Fixing your Debian NAS from within initramfs-tools

The following article was contributed by John Cass. Note that John's instructions will also work on other NAS machines running Debian, such as QNAP devices.

I had a problem upgrading my Debian kernel to 2.6.32 on the D-Link DNS-323. The upgrade looked like it worked but on reboot I had a hung machine. I tried taking the disk out, putting it in my main machine and remaking the links in /boot to the previous kernel and initrd.img but it turns out that on the DNS-323 both kernel and initrd are actually stored in flash memory (the /boot disk files are where they are built and a useful archive but are not used during the boot process).

So I was stuck, and had to make a serial cable in order to find out what was going on. The instructions here and here were very useful and the CA-42 clone cost me 4 GBP on eBay and arrived within a couple of days. A delicate bit of soldering (and installing ckermit on my main machine) and I had a serial connection - I could finally see what was was happening.

The upshot was, during the boot process the DNS-323 failed to mount my root partition. This was because the partition was formatted ext3 and the initrd.img had not included the ext3 module. It probably did this because I had deliberately forced mounting it as ext2 in the /etc/fstab (in an attempt to limit the write access to the disk, I want it to stay in standby for a long time but that's another post). (Remark by Martin: this is a known issue with initramfs-tools.)

Anyway, having seen this, Martin was able to guide me to getting it fixed:

• First of all, I issued modprobe ext2 to get the ext2 module loaded (luckily this was included in initrd.img).
• Then you can mount the root partition on /root: mount -t ext2 /dev/sda2 /root - it will complain a bit but will work fine.
• Then run /scripts/init-bottom/udev - this moves /dev to within the new root directory.
• chroot /root /bin/sh - shift the root around so we are working from the proper root disk
• mount -t proc none /proc - mount the proc tree
• mount -t sys none /sys - and the sys tree
• nano /etc/fstab - edit this to change ext2 back to ext3 for the root partition
• mount -t ext2 /dev/sda1 /boot - mount the boot partition if it's a different device (mine is)
• Edit /etc/initramfs-tools/modules and add both ext2 and ext3 - this will force their inclusion in the updated initrd.img
• run update-initramfs -u to regenerate the kernel and initrd.img - this will rebuild the files in /boot and it will use flash-kernel to put them into the flash memory
• umount /proc /sys /dev /boot - unmount filesystems
• exit - leave the chroot
• umount /root - unmount the root partition
• reboot

And it should all be working!