Difference between revisions of "Diskless network boot NBD root"

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[[Category:Boot process (English)]]
[[Category:Boot process]]
[[Category:Networking (English)]]
[[Category:Networking (English)]]
==Boot from a NBD root device==
==Boot from a NBD root device==

Revision as of 13:50, 23 April 2012

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/Disadvantages over NFS


  • NBD is faster
  • allows you to boot from an encrypted or LVM-based root device


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 Template:Keypress 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

Create directories for the pacman database and a few mount points that will be needed:

mkdir -p /mnt/var/lib/pacman/
mkdir /mnt/proc/
mkdir /mnt/dev/
mkdir /mnt/sys/

Mount proc, sysfs and dev. These will be neccessary to build the kernel image in the chroot environment:

mount -t proc none /mnt/proc
mount -t sysfs none /mnt/sys
mount -o bind /dev /mnt/dev

Install the Arch Linux base packages into the root directory:

pacman -Sy --root /mnt --dbpath /mnt/var/lib/pacman base

Build the mkinitcpio-nbdAUR package from the AUR. Then install it into the root directory as well:

pacman -U --root /mnt --dbpath /mnt/var/lib/pacman mkinitcpio-nbd-[...]-any.pkg.tar.xz

Configuring the Client system

The following configurations are neccessary for the client system to work properly, everything else can be configured later:

  • set NETWORK_PERSIST="yes" and the other client settings in /mnt/etc/rc.conf. You will also have to remove/disable the network daemon. The net hook will set the IP on boot, so there is no point in using it. Additionally, if you would change your IP with the network daemon, the connection to the root device would break. If you want a static IP, use the ip kernel parameter (see Boot Configuration).
  • set a mirror in /mnt/etc/pacman.d/mirrorlist.
  • enable your locales in /mnt/etc/locale.gen.
  • set your DNS server in /mnt/etc/resolv.conf, see man 5 resolv.conf for details.
  • add a line for / in /mnt/etc/fstab (use /dev/mapper/nbdcrypt instead of /dev/nbd0 if your root device is encrypted):
/dev/nbd0	/	ext4	defaults	0	0
building the kernel image

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 you can chroot into /mnt and rebuild the kernel image. Remember that you mounted proc, sysfs and dev earlier. This is always neccessary if you want to rebuild the kernel image. locale-gen is only neccessary this one time to set the locales:

chroot /mnt
mkinitcpio -p linux
umount /mnt/dev
umount /mnt/proc
umount /mnt/sys

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

cp /mnt/boot/vmlinuz-linux /nbd/boot/
cp /mnt/boot/initramfs-linux.img /nbd/boot/

Configuring the NBD server

nbd-server is configured with the config file /etc/nbd-server/config and can be started with rc.d start nbd. You may also want to add nbd to your DAEMONS array.

A basic config file may look like this:

	exportname = /nbd/root

In this example, there is only a simple export with the name nbdroot, which makes the file /nbd/root available via NBD. The name of the export (nbdroot in this case, not /nbd/root!) is important because you will have to set it in the kernel boot parameters later. By default, nbd-server will make the export available to clients with read/write support. If you have more than one client you want to boot from, this will break your filesystem (see Updating the Client System for details). You could enable read-only by setting readonly = true, but this is not recommended because there are just too many things that want to write to /var, /etc or other directories. You could partially work around this with tmpfs, but some things might still break.

Another interesting option is NBD's "copy on write"-mode (copyonwrite = true). From man 5 nbd-server:

"[...] any writes to this export will not be written to the master file, but to a separate file which will be removed upon disconnect. The result of using this option is that nbd-server will be somewhat slower, and that any writes will be lost upon disconnect."

It has not been tested yet how much this really affects performance, but if you want to boot from multiple clients, this is probably your best option.

For a detailed explanation of the config file, see the aforementioned man page man 5 nbd-server.


Install syslinux and dnsmasq on the server, then 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_host with the IP address of your NBD server and nbd_name with the export name you have specificed in /etc/nbd-server/config. If your nbd-server is not listening on the default port (10809), you can set it with nbd_port.

default linux

label linux
kernel vmlinuz-linux
append initrd=initramfs-linux.img ip=::::::dhcp nbd_host= nbd_name=nbdroot 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=initramfs-linux.img ip=::::::dhcp nbd_host= nbd_name=nbdroot cryptdevice=/dev/nbd0:nbdcrypt root=/dev/mapper/nbdcrypt


Before you boot your client, make sure you unmount everything on the server:

umount /mnt/proc /mnt/sys /mnt/dev /mnt

If your NBD device is encrypted, close the LUKS device as well:

cryptsetup luksClose nbdcrypt

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 partitions 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 unmount 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 and then update it (again, make sure it is not mounted anywhere else!). Note that proc, dev and sys should only be necessary for kernel upgrades, but just in case, they are included here.

mount /nbd/root /mnt
mount -t proc none /mnt/proc
mount -t sysfs none /mnt/sys
mount -o bind /dev /mnt/dev
chroot /mnt
pacman -Syu
umount /mnt/proc /mnt/sys /mnt/dev /mnt

Updating the kernel

Since the kernel the client system boots from is on the server and not on 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 NBD, Sshfs or NFS, mount it to /boot and then update your kernel
    • copy vmlinuz-linux and initramfs-linux.img from /boot back to the server to /nbd/boot after the kernel update
  • if you update from the server:
    • copy vmlinuz-linux and initramfs-linux.img from /mnt/boot to /nbd/boot after the kernel update

Known Problems

Resume from Suspend causes I/O errors

When resuming the machine from suspend, the NBD connection drops and the root device is lost, causing I/O errors. There is currently no fix or workaround for this.