Diskless network boot NBD root

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Boot from a NBD root device

This article will explain how to boot an ArchLinux Installation from a Network Block Device (NBD).

Much of the work to be done is based on the article Diskless network boot NFS root, so this will be referenced several times within the article.

Advantages over NFS

The main advantages are that NBD is faster and that you can boot from an encrypted or LVM-based NBD root device. One disadvantage is that you cannot easily update your kernel from within the running diskless client, although there is a workaround for this.

Server-Side Setup

Create the NBD File and Boot Directory

Create a directory that will hold the boot directory and the NBD file.

mkdir -p /nbd/boot/

Next, create the actual file that will be shared via NBD. Of course you can also use an actual block device (a hard drive) instead of creating a file on your filesystem. Just replace /nbd/root with the block device. In this example we are going to create a file with a size of 5GB.

dd if=/dev/zero of=/nbd/root bs=1M count=5000

Now you can create a filesystem on the file.

mkfs.ext4 /nbd/root

mkfs will show you warning about the fact that the file is no actual block device. You can ignore this and simply press y to continue.

Alternatively, if you want to create an encrypted NBD device:

cryptsetup luksFormat -s 256 /nbd/root
cryptsetup luksOpen /nbd/root nbdcrypt
mkfs.ext4 /dev/mapper/nbdcrypt
Note: Be aware that the rest of the article will use /nbd/root. If your NBD file is encrypted, replace it with /dev/mapper/nbdcrypt, if you use an actual block device, with /dev/sdX.

Install ArchLinux on the NBD filesystem

Mount the filesystem:

mount /nbd/root /mnt

Now follow the instructions here, but be aware of three things:

  1. make sure you use /mnt instead of /disklessroot
  2. you are going to have to install the mkinitcpio-nbd package from AUR before recreating the kernel image
  3. the editing of /mnt/etc/mkinitcpio.conf is different for NBD

Installing mkinitcpio-nbd

Editing mkinitcpio.conf

Set the following hook list in /mnt/etc/mkinitcpio.conf:

HOOKS="base udev net nbd filesystems"

If you use an encrypted NBD device, use this:

HOOKS="base udev net nbd usbinput keymap encrypt filesystems"

Then continue with the instructions about recreating the kernel image in the NFS article.

After leaving the chroot, the kernel image will be in /mnt/boot/. We are going to need it in /nbd/boot:

cp /mnt/boot/vmlinuz26 /nbd/boot/
cp /mnt/boot/kernel26.img /nbd/boot/

Editing rc.conf

Make sure you set NETWORK_PERSIST="yes" and your own settings in /mnt/etc/rc.conf. Also edit /mnt/etc/locale.gen and make sure your locales are enabled.

Note: Shutdown does currently not work, even if NETWORK_PERSIST is set to yes, see Known Problems/Shutdown Hang

PXE/TFTP Setup

Follow the instructions here. Just make sure you use /nbd/boot/ instead of /disklessroot/boot/ for the TFTP-Root.

Boot Configuration

Copy the pxelinux.0 boot file from syslinux to /nbd/boot and create the pxelinux.cfg directory:

cp /usr/lib/syslinux/pxelinux.0 /nbd/boot/
mkdir /nbd/boot/pxelinux.cfg/

Now create and edit /nbd/boot/pxelinux.cfg/default, which contains the boot configuration for the client. Replace the value for nbd_server with the IP and Port your NBD server will be running on.

default linux

label linux
kernel vmlinuz26
append initrd=kernel26.img ip=::::::dhcp nbd_server=192.168.0.1:10809 root=/dev/nbd0

See here for details about the ip option.

If your NBD device is encrypted, use the following append line instead:

append initrd=kernel26.img ip=::::::dhcp nbd_server=192.168.0.1:10809 cryptdevice=/dev/nbd0:nbdcrypt root=/dev/mapper/nbdcrypt

Configuring the NBD server

Using a Swap Partition

Although this has not been tested yet, you should be able to do this by creating a LVM volume group on /dev/nbd0 that contains the root and swap partition and adding lvm2 before the filesystems hook in /mnt/etc/mkinitcpio.conf.

Updating the Client System

Quote from the NBD homepage:

"[...] if someone has mounted NBD read/write, you must assure that no one else will have it mounted."

In other words, if you want to be able to update from your client system, you have to make sure that the NBD device is not mounted on any other system, not even read-only. If your NBD device is encrypted, make sure to not just dismount it, but also close it with 'cryptsetup luksClose'. Otherwise, you may break your filesystem. If you keep that in mind, everything except kernel updates should work fine.

Alternatively you can mount the NBD device on the server (again, make sure it is not mounted anywhere else!) and update your system with:

pacman -Syu --root /mnt --dbpath /mnt/var/lib/pacman

Kernel Updates

Since the kernel the client system boots from is on the server and not in the NBD device itself, you will not be able to simply update your kernel with pacman. There are several other ways to do this:

  • If you update from the client system:
    • share /nbd/boot on the server via NFS, mount it to /boot and then update your kernel
    • copy vmlinuz26 and kernel26.img from /boot back to the server to /nbd/boot after the kernel update
  • if you update from the server (make sure proc, dev and sys in /mnt are mounted!):
    • symlink /nbd/boot to /mnt/boot and then update your kernel
    • copy vmlinuz26 and kernel26.img from /mnt/boot to /nbd/boot after the kernel update

Known Problems

Shutdown Hang

Shutdown will hang at "Sending SIGTERM to processes". This probably happens because this also kills the nbd-client process and the connection to the root device is therefore lost. Unfortunately, there is currently no known workaround for this.