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Proposal idea: User path configuration

Filename: xxx-user-path-config.txt
Title: Configuration options regarding circuit building
Author: Sebastian Hahn
Created: 01-March-2010
Status: Draft


This document outlines how Tor handles the user configuration options
   to influence the circuit building process.


Tor's treatment of the configuration *Nodes options was surprising to many users, and quite a few conspiracy theories have crept up. We should update our specification and code to better describe and communicate what is going during circuit building, and how we're honoring configuration. So far,
   we've been tracking a bugreport about this behaviour (
https://bugs.torproject.org/flyspray/index.php? do=details&id=1090 ) and
   Nick replied in a thread on or-talk (
http://archives.seul.org/or/talk/Feb-2010/msg00117.html ). This proposal
   tries to document our intention for those configuration options.


   Five configuration options are available to users to influence Tor's
circuit building. EntryNodes and ExitNodes define a list of nodes that are for the Entry/Exit position in all circuits. ExcludeNodes is a list of nodes that are used for no circuit, and ExcludeExitNodes is a list of nodes that aren't used as the last hop. StrictNodes defines Tor's behaviour in case of a conflict, for example when a node that is excluded is the only
   available introduction point. Setting StrictNodes to 1 breaks Tor's
functionality in that case, and it will refuse to build such a circuit.

Neither Nick's email nor bug 1090 have clear suggestions how we should
   behave in each case, so I tried to come up with something that made
   sense to me.

Security implications:

Deviating from normal circuit building can break one's anonymity, so the documentation of the above option should contain a warning to make users
   aware of the pitfalls.


It is proposed that the "User configuration" part of path-spec (section
   2.2.2) be replaced with this:

Users can alter the default behavior for path selection with configuration options. In case of conflicts (excluding and requiring the same node) the "StrictNodes" option is used to determine behaviour. If a nodes is both excluded and required via a configuration option, the exclusion takes

- If "ExitNodes" is provided, then every request requires an exit node on the ExitNodes list. If a request is supported by no nodes on that list, and "StrictNodes" is false, then Tor treats that request as if ExitNodes
     were not provided.

   - "EntryNodes" behaves analogously.

- If "ExcludeNodes" is provided, then no circuit uses any of the nodes
     listed. If a circuit requires an excluded node to be used, and
"StrictNodes" is false, then Tor uses the node in that position while
     not using any other of the excluded nodes.

   - If "ExcludeExitNodes" is provided, then Tor will not use the nodes
listed for the exit position in a circuit. If a circuit requires an
     excluded node to be used in the exit position and "StrictNodes" is
false, then Tor builds that circuit as if ExcludeExitNodes were not

   - If a user tries to connect to or resolve a hostname of the form
<target>.<servername>.exit and the "AllowDotExit" configuration option is set to 1, the request is rewritten to a request for <target>, and the request is only supported by the exit whose nickname or fingerprint is <servername>. If "AllowDotExit" is set to 0 (default), any request for
     <anything>.exit is denied.

- When any of the *Nodes settings are changed, all circuits are expired immediately, to prevent a situation where a previously built circuit
     is used even though some of its nodes are now excluded.


The old Strict*Nodes options are deprecated, and the StrictNodes option is
   new. Tor users may need to update their configuration file.