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+Title: Configuration options regarding circuit building
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+Filename: 170-user-path-config.txt
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+Author: Sebastian Hahn
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+Created: 01-March-2010
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+Status: Draft
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+
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+Overview:
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+
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+ This document outlines how Tor handles the user configuration
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+ options to influence the circuit building process.
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+
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+Motivation:
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+
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+ Tor's treatment of the configuration *Nodes options was surprising
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+ to many users, and quite a few conspiracy theories have crept up. We
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+ should update our specification and code to better describe and
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+ communicate what is going during circuit building, and how we're
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+ honoring configuration. So far, we've been tracking a bugreport
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+ about this behaviour (
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+ https://bugs.torproject.org/flyspray/index.php?do=details&id=1090 )
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+ and Nick replied in a thread on or-talk (
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+ http://archives.seul.org/or/talk/Feb-2010/msg00117.html ).
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+
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+ This proposal tries to document our intention for those configuration
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+ options.
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+
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+Design:
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+
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+ Five configuration options are available to users to influence Tor's
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+ circuit building. EntryNodes and ExitNodes define a list of nodes
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+ that are for the Entry/Exit position in all circuits. ExcludeNodes
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+ is a list of nodes that are used for no circuit, and
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+ ExcludeExitNodes is a list of nodes that aren't used as the last
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+ hop. StrictNodes defines Tor's behaviour in case of a conflict, for
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+ example when a node that is excluded is the only available
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+ introduction point. Setting StrictNodes to 1 breaks Tor's
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+ functionality in that case, and it will refuse to build such a
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+ circuit.
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+
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+ Neither Nick's email nor bug 1090 have clear suggestions how we
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+ should behave in each case, so I tried to come up with something
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+ that made sense to me.
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+
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+Security implications:
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+
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+ Deviating from normal circuit building can break one's anonymity, so
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+ the documentation of the above option should contain a warning to
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+ make users aware of the pitfalls.
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+
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+Specification:
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+
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+ It is proposed that the "User configuration" part of path-spec
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+ (section 2.2.2) be replaced with this:
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+
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+ Users can alter the default behavior for path selection with
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+ configuration options. In case of conflicts (excluding and requiring
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+ the same node) the "StrictNodes" option is used to determine
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+ behaviour. If a nodes is both excluded and required via a
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+ configuration option, the exclusion takes preference.
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+
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+ - If "ExitNodes" is provided, then every request requires an exit
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+ node on the ExitNodes list. If a request is supported by no nodes
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+ on that list, and "StrictNodes" is false, then Tor treats that
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+ request as if ExitNodes were not provided.
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+
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+ - "EntryNodes" behaves analogously.
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+
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+ - If "ExcludeNodes" is provided, then no circuit uses any of the
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+ nodes listed. If a circuit requires an excluded node to be used,
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+ and "StrictNodes" is false, then Tor uses the node in that
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+ position while not using any other of the excluded nodes.
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+
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+ - If "ExcludeExitNodes" is provided, then Tor will not use the nodes
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+ listed for the exit position in a circuit. If a circuit requires
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+ an excluded node to be used in the exit position and "StrictNodes"
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+ is false, then Tor builds that circuit as if ExcludeExitNodes were
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+ not provided.
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+
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+ - If a user tries to connect to or resolve a hostname of the form
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+ <target>.<servername>.exit and the "AllowDotExit" configuration
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+ option is set to 1, the request is rewritten to a request for
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+ <target>, and the request is only supported by the exit whose
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+ nickname or fingerprint is <servername>. If "AllowDotExit" is set
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+ to 0 (default), any request for <anything>.exit is denied.
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+
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+ - When any of the *Nodes settings are changed, all circuits are
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+ expired immediately, to prevent a situation where a previously
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+ built circuit is used even though some of its nodes are now
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+ excluded.
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+
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+
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+Compatibility:
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+
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+ The old Strict*Nodes options are deprecated, and the StrictNodes
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+ option is new. Tor users may need to update their configuration file.
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Sebastian Hahn