The Tor network, which is free and open source, provides its users with online anonymity, privacy, and censorship circumvention. Tor software is designed to bounce communications around a distributed network of relays run by volunteers around the world, thereby hiding its users’ IP addresses and enabling them to circumvent online tracking and internet censorship.
In an attempt to limit online anonymity and circumvention, some governments have blocked access to Tor. The Tor Project has therefore created Tor bridges, enabling the circumvention of Tor blocking. Tor Browser – the main way that Tor is used by millions of users around the world – is shipped with a set of public bridges (called obfs4) that users in censored environments can enable.
The OONI Probe Tor test provides an automated way of examining whether Tor works in a tested network.
To this end, this test examines the reachability of a set of services that allow Tor to work. These include the following services:
Tor directory authorities (used by Tor relays)
OR port (used by Tor bridges)
OR port of directory authorities (used by Tor clients)
obfs4 (Tor bridge that speaks the OBFS4 protocol)
By measuring the reachability of selected Tor directory authorities and bridges, this test evaluates whether they can be used within the tested network.
More specifically, this test attempts to perform the following actions from the vantage point of the user:
HTTP GET request to the
tor/status-vote/current/consensus.z resource, which
is required for Tor directory authorities;
Connect to OR ports and OR ports of directory authorities and perform a TLS handshake;
Connect to obfs4 addresses and perform an OBFS4 handshake (an obfuscated handshake).
If the test succeeds in performing all of the above, Tor may work on this network (unless if it’s blocked in ways that aren’t being measured).
If the test fails in performing any of the above, Tor does not work in the tested network and may be blocked.
Read the Tor test specification.