[tor-dev] Proposal 305: ESTABLISH_INTRO Cell DoS Defense Extension

George Kadianakis desnacked at riseup.net
Wed Jun 12 12:39:55 UTC 2019

David Goulet <dgoulet at torproject.org> writes:

> Filename: 305-establish-intro-dos-defense-extention.txt
> Title: ESTABLISH_INTRO Cell DoS Defense Extension
> Author: David Goulet, George Kadianakis
> Created: 06-June-2019
> Status: Draft

Thanks for this proposal, it's most excellent and an essential building
block for future work on intro point related defences.

>    We propose a new EXT_FIELD_TYPE value:
>       [01] -- DOS_PARAMETERS.
>               If this flag is set, the extension should be used by the
>               introduction point to learn what values the denial of service
>               subsystem should be using.

Perhaps we can name it "rate-limiting parameters"? But no strong opinion.

>    The EXT_FIELD content format is:
>       N_PARAMS    [1 byte]
>       N_PARAMS times:
>          PARAM_TYPE  [1 byte]
>          PARAM_VALUE [8 byte]
>    The PARAM_TYPE proposed values are:
>               The rate per second of INTRODUCE2 cell relayed to the service.
>               The burst per second of INTRODUCE2 cell relayed to the service.
>    The PARAM_VALUE size is 8 bytes in order to accomodate 64bit values
>    (uint64_t). It MUST match the specified limit for the following PARAM_TYPE:
>       [01] -- Min: 0, Max: INT_MAX
>       [02] -- Min: 0, Max: INT_MAX

How would this new addition to the cell impact the size of the cell? How
much free space do we have for additional features to this cell (e.g. to
do the PoW stuff of the other thread)?

>    A value of 0 means the defense is disabled which has precedence over the
>    network wide consensus parameter.
>    In this case, if the rate per second is set to 0 (param 0x01) then the
>    burst value should be ignored. And vice-versa, if the burst value is 0,
>    then the rate value should be ignored. In other words, setting one single
>    parameter to 0 disables the INTRODUCE2 rate limiting defense.

I think it could be cool to add a discussion section where we introduce
a new cell from the intro to the service which informs the service that
rate limiting limits have been hit. So that there is a way for the
service to get feedback that it's under attack or capped by
limits. Otherwise, there is simply no way to learn it.

This can be a later feature fwiw.

> 3. Protocol Version
>    We introduce a new protocol version in order for onion service that wants
>    to specifically select introduction points supporting this new extension.
>    But also, it should be used to know when to send this extension or not.
>    The new version for the "HSIntro" protocol is:
>       "5" -- support ESTABLISH_INTRO cell DoS parameters extension for onion
>              service version 3 only.
> 4. Configuration Options
>    We also propose new torrc options in order for the operator to control
>    those values passed through the ESTABLISH_INTRO cell.
>       "HiddenServiceEnableIntroDoSDefense 0|1"
>          If this option is set to 1, the onion service will always send to the
>          introduction point denial of service defense parameters regardless of
>          what the consensus enables it or not. The value will be taken from
>          the consensus and if not present, the default values will be used.
>          (Default: 0)
>       "HiddenServiceEnableIntroDoSRatePerSec N sec"
>          Controls the introduce rate per second the introduction point should
>          impose on the introduction circuit.
>          (Default: 25, Min: 0, Max: 4294967295)
>       "HiddenServiceEnableIntroDoSBurstPerSec N sec"
>          Controls the introduce burst per second the introduction point should
>          impose on the introduction circuit.
>          (Default: 200, Min: 0, Max: 4294967295)
>    They respectively control the parameter type 0x01 and 0x02 in the
>    ESTABLISH_INTRO cell detailed in section 2.
>    The default values of the rate and burst are taken from ongoing anti-DoS
>    implementation work [1][2]. They aren't meant to be defined with this
>    proposal.
> 5. Security Considerations
>    Using this new extension leaks to the introduction point the service's tor
>    version. This could in theory help any kind of de-anonymization attack on a
>    service since at first it partitions it in a very small group of running
>    tor.
>    Furthermore, when the first tor version supporting this extension will be
>    released, very few introduction points will be updated to that version.
>    Which means that we could end up in a situation where many services want to
>    use this feature and thus will only select a very small subset of relays
>    supporting it overloading them but also making it an easier vector for an
>    attacker that whishes to be the service introduction point.

Interesting idea.

I'm not that worried about the service leaking its version to the intro,
but I am worried about all attacked services saturating the few upgraded
intro points, so I agree that such a switch makes sense.

>    For the above reasons, we propose a new consensus parameters that will
>    provide a "go ahead" for all service out there to start using this
>    extension only if the introduction point supports it.
>       "enable_establish_intro_dos_extension"
>          If set to 1, this makes tor start using this new proposed extension
>          if available by the introduction point (looking at the new protover).
>    This parameter should be switched on when a majority of relays have
>    upgraded to a tor version that supports this extension for which we believe
>    will also give enough time for most services to move to this new stable
>    version making the anonymity set much bigger.
>    We propose to add a torrc option to ignore this parameter and force tor to
>    select introduction points supporting this extension which will
>    effectively, in the beginning, toss away these security considerations.
>    We believe that there are services that do not care about anonymity on the
>    service side and thus could benefit from this feature right away if they
>    wish to use it.
> References:
> [1] https://lists.torproject.org/pipermail/tor-dev/2019-May/013837.html
> [2] https://trac.torproject.org/15516
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