add settings to disable server STAT and SYST

driver.go

@@ -186,5 +186,7 @@ type Settings struct {
 	DisableSite              bool             // Disable SITE command
 	DisableActiveMode        bool             // Disable Active FTP
 	EnableHASH               bool             // Enable support for calculating hash value of files
+	DisableSTAT              bool             // Disable Server STATUS, STAT on files and directories will still work
+	DisableSYST              bool             // Disable SYST
 	EnableCOMB               bool             // Enable COMB support
 }

handle_auth_test.go

@@ -64,6 +64,11 @@ func TestLoginSuccess(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, StatusSystemType, rc)
 	require.Equal(t, "UNIX Type: L8", response)
+
+	s.settings.DisableSYST = true
+	rc, response, err = raw.SendCommand("SYST")
+	require.NoError(t, err)
+	require.Equal(t, StatusCommandNotImplemented, rc, response)
 }
 
 func TestLoginFailure(t *testing.T) {

handle_misc.go

@@ -40,7 +40,13 @@ func (c *clientHandler) handlePBSZ() error {
 }
 
 func (c *clientHandler) handleSYST() error {
+	if c.server.settings.DisableSYST {
+		c.writeMessage(StatusCommandNotImplemented, "SYST is disabled")
+		return nil
+	}
+
 	c.writeMessage(StatusSystemType, "UNIX Type: L8")
+
 	return nil
 }
 
@@ -80,6 +86,14 @@ func (c *clientHandler) handleSITE() error {
 }
 
 func (c *clientHandler) handleSTATServer() error {
+	if c.server.settings.DisableSTAT {
+		c.writeMessage(StatusCommandNotImplemented, "STAT is disabled")
+		return nil
+	}
+
+	// drakkan(2020-12-17): we don't handle STAT properly,
+	// we should return the status for all the transfers and we should allow
+	// stat while a transfer is in progress, see RFC 959
 	defer c.multilineAnswer(StatusSystemStatus, "Server status")()
 
 	duration := time.Now().UTC().Sub(c.connectedAt)

handle_misc_test.go

@@ -88,6 +88,12 @@ func TestStat(t *testing.T) {
 	count := strings.Count(str, "\n")
 	require.GreaterOrEqual(t, count, 4)
 	require.NotEqual(t, ' ', str[0])
+
+	s.settings.DisableSTAT = true
+
+	rc, str, err = raw.SendCommand("STAT")
+	require.NoError(t, err)
+	require.Equal(t, StatusCommandNotImplemented, rc, str)
 }
 
 func TestCLNT(t *testing.T) {