve this
+  image itself, without attempting to retrieve anything from the Internet.
+
+  If Tor can identify the name of the proxy application requesting the
+  resource then it could store and serve an image identifying the proxy to the
+  user.
+
+  3. Tor Connectivity Test Image
+
+  This is a HTML element embedded in the page served by Tor at
+  http://127.0.0.1:9999:
+
+  <IMG src="http://torproject.org/[uniquesessionid]-torlogo.jpg" alt="If you
+  can see this text, your Tor installation cannot connect to the Internet."
+  width="200" height="200" align="middle" border="2">
+
+  The referenced image should actually exist on the Tor project website. If
+  Tor receives the request for the above resource it should remove the random
+  base64 encoded digest from the request (i.e. [uniquesessionid]-) and attempt
+  to retrieve the real image.
+
+  Even on a fully operational Tor client this test may not always succeed. The
+  user should be advised that one or more attempts to retrieve this image may
+  be necessary to confirm a genuine problem.
+
+Open Issues:
+
+  The final connectivity test relies on an externally maintained resource, if
+  this resource becomes unavailable the connectivity test will always fail.
+  Either the text accompanying the test should advise of this possibility or
+  Tor clients should be advised of the location of the test resource in the
+  main network directory listings.
+
+  Any number of misconfigurations may make the web service unreachable, it is
+  the responsibility of the user's controller to recognize these and assist
+  the user in eliminating them. Tor can mitigate against the specific
+  misconfiguration of routing HTTP traffic to 127.0.0.1 to Tor itself by
+  serving such requests through the SOCKS port as well as the configured web
+  service report.
+
+  Now Tor is inspecting the URLs requested on its SOCKS port and 'dropping'
+  them. It already inspects for raw IP addresses (to warn of DNS leaks) but
+  maybe the behaviour proposed here is qualitatively different. Maybe this is
+  an unwelcome precedent that can be used to beat the project over the head in
+  future. Or maybe it's not such a bad thing, Tor is merely attempting to make
+  normally invalid resource requests valid for a given purpose.
+