[tor-dev] Proposal 207: Directory guards
nickm at torproject.org
Thu Oct 11 03:46:51 UTC 2012
Title: Directory guards
Author: Nick Mathewson
When we added guard nodes to resist profiling attacks, we made it so
that clients won't build general-purpose circuits through just any
node. But clients don't use their guard nodes when downloading
general-purpose directory information from the Tor network. This
allows a directory cache, over time, to learn a large number of IPs
for non-bridge-using users of the Tor network.
In the same way as they currently pick guard nodes as needed, adding more
guards as those nodes are down, clients should also pick a small-ish set
of directory guard nodes, to persist in Tor's state file.
Clients should not pick their own guards as directory guards, or pick
their directory guards as regular guards.
When downloading a regular directory object (that is, not a hidden
service descriptor), clients should prefer their directory guards
first. Then they should try more directories from a recent consensus
(if they have one) and pick one of those as a new guard if the
existing guards are down and a new one is up. Failing that, they
should fall back to a directory authority (or a directory source, if
those get implemented-- see proposal 206).
If a client has only one directory guard running, they should add new
guards and try them, and then use their directory guards to fetch multiple
descriptors in parallel.
The rule that the set of guards and the set of directory guards need to
be disjoint, and the rule that multiple directory guards need to be
providing descriptors, are both attempts to make it harder for a
single node to capture a route.
Open questions and notes:
What properties does a node need to be a suitable directory guard?
If we require that it have the Guard flag, we'll lose some nodes:
only 74% of the directory caches have it (weighted by bandwidth).
We may want to tune the algorithm used to update guards.
For future-proofing, we may want to have the DirCache flag from 185
be the one that nodes must have in order to be directory guards. For
now, we could have authorities set it to Guard && DirPort!=0, with a
better algorithm to follow. Authorities should never get the
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