Filename: 135-private-tor-networks.txt
Title: Simplify Configuration of Private Tor Networks
Author: Karsten Loesing
Created: 29-Apr-2008
Status: Closed
Target: 0.2.1.x
Implemented-In: 0.2.1.2-alpha

Change history:

  29-Apr-2008  Initial proposal for or-dev
  19-May-2008  Included changes based on comments by Nick to or-dev and
               added a section for test cases.
  18-Jun-2008  Changed testing-network-only configuration option names.

Overview:

  Configuring a private Tor network has become a time-consuming and
  error-prone task with the introduction of the v3 directory protocol. In
  addition to that, operators of private Tor networks need to set an
  increasing number of non-trivial configuration options, and it is hard
  to keep FAQ entries describing this task up-to-date. In this proposal we
  (1) suggest to (optionally) accelerate timing of the v3 directory voting
  process and (2) introduce an umbrella config option specifically aimed at
  creating private Tor networks.

Design:

  1. Accelerate Timing of v3 Directory Voting Process

  Tor has reasonable defaults for setting up a large, Internet-scale
  network with comparably high latencies and possibly wrong server clocks.
  However, those defaults are bad when it comes to quickly setting up a
  private Tor network for testing, either on a single node or LAN (things
  might be different when creating a test network on PlanetLab or
  something). Some time constraints should be made configurable for private
  networks. The general idea is to accelerate everything that has to do
  with propagation of directory information, but nothing else, so that a
  private network is available as soon as possible. (As a possible
  safeguard, changing these configuration values could be made dependent on
  the umbrella configuration option introduced in 2.)

  1.1. Initial Voting Schedule

  When a v3 directory does not know any consensus, it assumes an initial,
  hard-coded VotingInterval of 30 minutes, VoteDelay of 5 minutes, and
  DistDelay of 5 minutes. This is important for multiple, simultaneously
  restarted directory authorities to meet at a common time and create an
  initial consensus. Unfortunately, this means that it may take up to half
  an hour (or even more) for a private Tor network to bootstrap.

  We propose to make these three time constants configurable (note that
  V3AuthVotingInterval, V3AuthVoteDelay, and V3AuthDistDelay do not have an
  effect on the _initial_ voting schedule, but only on the schedule that a
  directory authority votes for). This can be achieved by introducing three
  new configuration options: TestingV3AuthInitialVotingInterval,
  TestingV3AuthInitialVoteDelay, and TestingV3AuthInitialDistDelay.

  As first safeguards, Tor should only accept configuration values for
  TestingV3AuthInitialVotingInterval that divide evenly into the default
  value of 30 minutes. The effect is that even if people misconfigured
  their directory authorities, they would meet at the default values at the
  latest. The second safeguard is to allow configuration only when the
  umbrella configuration option TestingTorNetwork is set.

  1.2. Immediately Provide Reachability Information (Running flag)

  The default behavior of a directory authority is to provide the Running
  flag only after the authority is available for at least 30 minutes. The
  rationale is that before that time, an authority simply cannot deliver
  useful information about other running nodes. But for private Tor
  networks this may be different. This is currently implemented in the code
  as:

  /** If we've been around for less than this amount of time, our
   * reachability information is not accurate. */
  #define DIRSERV_TIME_TO_GET_REACHABILITY_INFO (30*60)

  There should be another configuration option
  TestingAuthDirTimeToLearnReachability with a default value of 30 minutes
  that can be changed when running testing Tor networks, e.g. to 0 minutes.
  The configuration value would simply replace the quoted constant. Again,
  changing this option could be safeguarded by requiring the umbrella
  configuration option TestingTorNetwork to be set.

  1.3. Reduce Estimated Descriptor Propagation Time

  Tor currently assumes that it takes up to 10 minutes until router
  descriptors are propagated from the authorities to directory caches.
  This is not very useful for private Tor networks, and we want to be able
  to reduce this time, so that clients can download router descriptors in a
  timely manner.

  /** Clients don't download any descriptor this recent, since it will
   * probably not have propagated to enough caches. */
  #define ESTIMATED_PROPAGATION_TIME (10*60)

  We suggest to introduce a new config option
  TestingEstimatedDescriptorPropagationTime which defaults to 10 minutes,
  but that can be set to any lower non-negative value, e.g. 0 minutes. The
  same safeguards as in 1.2 could be used here, too.

  2. Umbrella Option for Setting Up Private Tor Networks

  Setting up a private Tor network requires a number of specific settings
  that are not required or useful when running Tor in the public Tor
  network. Instead of writing down these options in a FAQ entry, there
  should be a single configuration option, e.g. TestingTorNetwork, that
  changes all required settings at once. Newer Tor versions would keep the
  set of configuration options up-to-date. It should still remain possible
  to manually overwrite the settings that the umbrella configuration option
  affects.

  The following configuration options are set by TestingTorNetwork:

  - ServerDNSAllowBrokenResolvConf 1
      Ignore the situation that private relays are not aware of any name
      servers.

  - DirAllowPrivateAddresses 1
      Allow router descriptors containing private IP addresses.

  - EnforceDistinctSubnets 0
      Permit building circuits with relays in the same subnet.

  - AssumeReachable 1
      Omit self-testing for reachability.

  - AuthDirMaxServersPerAddr 0
  - AuthDirMaxServersPerAuthAddr 0
      Permit an unlimited number of nodes on the same IP address.

  - ClientDNSRejectInternalAddresses 0
      Believe in DNS responses resolving to private IP addresses.

  - ExitPolicyRejectPrivate 0
      Allow exiting to private IP addresses. (This one is a matter of
      taste---it might be dangerous to make this a default in a private
      network, although people setting up private Tor networks should know
      what they are doing.)

  - V3AuthVotingInterval 5 minutes
  - V3AuthVoteDelay 20 seconds
  - V3AuthDistDelay 20 seconds
      Accelerate voting schedule after first consensus has been reached.

  - TestingV3AuthInitialVotingInterval 5 minutes
  - TestingV3AuthInitialVoteDelay 20 seconds
  - TestingV3AuthInitialDistDelay 20 seconds
      Accelerate initial voting schedule until first consensus is reached.

  - TestingAuthDirTimeToLearnReachability 0 minutes
      Consider routers as Running from the start of running an authority.

  - TestingEstimatedDescriptorPropagationTime 0 minutes
      Clients try downloading router descriptors from directory caches,
      even when they are not 10 minutes old.

  In addition to changing the defaults for these configuration options,
  TestingTorNetwork can only be set when a user has manually configured
  DirServer lines.

Test:

  The implementation of this proposal must pass the following tests:

  1. Set TestingTorNetwork and see if dependent configuration options are
     correctly changed.

     tor DataDirectory . ControlPort 9051 TestingTorNetwork 1 DirServer \
       "mydir 127.0.0.1:1234 0000000000000000000000000000000000000000"
     telnet 127.0.0.1 9051
     AUTHENTICATE
     GETCONF TestingTorNetwork TestingAuthDirTimeToLearnReachability
     250-TestingTorNetwork=1
     250 TestingAuthDirTimeToLearnReachability=0
     QUIT

  2. Set TestingTorNetwork and a dependent configuration value to see if
     the provided value is used for the dependent option.

     tor DataDirectory . ControlPort 9051 TestingTorNetwork 1 DirServer \
       "mydir 127.0.0.1:1234 0000000000000000000000000000000000000000" \
       TestingAuthDirTimeToLearnReachability 5
     telnet 127.0.0.1 9051
     AUTHENTICATE
     GETCONF TestingTorNetwork TestingAuthDirTimeToLearnReachability
     250-TestingTorNetwork=1
     250 TestingAuthDirTimeToLearnReachability=5
     QUIT

  3. Start with TestingTorNetwork set and change a dependent configuration
     option later on.

     tor DataDirectory . ControlPort 9051 TestingTorNetwork 1 DirServer \
       "mydir 127.0.0.1:1234 0000000000000000000000000000000000000000"
     telnet 127.0.0.1 9051
     AUTHENTICATE
     SETCONF TestingAuthDirTimeToLearnReachability=5
     GETCONF TestingAuthDirTimeToLearnReachability
     250 TestingAuthDirTimeToLearnReachability=5
     QUIT

  4. Start with TestingTorNetwork set and a dependent configuration value,
     and reset that dependent configuration value. The result should be
     the testing-network specific default value.

     tor DataDirectory . ControlPort 9051 TestingTorNetwork 1 DirServer \
       "mydir 127.0.0.1:1234 0000000000000000000000000000000000000000" \
       TestingAuthDirTimeToLearnReachability 5
     telnet 127.0.0.1 9051
     AUTHENTICATE
     GETCONF TestingAuthDirTimeToLearnReachability
     250 TestingAuthDirTimeToLearnReachability=5
     RESETCONF TestingAuthDirTimeToLearnReachability
     GETCONF TestingAuthDirTimeToLearnReachability
     250 TestingAuthDirTimeToLearnReachability=0
     QUIT

  5. Leave TestingTorNetwork unset and check if dependent configuration
     options are left unchanged.

     tor DataDirectory . ControlPort 9051 DirServer \
       "mydir 127.0.0.1:1234 0000000000000000000000000000000000000000"
     telnet 127.0.0.1 9051
     AUTHENTICATE
     GETCONF TestingTorNetwork TestingAuthDirTimeToLearnReachability
     250-TestingTorNetwork=0
     250 TestingAuthDirTimeToLearnReachability=1800
     QUIT

  6. Leave TestingTorNetwork unset, but set dependent configuration option
     which should fail.

     tor DataDirectory . ControlPort 9051 DirServer \
       "mydir 127.0.0.1:1234 0000000000000000000000000000000000000000" \
       TestingAuthDirTimeToLearnReachability 0
     [warn] Failed to parse/validate config:
     TestingAuthDirTimeToLearnReachability may only be changed in testing
     Tor networks!

  7. Start with TestingTorNetwork unset and change dependent configuration
     option later on which should fail.

     tor DataDirectory . ControlPort 9051 DirServer \
       "mydir 127.0.0.1:1234 0000000000000000000000000000000000000000"
     telnet 127.0.0.1 9051
     AUTHENTICATE
     SETCONF TestingAuthDirTimeToLearnReachability=0
     513 Unacceptable option value: TestingAuthDirTimeToLearnReachability
     may only be changed in testing Tor networks!

  8. Start with TestingTorNetwork unset and set it later on which should
     fail.

     tor DataDirectory . ControlPort 9051 DirServer \
       "mydir 127.0.0.1:1234 0000000000000000000000000000000000000000"
     telnet 127.0.0.1 9051
     AUTHENTICATE
     SETCONF TestingTorNetwork=1
     553 Transition not allowed: While Tor is running, changing
     TestingTorNetwork is not allowed.

  9. Start with TestingTorNetwork set and unset it later on which should
     fail.

     tor DataDirectory . ControlPort 9051 TestingTorNetwork 1 DirServer \
       "mydir 127.0.0.1:1234 0000000000000000000000000000000000000000"
     telnet 127.0.0.1 9051
     AUTHENTICATE
     RESETCONF TestingTorNetwork
     513 Unacceptable option value: TestingV3AuthInitialVotingInterval may
     only be changed in testing Tor networks!

 10. Set TestingTorNetwork, but do not provide an alternate DirServer
     which should fail.

     tor DataDirectory . ControlPort 9051 TestingTorNetwork 1
     [warn] Failed to parse/validate config: TestingTorNetwork may only be
     configured in combination with a non-default set of DirServers.

