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Filename: 135-private-tor-networks.txt
Title: Simplify Configuration of Private Tor Networks
Version: $LastChangedRevision$
Last-Modified: $LastChangedDate$
Author: Karsten Loesing
Created: 29-Apr-2008
Status: Open
Change history:
29-Apr-2008 Initial proposal for or-dev
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: V3AuthInitialVotingInterval,
V3AuthInitialVoteDelay, and V3AuthInitialDistDelay.
As first safeguards, Tor should only accept configuration values for
V3AuthInitialVotingInterval 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 PrivateTorNetwork 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 DirAssumeRunningDelay with
a default value of 30 minutes that can be changed when running private
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
PrivateTorNetwork 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
EstimatedDescriptorPropagationTime 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. PrivateTorNetwork, 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 PrivateTorNetwork:
- 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.
V3AuthInitialVotingInterval 5 minutes
V3AuthInitialVoteDelay 20 seconds
V3AuthInitialDistDelay 20 seconds
Accelerate initial voting schedule until first consensus is reached.
DirAssumeRunningDelay 0 minutes
Consider routers as Running from the start of running an authority.
EstimatedDescriptorPropagationTime 0 minutes
Clients try downloading router descriptors from directory caches,
even when they are not 10 minutes old.
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