geolocate: add cloudflare-based IP lookup (#676)

Cloudflare hosted services provide a certain service of `/cdn-cgi/trace` with their base url (for example, `www.cloudflare.com` or `www.nginx.com`), which can be used to obtain `ip` in the probe's `geolocate` feature.

The same feature was added in this pr, hence, increasing the number of `baseURL`s in `geolocate`.

Co-authored-by: Simone Basso <bassosimone@gmail.com>

internal/engine/geolocate/cloudflare.go

@@ -0,0 +1,33 @@
+package geolocate
+
+import (
+	"context"
+	"net/http"
+	"regexp"
+	"strings"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/httpheader"
+	"github.com/ooni/probe-cli/v3/internal/httpx"
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+func cloudflareIPLookup(
+	ctx context.Context,
+	httpClient *http.Client,
+	logger model.Logger,
+	userAgent string,
+) (string, error) {
+	data, err := (&httpx.APIClientTemplate{
+		BaseURL:    "https://www.cloudflare.com",
+		HTTPClient: httpClient,
+		Logger:     logger,
+		UserAgent:  httpheader.CLIUserAgent(),
+	}).WithBodyLogging().Build().FetchResource(ctx, "/cdn-cgi/trace")
+	if err != nil {
+		return DefaultProbeIP, err
+	}
+	r := regexp.MustCompile("(?:ip)=(.*)")
+	ip := strings.Trim(string(r.Find(data)), "ip=")
+	logger.Debugf("cloudflare: body: %s", ip)
+	return ip, nil
+}

internal/engine/geolocate/cloudflare_test.go

@@ -0,0 +1,26 @@
+package geolocate
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/httpheader"
+)
+
+func TestIPLookupWorksUsingcloudlflare(t *testing.T) {
+	ip, err := cloudflareIPLookup(
+		context.Background(),
+		http.DefaultClient,
+		log.Log,
+		httpheader.UserAgent(),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if net.ParseIP(ip) == nil {
+		t.Fatalf("not an IP address: '%s'", ip)
+	}
+}

internal/engine/geolocate/iplookup.go

@@ -40,6 +40,10 @@ var (
 			name: "avast",
 			fn:   avastIPLookup,
 		},
+		{
+			name: "cloudflare",
+			fn:   cloudflareIPLookup,
+		},
 		{
 			name: "ipconfig",
 			fn:   ipConfigIPLookup,