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60 lines
2.4 KiB
Plaintext
60 lines
2.4 KiB
Plaintext
Filename: 148-uniform-client-end-reason.txt
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Title: Stream end reasons from the client side should be uniform
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Version: $Revision$
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Last-Modified: $Date$
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Author: Roger Dingledine
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Created: 2-Jul-2008
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Status: Accepted
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Target: 0.2.1.x
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Overview
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When a stream closes before it's finished, the end relay cell that's
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sent includes an "end stream reason" to tell the other end why it
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closed. It's useful for the exit relay to send a reason to the client,
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so the client can choose a different circuit, inform the user, etc. But
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there's no reason to include it from the client to the exit relay,
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and in some cases it can even harm anonymity.
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We should pick a single reason for the client-to-exit-relay direction
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and always just send that.
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Motivation
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Back when I first deployed the Tor network, it was useful to have
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the Tor relays learn why a stream closed, so I could debug both ends
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of the stream at once. Now that streams have worked for many years,
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there's no need to continue telling the exit relay whether the client
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gave up on a stream because of "timeout" or "misc" or what.
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Then in Tor 0.2.0.28-rc, I fixed this bug:
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- Fix a bug where, when we were choosing the 'end stream reason' to
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put in our relay end cell that we send to the exit relay, Tor
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clients on Windows were sometimes sending the wrong 'reason'. The
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anonymity problem is that exit relays may be able to guess whether
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the client is running Windows, thus helping partition the anonymity
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set. Down the road we should stop sending reasons to exit relays,
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or otherwise prevent future versions of this bug.
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It turned out that non-Windows clients were choosing their reason
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correctly, whereas Windows clients were potentially looking at errno
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wrong and so always choosing 'misc'.
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I fixed that particular bug, but I think we should prevent future
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versions of the bug too.
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(We already fixed it so *circuit* end reasons don't get sent from
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the client to the exit relay. But we appear to be have skipped over
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stream end reasons thus far.)
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Design:
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One option would be to no longer include any 'reason' field in end
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relay cells. But that would introduce a partitioning attack ("users
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running the old version" vs "users running the new version").
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Instead I suggest that clients all switch to sending the "misc" reason,
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like most of the Windows clients currently do and like the non-Windows
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clients already do sometimes.
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