[tor-bugs] #24050 [Core Tor/Tor]: We still do client-side caching. We just don't use the cache.

[Tor Bug Tracker & Wiki]

#24050: We still do client-side caching. We just don't use the cache.
----------------------------------------+----------------------------------
 Reporter:  arma                        |          Owner:  (none)
     Type:  defect                      |         Status:  new
 Priority:  Medium                      |      Milestone:  Tor:
                                        |  0.3.3.x-final
Component:  Core Tor/Tor                |        Version:
 Severity:  Normal                      |     Resolution:
 Keywords:  032-backport, ???-backport  |  Actual Points:
Parent ID:                              |         Points:
 Reviewer:                              |        Sponsor:
----------------------------------------+----------------------------------

Comment (by nickm):

 One big problem with client-side DNS caching in Tor is that if the exit on
 the first circuit lies about the IP address, the exit on the second
 circuit will be told about the false IP address too.  In this way, one bad
 exit can set up a "sticky" MITM that will persist even on a new circuit if
 the user is using the same DNS cache.  Similarly, IPv6 addresses can
 trivially be used to set up unique client identifiers that will last for
 as long as the DNS cache lasts.

 How bad is this attack?  Consider:

 1. The more we reuse DNS caches across multiple circuits, the worse this
 attack gets... but on the other hand, the DNS cache is only beneficial to
 the extent that we can reuse it.

 2. These attacks seem especially bad when performed against uncommon
 sites... but common sites are likely to be in the exit-side DNS cache,
 making client-side caching unnecessary.

 So it seems to me that client-side DNS caching is risky to the extent that
 it is useful, and vice versa. :)

 One more consideration: client-side DNS caching can also lower performance
 for big sites that use CDNs to match exits with nearby servers.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24050#comment:3>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online