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+Filename: 163-detecting-clients.txt
+Title: Detecting whether a connection comes from a client
+Author: Nick Mathewson
+Created: 22-May-2009
+Target: 0.2.2
+Status: Open
+
+
+Overview:
+
+   Some aspects of Tor's design require relays to distinguish
+   connections from clients from connections that come from relays.
+   The existing means for doing this is easy to spoof.  We propose
+   a better approach.
+
+Motivation:
+
+   There are at least two reasons for which Tor servers want to tell
+   which connections come from clients and which come from other
+   servers:
+
+     1) Some exits, proposal 152 notwithstanding, want to disallow
+        their use as single-hop proxies.
+     2) Some performance-related proposals involve prioritizing
+        traffic from relays, or limiting traffic per client (but not
+        per relay).
+
+   Right now, we detect client vs server status based on how the
+   client opens circuits.  (Check out the code that implements the
+   AllowSingleHopExits option if you want all the details.)  This
+   method is depressingly easy to fake, though.  This document
+   proposes better means.
+
+Goals:
+
+   To make grabbing relay privileges at least as difficult as just
+   running a relay.
+
+   In the analysis below, "using server privileges" means taking any
+   action that only servers are supposed to do, like delivering a
+   BEGIN cell to an exit node that doesn't allow single hop exits,
+   or claiming server-like amounts of bandwidth.
+
+Passive detection:
+
+   A connection is definitely a client connection if it takes one of
+   the TLS methods during setup that does not establish an identity
+   key.
+
+   A circuit is definitely a client circuit if it is initiated with
+   a CREATE_FAST cell, though the node could be a client or a server.
+
+   A node that's listed in a recent consensus is probably a server.
+
+   A node to which we have successfully extended circuits from
+   multiple origins is probably a server.
+
+Active detection:
+
+   If a node doesn't try to use server privileges at all, we never
+   need to care whether it's a server.
+
+   When a node or circuit tries to use server privileges, if it is
+   "definitely a client" as per above, we can refuse it immediately.
+
+   If it's "probably a server" as per above, we can accept it.
+
+   Otherwise, we have either a client, or a server that is neither
+   listed in any consensus or used by any other clients -- in other
+   words, a new or private server.
+
+   For these servers, we should attempt to build one or more test
+   circuits through them.  If enough of the circuits succeed, the
+   node is a real relay.  If not, it is probably a client.
+
+   While we are waiting for the test circuits to succeed, we should
+   allow a short grace period in which server privileges are
+   permitted.  When a test is done, we should remember its outcome
+   for a while, so we don't need to do it again.
+
+Why it's hard to do good testing:
+
+   Doing a test circuit starting with an unlisted router requires
+   only that we have an open connection for it.  Doing a test
+   circuit starting elsewhere _through_ an unlisted router--though
+   more reliable-- would require that we have a known address, port,
+   identity key, and onion key for the router.  Only the address and
+   identity key are easily available via the current Tor protocol in
+   all cases.
+
+   We could fix this part by requiring that all servers support
+   BEGIN_DIR and support downloading at least a current descriptor
+   for themselves.
+
+Open questions:
+
+   What are the thresholds for the needed numbers of circuits
+   for us to decide that a node is a relay?
+
+      [Suggested answer: two circuits from two distinct hosts.]
+
+   How do we pick grace periods?  How long do we remember the
+   outcome of a test?
+
+      [Suggested answer: 10 minute grace period; 48 hour memory of
+      test outcomes.]
+
+   If we can build circuits starting at a suspect node, but we don't
+   have enough information to try extending circuits elsewhere
+   through the node, should we conclude that the node is
+   "server-like" or not?
+
+      [Suggested answer: for now, just try making circuits through
+      the node.  Extend this to extending circuits as needed.]
+