Recovery mode of the QNAP TS-41x/TS-42x

QNAP TS-41x/TS-42x devices have a recovery mode that can be used when there is a problem with your installation of Debian that renders your device unbootable. The system recovery mode allows you write a recovery image to flash via the network using the TFTP protocol. This pages describes how how to create recovery images and how to use the recovery mode. As an alternative to the instructions on this page, you can use a Live CD provided by QNAP.

Creating recovery images

In order to create a recovery image for your QNAP TS-41x/TS-42x, you have to take an exact copy of your flash memory. That is, the recovery image consists of the following parts of your flash in this order: mtd0, mtd4, mtd5, mtd1, mtd2, mtd3. You may wonder about this strange order but this is the way in which the data is physically stored in the flash memory. During recovery mode, mtd0 (the boot loader), mtd4 (the boot loader configuration) and on some devices mtd5 (device configuration) are ignored and the other parts of flash are overwritten with the data from your recovery image. In order to make a valid QNAP recovery image, you therefore have to put all mtd partitions into one file in the order described above.

Making a recovery image of the QNAP backup

Before installing Debian, you should have made a backup of the QNAP firmware and have several mtdX files. You can now make a recovery image out of the backup with this command:

cat mtd0 mtd4 mtd5 mtd1 mtd2 mtd3 > F_TS-419P_qnap

Making a recovery image from a running system

You can easily make a valid recovery image with the following command:

modprobe mtdblock
cat /dev/mtdblock0 /dev/mtdblock4 /dev/mtdblock5 /dev/mtdblock1 /dev/mtdblock2 /dev/mtdblock3 > F_TS-419P_debian

It is recommended to create periodic recovery images of your flash, so you have a recovery image in case something goes wrong.

Making a recovery image containing the Debian installer

If you want to make a QNAP recovery image containing the Debian installer, you can follow these steps.

Since there are two different kernel variants for QNAP devices, you first have to determine which variant you need. You can do this by calling this script:

/usr/share/flash-kernel/dtb-probe/kirkwood-qnap

Alternatively, you can look up the CPU model in the table listing all supported QNAP devices.

If the output is kirkwood-ts419-6281.dtb, you need the kernel-6281 file. If the output is kirkwood-ts419-6282.dtb, you need the kernel-6282 file.

You also have to download the initrd of the installer (make sure the initrd does not get uncompressed when you download it).

You now have to pad the kernel so it is the right size:

dd if=kernel628x of=kernel.pad ibs=2097152 conv=sync

Finally, you can create a recovery image:

cat mtd0 mtd4 mtd5 kernel.pad initrd mtd3 > F_TS-419P_di

Note that the mtdX files in this command refer to the backup you created earlier (before the installation).

Using the recovery mode

In case your Debian system no longer boots after a system upgrade, you can use the QNAP system recovery mode to restore a previous image, an image of the Debian installer, or the QNAP firmware.

The recovery mode will first request an IP address via DHCP and then request a recovery image by TFTP from a server on your network. You have to configure your DHCP server so it will request the right file from the right machine. In particular, you have to create an entry for your QNAP that specifies filename and next-server. The latter refers to the IP address of your TFTP server (likely the same as your DHCP server). The filename of the recovery image has to start with a specific string depending on your machine (otherwise the machine won't load the image!). You also have to install a TFTP server (such as tftpd-hpa) to serve the file.

You can check the following table to find out which string to use:

One some devices (but not on the TS-419P+), you can check the backup you made of the flash to find out what the right string is for your machine:

strings mtd4 | grep bootp_vendor_class

Please note that by default the MAC address used by your QNAP device in the recovery mode is different to the one used while running the QNAP firmware. When you install Debian, a script is run to ensure that the same MAC address is used everywhere. Therefore, depending on whether you ran that script, the MAC address will either be that from the QNAP firmware (the one printed on your device; it probably starts with 00:08:9B) or an address starting with 00:50:43. You can run the recovery mode and then look for DHCPDISCOVER messages in /var/log/syslog to find out the correct MAC address.

Once you have the right MAC address, you can configure your DHCP server. I used the following entry in /etc/dhcp/dhcpd.conf:

host ts419p {
    hardware ethernet 00:50:43:3c:3b:5d;
    filename "F_TS-419P_debian";
    fixed-address 192.168.1.71;
    next-server 192.168.1.2;
}

Reload your DHCP server so the configuration change will take effect.

You can activate the recovery mode by pressing the reset button on the back of your QNAP for about 10 seconds. I suggest you press the reset button and keep it pressed, and then turn on your QNAP device. After about 10 seconds, you will hear two short beeps. Your QNAP will now request a recovery image via TFTP. When it has obtained the recovery image and written it to flash (which takes about 3.5 minutes), your QNAP will make two short beeps again and restart.