feat(transfer): add DisableASCIIConversion to skip CRLF/LF rewriting

driver.go

@@ -328,6 +328,12 @@ type Settings struct {
 	EnableCOMB              bool         // Enable COMB support
 	DeflateCompressionLevel int          // Deflate compression level (0-9). 0 means disabled
 	DefaultTransferType     TransferType // Transfer type to use if the client don't send the TYPE command
+	// DisableASCIIConversion disables CRLF/LF line-ending conversion for TYPE A (ASCII) transfers.
+	// When true, files are transferred byte-for-byte even in ASCII mode, and SIZE is allowed in
+	// ASCII mode. This is not RFC 959-compliant but matches the behavior of vsftpd's
+	// ascii_download_enable/ascii_upload_enable=NO and is useful when clients do not want the
+	// server to modify their files.
+	DisableASCIIConversion bool
 	// ActiveConnectionsCheck defines the security requirements for active connections
 	ActiveConnectionsCheck DataConnectionRequirement
 	// PasvConnectionsCheck defines the security requirements for passive connections

handle_files.go

@@ -140,7 +140,7 @@ func (c *clientHandler) doFileTransfer(transferConn io.ReadWriter, file io.ReadW
 		writer = transferConn
 	}
 
-	if c.currentTransferType == TransferTypeASCII {
+	if c.currentTransferType == TransferTypeASCII && !c.server.settings.DisableASCIIConversion {
 		reader = newASCIIConverter(reader, conversionMode)
 	}
 
@@ -396,9 +396,12 @@ func (c *clientHandler) handleRNTO(param string) error {
 // the current TYPE is ASCII.
 // However, clients in general should not be resuming downloads
 // in ASCII mode. Resuming downloads in binary mode is the
-// recommended way as specified in RFC-3659
+// recommended way as specified in RFC-3659.
+// When Settings.DisableASCIIConversion is true, no conversion is
+// performed and the on-disk size matches the transfer size, so
+// SIZE is allowed in ASCII mode.
 func (c *clientHandler) handleSIZE(param string) error {
-	if c.currentTransferType == TransferTypeASCII {
+	if c.currentTransferType == TransferTypeASCII && !c.server.settings.DisableASCIIConversion {
 		c.writeMessage(StatusActionNotTaken, "SIZE not allowed in ASCII mode")
 
 		return nil

transfer_test.go

@@ -1036,6 +1036,62 @@ func TestASCIITransfersInvalidFiles(t *testing.T) {
 	require.Equal(t, localHash, remoteHash)
 }
 
+func TestASCIITransfersWithConversionDisabled(t *testing.T) {
+	driver := &TestServerDriver{
+		Settings: &Settings{DisableASCIIConversion: true},
+	}
+	s := NewTestServerWithTestDriver(t, driver)
+	conf := goftp.Config{
+		User:     authUser,
+		Password: authPass,
+	}
+	client, err := goftp.DialConfig(conf, s.Addr())
+	require.NoError(t, err, "Couldn't connect")
+
+	defer func() { require.NoError(t, client.Close()) }()
+
+	raw, err := client.OpenRawConn()
+	require.NoError(t, err)
+
+	defer func() { require.NoError(t, raw.Close()) }()
+
+	file, err := os.CreateTemp("", "ftpserver")
+	require.NoError(t, err)
+
+	defer func() { require.NoError(t, file.Close()) }()
+
+	// A mix of CRLF and lone LF that the converter would normally rewrite.
+	contents := []byte("line1\r\n\r\nline3\r\n,line4\nline5")
+	_, err = file.Write(contents)
+	require.NoError(t, err)
+
+	returnCode, response, err := raw.SendCommand("TYPE A")
+	require.NoError(t, err)
+	require.Equal(t, StatusOK, returnCode, response)
+
+	_, err = file.Seek(0, io.SeekStart)
+	require.NoError(t, err)
+
+	ftpUploadWithRawConnection(t, raw, file, "file.txt", nil)
+
+	// On disk size must match the input exactly: no \r stripping on upload.
+	files, err := client.ReadDir("/")
+	require.NoError(t, err)
+	require.Len(t, files, 1)
+	require.Equal(t, int64(len(contents)), files[0].Size())
+
+	// SIZE must be allowed in ASCII mode when conversion is disabled.
+	returnCode, response, err = raw.SendCommand("SIZE file.txt")
+	require.NoError(t, err)
+	require.Equal(t, StatusFileStatus, returnCode, response)
+	require.Equal(t, strconv.Itoa(len(contents)), response)
+
+	// Round-trip hash must match: no line-ending mangling on download either.
+	remoteHash := ftpDownloadAndHashWithRawConnection(t, raw, "file.txt", nil)
+	localHash := hashFile(t, file)
+	require.Equal(t, localHash, remoteHash)
+}
+
 func TestPASVWrappedListenerError(t *testing.T) {
 	server := NewTestServerWithTestDriver(t, &TestServerDriver{
 		Debug:              true,