feat: tlsmiddlebox experiment (#817)

See https://github.com/ooni/probe/issues/2124

internal/engine/experiment/tlsmiddlebox/config.go

@@ -0,0 +1,79 @@
+package tlsmiddlebox
+
+//
+// Config for the tlsmiddlebox experiment
+//
+
+import (
+	"net/url"
+	"time"
+)
+
+// Config contains the experiment configuration.
+type Config struct {
+	// ResolverURL is the default DoH resolver
+	ResolverURL string `ooni:"URL for DoH resolver"`
+
+	// SNIPass is the SNI value we don't expect to be blocked
+	SNIControl string `ooni:"control SNI value for testhelper"`
+
+	// Delay is the delay between each iteration (in milliseconds).
+	Delay int64 `ooni:"delay between consecutive iterations"`
+
+	// MaxTTL is the default number of interations we trace
+	MaxTTL int64 `ooni:"maximum TTL value to iterate upto"`
+
+	// TestHelper is the testhelper host for iterative tracing
+	TestHelper string `ooni:"testhelper URL to use for tracing"`
+
+	// ClientId is the client fingerprint to use
+	ClientId int `ooni:"ClientHello fingerprint to use"`
+}
+
+func (c Config) resolverURL() string {
+	if c.ResolverURL != "" {
+		return c.ResolverURL
+	}
+	return "https://mozilla.cloudflare-dns.com/dns-query"
+}
+
+func (c Config) snicontrol() string {
+	if c.SNIControl != "" {
+		return c.SNIControl
+	}
+	return "example.com"
+}
+
+func (c Config) delay() time.Duration {
+	if c.Delay > 0 {
+		return time.Duration(c.Delay) * time.Millisecond
+	}
+	return 100 * time.Millisecond
+}
+
+func (c Config) maxttl() int64 {
+	if c.MaxTTL > 0 {
+		return c.MaxTTL
+	}
+	return 20
+}
+
+func (c Config) testhelper(address string) (URL *url.URL, err error) {
+	// TODO(DecFox, bassosimone): We want to replace this with a generic input parser
+	// Issue: https://github.com/ooni/probe/issues/2239
+	if c.TestHelper != "" {
+		return url.Parse(c.TestHelper)
+	}
+	URL = &url.URL{
+		Host:   address,
+		Scheme: "tlshandshake",
+	}
+	return
+}
+
+func (c Config) clientid() int {
+	if c.ClientId > 0 {
+		return c.ClientId
+	}
+	return 0
+}