[tor-dev] Acceptable clock skew in tor 0.4.1

George Kadianakis desnacked at riseup.net
Mon Nov 11 09:23:58 UTC 2019

intrigeri <intrigeri at boum.org> writes:

> Hi,
> recently, tor has become more tolerant to skewed system clocks;
> great, thanks!
> At Tails, we would like to take advantage of these improvements in
> order to remove as much as we can of our not-quite-safe clock fixing
> code. Our testing suggests that:
>  - A ±24h clock skew is now acceptable in most cases¹: tor
>    bootstraps successfully.
>  - While with a ±48h clock skew, tor fails to bootstrap.
> Could someone on the network team please confirm that these empirical
> results match what the code is currently supposed to do?

Hello intri!

I'm not really 100% up to date with the clock skew tolerance of Tor, but
the ±24h value seems plausible since that's the range where tor
considers a consensus to be "reasonably live" [0], which is what most of
its subsystems require to work.

An unfortunate exception here is v3 onion services: v3 onion services
only tolerate skews of maximum ±3 hours [1] but in most cases even
tighter than that. This is to assure that v3 clients and services have a
recent and accurate view of the network. In theory all of Tor needs a
recent and accurate view of the network, but v3 is particularly fragile
because of the shared random value and the precise time periods.

Sorry to break the bad news, but it is what it is. In theory we could
potentially do improvements for v3 here, but this is not in the scope
right now.


[0]: see networkstatus_consensus_reasonably_live()
[1]: see networkstatus_get_live_consensus() which is used in the v3
     system, and basically checks if the current time is between the
     consensus valid-after and valid-until.

> [1] In some corner cases I see weird behavior (#32438).
>     And obfs4proxy is stricter than that, which I should report on Trac.
> Cheers,
> -- 
> intrigeri
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