[tor-dev] [prop305] Introduction Point Behavior

David Goulet dgoulet at torproject.org
Thu Aug 15 12:56:19 UTC 2019


This is part of the many discussions about proposal 305 which is the
ESTABLISH_INTRO DoS defenses cell extension.

Implementation is close to done and under review in ticket #30924. However,
there is one part that is yet to be cleared out. asn and I thought it would be
better to bring it to tor-dev@ to get a more informed decision.

As a reminder, the service operator will be able set torrc options that are
the DoS defenses parameters. Those values are validated (bound check) and then
sent to an introduction point, supporting the extension (protover HSIntro=5),
in the ESTABLISH_INTRO cell. The intro point then gets them and apply them
only to that specific circuit. If no cell extension is seen, the intro point
will honor the consensus parameters for these DoS defenses.

What we want to discuss is what happened when the introduction points receives
bad values. What does it do with the circuit? Below is the list of possible
bad values and the proposed behavior:

1) One of the paramater (at this point in time, only 2 exists) is out of bound
   that is above INT32_MAX.

   Behavior: We propose to ignore the cell extension, and fallback to follow
             the consensus parameters. Keeping the circuit alive and working.

   The reason for this is because if let say the intro point would close the
   circuit due to "bad protocol", then the service would open a new circuit to
   an intro point supporting the extension and it would fail again.
   Effectively turning the service into a "zombie" and "DoS" weapon itself ;).

   At this point, there is really no reason on why the service would send bad
   values since torrc options are validated and then sent to the intro point.
   But this doesn't protect us from our future-developer-self making coding
   or protocol mistake ;).

2) A parameter type is unrecognized. This one is mostly about forward

   Behavior: Ignore the parameter type, and continue parsing the cell
             extension. If all the right parameters are found, apply defenses.

3) None or 1 parameter out of 2 is given.

   Behavior: This would be considered bad protocol so it would be like (1)
             where the intro point uses the consensus parameters.

4) <insert whatever case you can think about>

So, for (1) and (3), there lies mostly the discussion. There is no way to send
feedback to the service if a problem occurred and that the DoS defenses
weren't applied due to reason X.

If we had that, we could not only try to warn the operator (log or control
event) but make tor smarter when an error is returned so it doesn't try to
infinity other intro points with values that makes them fail.

One avenue there is to pass down such error code into the INTRO_ESTABLISHED
cell with probably a new extension. That requires more work and likely either
extending prop305 or a new proposal.

Thus, what is the minimal viable product here we are ok with?

I'm leaning towards not closing the circuit and falling back on the consensus
parameters. And at some point in time, we'll be able to implement the
INTRO_ESTABLISHED response. In the meantime, there is little chances that tor
vanilla start sending bad values since they are validated from the torrc file.


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