fix: enable HTTP/2 for CLI RDAP requests

cli.go

@@ -488,10 +488,15 @@ func RunCLI(args []string, stdout io.Writer, stderr io.Writer, options CLIOption
 	}
 
 	// Custom HTTP client. Used to disable TLS certificate verification.
-	transport := &http.Transport{
-		Proxy:           http.ProxyFromEnvironment,
-		TLSClientConfig: tlsConfig,
-	}
+	// Clone keeps the defaults (proxy, timeouts, HTTP/2) without mutating the
+	// shared global. HTTP/2 matters: some servers, including TWNIC, reject
+	// HTTP/1.1 with 426 Upgrade Required.
+	transport := &http.Transport{Proxy: http.ProxyFromEnvironment, ForceAttemptHTTP2: true}
+	// Embedders may have replaced the global with a non-*http.Transport.
+	if t, ok := http.DefaultTransport.(*http.Transport); ok {
+		transport = t.Clone()
+	}
+	transport.TLSClientConfig = tlsConfig
 
 	// Setup http.RoundTripper for http clients
 	bs.HTTP = &http.Client{

cli_test.go

@@ -39,6 +39,47 @@ func fixtureServer(t *testing.T) *httptest.Server {
 	return srv
 }
 
+func http2FixtureServer(t *testing.T) *httptest.Server {
+	t.Helper()
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("/domain/example.cz", func(w http.ResponseWriter, r *http.Request) {
+		if r.ProtoMajor != 2 {
+			w.WriteHeader(http.StatusUpgradeRequired)
+			return
+		}
+
+		w.Header().Set("Content-Type", "application/rdap+json")
+		_, _ = w.Write(test.LoadFile("rdap/rdap.nic.cz/domain-example.cz.json"))
+	})
+
+	srv := httptest.NewUnstartedServer(mux)
+	srv.EnableHTTP2 = true
+	srv.StartTLS()
+	t.Cleanup(srv.Close)
+
+	return srv
+}
+
+func TestRunCLIUsesHTTP2WhenTLSConfigIsCustomised(t *testing.T) {
+	srv := http2FixtureServer(t)
+
+	var stdout, stderr bytes.Buffer
+	code := RunCLI([]string{
+		"--insecure",
+		"--cache-dir", "",
+		"--server", srv.URL,
+		"example.cz",
+	}, &stdout, &stderr, CLIOptions{})
+
+	if code != 0 {
+		t.Fatalf("RunCLI() exit code = %d, want 0; stderr = %s", code, stderr.String())
+	}
+	if !strings.Contains(stdout.String(), "Domain Name: example.cz") {
+		t.Errorf("RunCLI() output does not contain the decoded domain: %s", stdout.String())
+	}
+}
+
 // TestRunCLIGolden drives RunCLI end-to-end for a range of commands and output
 // modes, comparing exit code, stdout, and stderr against committed golden files.
 // Network-backed cases point --server at an in-process fixture server and use an