Tor design proposals

Title: Configuration options regarding circuit building
Filename: 170-user-path-config.txt
Author: Sebastian Hahn
Created: 01-March-2010
Status: Superseded

Overview:

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

Motivation:

    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.

Design:

    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.

Specification:

    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 preference.

    - 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 provided.

    - 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.


Compatibility:

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