[tor-dev] Brainstorming ideas for controller features for improved testing; want feedback

Nick Mathewson nickm at torproject.org
Fri Mar 20 15:55:03 UTC 2015

Hi!  I've got an end-of-month deliverable to flesh out as many good
ideas here as I can, and I'd appreciate feedback on what kind of
features it would be good to add to the controller protocol in order
to better support testing.

More ideas would be most welcome.

Yes, some of these ideas are probably foolish or pointless or
half-baked or useless or even dangerous; this is a brainstorming
exercise, not a declaration of intent.  The goal right now is to
generate a lot of ideas and thoughts now, and to make decisions about
what to build later.


1. Step-by-step hidden service connections

   Add the ability to create connections to hidden services step by
   step, to best

   What's necessary here is commands to:
      * Establish a rendezvous point on a given circuit.
      * Construct and send an introduce2 cell on a given circuit.
      * Realize that a rendezvous circuit has been constructed.

2. Send a single cell on a circuit


   For fuzzing and low-level testing purposes, it would be handy to be
   able to send a single cell on a tor circuit.

   This might be better to expose via a low-level modular API than via
   the control port.

3. Intercept cell by cell on a circuit


   For fuzzing, testing, and debugging purposes, it might be handy for
   a controller to be able to observe data cell by cell on a circuit of

   This might be better to expose via a low-level modular API than via
   the control port.

4. Send a single cell on a connection.


   As 2, but for connections.  Note that we might even, for testing,
   expose this at a sub-cell level.

5. Intercept all cells on a connection


   As 2, but for connections.

6. Plug-in to handle a relay or other command.

   Right now, all Tor's features need to be baked into Tor; it's not
   easy to write extensions.  We could change that by having the
   controller able to intersect particular relay or extension commands
   and act accordingly.  This could be used for prototyping new
   features, etc.

7. Force a given protocol on a given connection

   We could add a feature to restrict what protocols can be negotiated
   on a given connection we create.  This could help us better test our
   protocols for interoperatbility.

8. Examine fine-grained connection detail.

   There are many data available for a given connection (such as
   fine-grained TLS information) that are not currently exposed on the
   GETINFO interface.  We could make most of this available for testing,
   pending security analysis.

9. Examine cache in detail

   In the past we've seen crazy issues with our descriptor caching
   code.  It might be good to expose for testing information about
   where exactly descriptors are stored, what attributes are set on
   them, and so on.  We could also expose events for cache compaction
   and discarded expired descriptors.

10. Fetch literal documents

   Currently there's no way for a controller to ask Tor to download a
   given descriptor or microdescriptor or networkstatus.  That could

11. OOM stats

   To resist out-of-memory attacks, we track our memory usage and kill
   off circuits as needed when memory gets low.  We could expose the
   memory thresholds and current sizes via one or more controller

12. Timeout values

   Tor has a truly huge variety of internal timers to ensure that given
   periodic events happen enough; we could expose those, and (TESTING
   ONLY) allow controllers to adjust them or trigger their corresponding

13. Detailed connection debugging info

   Current connection events expose only large-scale state changes in
   connections; we could instead expose every state transition at the
   cell-by-cell handshake level.

14. Detailed circuit debugging info

   As 13 but for circuits.

15. Halt main loop except for control layer.


   For inspection/debugging purposes, it might be clever to have Tor be
   able to freeze itself, except for the control layer, and let the
   controller inquire about information.

   This presents implementation challenges, and is probably not a great
   idea to do before a _big_ refactoring.

16. Service a single connection


   Currently controllers can disable circuit construction or stream
   attachment, and do them manually.  We might also do this for
   connections, allowing a testing controller to trace what Tor does
   cell by cell on a single connection.

17. All rephist data

    There are many data about history and usage in rephist.c (which
    stands for 'reputation and history!').  We could expose them, to let
    us better test them.  Some of this might be useful for Seth
    (previously arm) users.

    Spec: This would use GETINFO extensions, and probably some new

18. All ratelim data

    Sometimes our rate-limiting code can get wonky.  It would be great
    to expose it to Tor controllers in order to help ensure it's
    behaving correctly.  This would include send/receive windows and bw

    Spec: This would use GETINFO extensions, and probably some new

19. All accounting data

    As 17, but for hibernate.c, which performs bandwidth accounting.

20. All guard transitions

    Our guard node state logic is very complicated, and much in need of
    testing and refactoring.  Exposing more state transitions and guard
    selection transitions to the controller might help.  (We have a
    "GUARD" event now, but it is a bit out of sync with the main

21. All key transitions

    We could generate events every time we change keys, and (TESTING
    ONLY) allow a controller to time-out a key early.

22. Examine mux settings


    The circuitmux code that we use to decide which cell to send next is
    very complex; it would be good to expose its thinking and
    decisionmaking to a testing controller for better observation.

23. Examine pathbias settings

    Our pathbias code is also complex, and a bit flakier than the
    circuitmux code.  We could do for it as with 22 above.

24. Examine cpuworker queues

    As of 0.2.6, we have a new cpuworker infrastructure that better
    sends data to worker threads, but not much visibility into how well
    it's working.  Exposing some information about this to the
    controller could help us tune better.

25. All geoip data

    As 17, but for any geoip information we aren't currently exporting.

26. Replay detection

    For hidden service security (and maybe eventually for secure ntor Y
    reuse) we have to keep replay caches to prevent us from being
    tricked into handling the same value twice.  We don't expose the
    load on these caches to the controller, however.  And we could, to
    help us better tune them into using a good memory/error-rate

27. Hidden service intropoint changes, desc changes, uploads

    Many hidden service transitions currently generate no events.  We
    could at minimum generate events for changed inroduction points,
    changed hidden service descriptors, uploading our own HS descriptor.

28. Descriptor uploads.

    We have an event for when our descriptor has changed, but not for a
    successful upload for it.  We could fix that.

29. Path generation logic -- expose, allow.

    Currently a controller's only visibility into path selection logic
    is in its outputs, and in the opportunity to replace path selection
    logic entirely.  We could expose more details about the algorithm's
    operation in order to help better test our path selection.

30. All PT status information.

    Pluggable transport feedback is, at present, very coarse-grained.
    For testing we might expose more.

31. Crypto operation counts.

    We ought to keep count of our various cryptographic operations, and
    expose them to the controller.   This would help us know where to
    spend our optimization efforts.

32. Forget cached information

    To better test our download logic, it would be helpful to have a way
    to drop items from our caches.


Not lightly does one list 31 controller improvement areas.  If we're
hoping to do these without too much programmer time and , we need to
take a much more principled approach to implementing controller
commands.  For #8351 I worked on a branch called 'ticket8351' that has
some code we could use here.


Many of the features here are ones that we might not want to promise to
support indefinitely; we should gate them behind a USEFEATURE command,
and maybe place them in an annex of the control spec.

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