1 Title: Configuration options regarding circuit building
2 Filename: 170-user-path-config.txt
9 This document outlines how Tor handles the user configuration
10 options to influence the circuit building process.
14 Tor's treatment of the configuration *Nodes options was surprising
15 to many users, and quite a few conspiracy theories have crept up. We
16 should update our specification and code to better describe and
17 communicate what is going during circuit building, and how we're
18 honoring configuration. So far, we've been tracking a bugreport
19 about this behaviour (
20 https://bugs.torproject.org/flyspray/index.php?do=details&id=1090 )
21 and Nick replied in a thread on or-talk (
22 http://archives.seul.org/or/talk/Feb-2010/msg00117.html ).
24 This proposal tries to document our intention for those configuration
29 Five configuration options are available to users to influence Tor's
30 circuit building. EntryNodes and ExitNodes define a list of nodes
31 that are for the Entry/Exit position in all circuits. ExcludeNodes
32 is a list of nodes that are used for no circuit, and
33 ExcludeExitNodes is a list of nodes that aren't used as the last
34 hop. StrictNodes defines Tor's behaviour in case of a conflict, for
35 example when a node that is excluded is the only available
36 introduction point. Setting StrictNodes to 1 breaks Tor's
37 functionality in that case, and it will refuse to build such a
40 Neither Nick's email nor bug 1090 have clear suggestions how we
41 should behave in each case, so I tried to come up with something
42 that made sense to me.
44 Security implications:
46 Deviating from normal circuit building can break one's anonymity, so
47 the documentation of the above option should contain a warning to
48 make users aware of the pitfalls.
52 It is proposed that the "User configuration" part of path-spec
53 (section 2.2.2) be replaced with this:
55 Users can alter the default behavior for path selection with
56 configuration options. In case of conflicts (excluding and requiring
57 the same node) the "StrictNodes" option is used to determine
58 behaviour. If a nodes is both excluded and required via a
59 configuration option, the exclusion takes preference.
61 - If "ExitNodes" is provided, then every request requires an exit
62 node on the ExitNodes list. If a request is supported by no nodes
63 on that list, and "StrictNodes" is false, then Tor treats that
64 request as if ExitNodes were not provided.
66 - "EntryNodes" behaves analogously.
68 - If "ExcludeNodes" is provided, then no circuit uses any of the
69 nodes listed. If a circuit requires an excluded node to be used,
70 and "StrictNodes" is false, then Tor uses the node in that
71 position while not using any other of the excluded nodes.
73 - If "ExcludeExitNodes" is provided, then Tor will not use the nodes
74 listed for the exit position in a circuit. If a circuit requires
75 an excluded node to be used in the exit position and "StrictNodes"
76 is false, then Tor builds that circuit as if ExcludeExitNodes were
79 - If a user tries to connect to or resolve a hostname of the form
80 <target>.<servername>.exit and the "AllowDotExit" configuration
81 option is set to 1, the request is rewritten to a request for
82 <target>, and the request is only supported by the exit whose
83 nickname or fingerprint is <servername>. If "AllowDotExit" is set
84 to 0 (default), any request for <anything>.exit is denied.
86 - When any of the *Nodes settings are changed, all circuits are
87 expired immediately, to prevent a situation where a previously
88 built circuit is used even though some of its nodes are now
94 The old Strict*Nodes options are deprecated, and the StrictNodes
95 option is new. Tor users may need to update their configuration file.