Tor network configuration
Download Tails 0.23 March 19, 2014
  1. Rationale
  2. Implementation

Rationale

Certain networks are restricted in ways that prevent Tor from connecting properly without further configuration. Examples are:

  • Egress port filtering (solved with ReachableAddresses).

  • The Internet is only reachable via a proxy (solved with the *Proxy options).

  • Tor is explicitly blocked (the Tor people like the term "censored"), which can be circumvented by using Tor bridges (solved with UseBridges, Bridge, and possibly ClientTransportPlugin).

In addition, bridges can be used to obfuscate Tor's traffic so that it becomes harder for adversaries to identify Tor usage, especially in combination with pluggable transports. This makes Tor (and hence Tails) safer to use in regions where it's dangerous for some reason, or in other situations where users want to hide that they use Tor. In Tails we think this is a highly useful use case of Tor bridges beyond its more publicly announced "censorship circumvention" usage.

In Tails we need a user-friendly and safe way for users to configure these options before Tor tries to connect to the public Tor network.

Implementation

In Tails Greeter the "Network Configuration" section contains a non-default option called "My Internet Connection is censored...". When activated, the following deviations from normal Tails behaviour occur, in order:

  1. Tails Greeter adds DisableNetwork 1 to torrc so Tor will not connect to the network without user intervention.

  2. When we connect to the network, a NetworkManager hook starts Tor Launcher in the background, i.e. non-blocking.

  3. Time syncing waits until the user has committed their configuration via Tor Launcher and then does its usual magic to ensure that Tor bootstraps even if the clock was incorrect. That is the reason why we have to take the more complex approach of starting Tor Launcher in parallel.

Scripts: