chapter04

Chapter IV: writing minimal torsf experiment

In this chapter we will replace the code written in the previous chapter that simulates running the torsf experiment with code that uses the ooni/probe-cli library to run the real experiment.

(This file is auto-generated from the corresponding source file, so make sure you don't edit it manually.)

Updating the imports

We need to update the imports of torsf.go first to look like this:

import (

These are standard library imports.

	"context"
	"path"
	"time"

As we have already seen, the model package defines the generic data model used by all experiments.

	"github.com/ooni/probe-cli/v3/internal/model"

The tracex package contains code used to format internal measurements representations to the OONI data format.

	"github.com/ooni/probe-cli/v3/internal/legacy/tracex"

The ptx package contains pluggable transport code. It includes code to dial with obfs4 and snowflake and code to create a pluggable transport listener.

	"github.com/ooni/probe-cli/v3/internal/ptx"

The tunnel package contains code to create a tunnel. We will use this package to start a tor tunnel, which executes the tor binary using specified command line arguments.

	"github.com/ooni/probe-cli/v3/internal/tunnel"
)

Rewriting the run method

Let us now rewrite the run method to run a real torsf test rather than just pretending to do it.

func (m *Measurer) run(ctx context.Context,
	sess model.ExperimentSession, testkeys *TestKeys, errch chan<- error) {

As a first step, we create a dialer for snowflake using the ptx package. This dialer will allow us to create a net.Conn-like network connection where traffic is sent using the Snowflake pluggable transport. There are several optional fields in SnowflakeDialer; the NewSnowflakeDialer constructor will give us a suitable configured dialer with default settings.

	sfdialer := ptx.NewSnowflakeDialer()

Let us now create a listener. The ptx.Listener is a listener that listens on a local port and speaks the SOCKS5 protocol. When tor connect to this port, the listener will forward the traffic to the Snowflake dialer we previously created. We will also use the session's logger to emit logging messages.

	ptl := &ptx.Listener{
		PTDialer: sfdialer,
		Logger:   sess.Logger(),
	}

Now we start the listener. This entails opening a port on the local host. If this operation fails, we return an error. In fact, a failure here means a hard error that prevented us from even starting the experiment. Therefore, it's consistent with the Run's expectations to return an error here.

	if err := ptl.Start(); err != nil {
		testkeys.Failure = tracex.NewFailure(err)
		errch <- err
		return
	}
	defer ptl.Stop()

Next, we start tor using the tunnel package. Note how we pass specific TorArgs that cause tor to know about the pluggable transport created by ptl and sfdialer.

	tun, _, err := tunnel.Start(ctx, &tunnel.Config{
		Name:      "tor",
		Session:   sess,
		TunnelDir: path.Join(sess.TempDir(), "torsf"),
		Logger:    sess.Logger(),
		TorArgs: []string{
			"UseBridges", "1",
			"ClientTransportPlugin", ptl.AsClientTransportPluginArgument(),
			"Bridge", sfdialer.AsBridgeArgument(),
		},
	})

In case of error, we convert err to a OONI failure using the NewFailure function of tracex. This function reduces Go error strings to the error strings used by OONI. You can read the errors spec at the github.com/ooni/spec repo.

Note that, in this case, we return nil to the caller, because a failure here is not a fundamental failure in running the experiment, but rather a possibly interesting anomaly.

	if err != nil {
		testkeys.Failure = tracex.NewFailure(err)
		errch <- nil
		return
	}

Otherwise, we successfully created a tor tunnel using Snowflake, so we just close the tunnel and record the bootstrap time.

	defer tun.Stop()
	testkeys.BootstrapTime = tun.BootstrapTime().Seconds()
	errch <- nil
}

Running the code

We can now run the code as follows to obtain:

$ go run ./experiment/torsf/chapter04 | tail -n 1 | jq
[...]
Jun 21 23:40:50.000 [notice] Bootstrapped 100% (done): Done
2021/06/21 23:40:50  info [100.0%] torsf experiment is finished
Jun 21 23:40:50.000 [notice] Catching signal TERM, exiting cleanly
{
  "data_format_version": "",
  "input": null,
  "measurement_start_time": "",
  "probe_asn": "",
  "probe_cc": "",
  "probe_network_name": "",
  "report_id": "",
  "resolver_asn": "",
  "resolver_ip": "",
  "resolver_network_name": "",
  "software_name": "",
  "software_version": "",
  "test_keys": {
    "bootstrap_time": 48.122813,
    "failure": null
  },
  "test_name": "",
  "test_runtime": 0,
  "test_start_time": "",
  "test_version": ""
}

Concluding remarks

Congratulations, we have now rewritten together (a simplified version of) the torsf experiment! In this journey, we have learned how experiments interact with the rest of OONI Probe, how they are typically organized, and how to use lower-level libraries to implement them.