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Introduction

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ZFSBootMenu is a Linux bootloader that attempts to provide an experience similar to FreeBSD's bootloader. By taking advantage of ZFS features, it allows a user to have multiple "boot environments" (with different distributions, for example), manipulate snapshots before booting, and, for the adventurous user, even bootstrap a system installation via zfs recv.

In essence, ZFSBootMenu is a small, self-contained Linux system that knows how to find other Linux kernels and initramfs images within ZFS filesystems. When a suitable kernel and initramfs are identified (either through an automatic process or direct user selection), ZFSBootMenu launches that kernel using the kexec command.

screenshot

In broad strokes, it works as follows:

  • Via direct EFI booting, an EFI boot manager like refind, rEFInd, a BIOS bootloader like syslinux, or some other means, boot ZFSBootMenu (as either a self-contained UEFI application or a dedicated Linux kernel and initramfs image).
  • Find all healthy ZFS pools and import them.
  • If appropriate, select a preferred boot environment:
    • If the ZFSBootMenu command line specifies no pool preference, prefer the filesystem indicated by the bootfs property (if defined) on the first-found pool.
    • If the ZFSBootMenu command line specifies a pool preference, and that pool has been imported, prefer the filesystem indicated by its bootfs property (if defined).
    • If a bootfs value has been identified, start an interruptable countdown (by default, 10 seconds) to automatically boot that environment.
    • If no bootfs value can be identified or the automatic countdown was interrupted, search all imported pools for filesystems that set mountpoint=/ and contain a /boot subdirectory that contains Linux kernels and initramfs images. Present a list of identified environments for user selection via fzf.
  • Mount the filesystem representing the selected boot environment and find the highest versioned kernel in /boot in the selected boot environment.
  • Using kexec, load the selected kernel and initramfs into memory, setting the kernel command line with the contents of the org.zfsbootmenu:commandline property for that filesystem.
  • Unmount all ZFS filesystems.
  • Boot the final kernel and initramfs.

At this point, you'll be booting into your usual OS-managed kernel and initramfs, along with any arguments needed to correctly boot your system.

Whenever ZFSBootMenu encounters natively encrypted ZFS filesystems that it intends to scan for boot environments, it will prompt the user to enter a passphrase as necessary.

This tool makes uses of the following additional software:

The ZFSBootMenu may be created using your your regular system kernel, user-space utilities and initramfs generator. Image creation is known to work and explicitly supported with:

Note that ZFSBootMenu does not replace your regular initramfs image. In fact, it is possible to use one of the supported generators to produce a ZFSBootMenu image even on Linux distributions entirely different program to produce their initramfs images (e.g., initramfs-tools on Debian or Ubuntu).

ZFSBootMenu is capable of booting just about any Linux distribution. Major distributions that are known to boot without requiring any special configuration include:

  • Void
  • Arch
  • Alpine
  • Gentoo
  • Debian and its descendants (Ubuntu, Linux Mint, Devuan, etc.)

Red Hat and its descendants (RHEL, CentOS, Fedora, etc.) are expected to work as well but have never been tested. ZFSBootMenu also provides several configuration options that can be used to fine-tune the boot process for nonstandard configurations.

Each release includes pre-generated images (both a monolithic UEFI applications as well as separate kernel and initramfs components suitable for both UEFI and BIOS systems) based on Void Linux. Building a custom image is known to work in the following configurations:

  • With mkinitcpio or dracut on Void (the zfsbootmenu package will make sure all prerequisites are available)
  • With mkinitcpio or dracut on Arch
  • With dracut on Debian or Ubuntu (installed as dracut-core to avoid replacing the system initramfs-tools setup)

Community documentation

The ZFSBootMenu wiki contains additional documentation, provided both by the ZFSBootMenu development team and by community members.

Installation and integration guides are available, along with other live documents.

Containerized builds

If you run Docker or podman, it is also possible to build ZFSBootMenu images in a container. Build containers are based on Void Linux and provide a consistent and well-tested environment for creating images with custom configurations. The build guide provides a brief overview of the zbm-builder.sh script that provides a simple front-end for containerized builds. Advanced users with very specific needs may consult the container README for a more detailed description of ZFSBootMenu build containers.

ZFS boot environments

From the perspective of ZFSBootMenu, a "boot environment" is simply a ZFS filesystem that contains a Linux kernel and initramfs in its /boot subdirectory. More thorough consideration of the concept is presented in the boot environment primer.

The following example filesystem layout defines two boot environments as filesystems which define the property mountpoint=/:

NAME                           USED  AVAIL     REFER  MOUNTPOINT
zroot                          278G   582G       96K  none
zroot/ROOT                    10.9G   582G       96K  none
zroot/ROOT/void.2019.10.04    1.20M   582G     7.17G  /
zroot/ROOT/void.2019.11.01    10.9G   582G     7.17G  /
zroot/home                     120G   582G     11.8G  /home

It is generally advisable to set the canmount=noauto property on all ZFS root filesystems. Regardless of the value of this property, the initramfs for your environment will always explicitly mount the specified root filesystem. Leaving this property set to the default canmount=auto may cause your distribution to attempt to mount multiple conflicting roots at startup, leaving your system in an inconsistent or unbootable state.

If the zroot pool defines a bootfs property that points to one of the two boot environments, ZFSBootMenu will attempt to boot that environment by default:

NAME   PROPERTY  VALUE                       SOURCE
zroot  bootfs    zroot/ROOT/void.2019.11.01  local

Unless the org.zfsbootmenu:kernel property of a boot environment specifies a version restriction, ZFSBootMenu will find and boot the highest versioned kernel in zroot/ROOT/void.2019.11.01/boot that also includes a matching initramfs.

Boot environments may also reside on filesystems that define the property mountpoint=legacy. To avoid time-consuming searches for boot environments on arbitrary legacy-mounted filesystems, such boot environments must opt into recognition by defining the custom property org.zfsbootmenu:active=on.

Filesystems which define mountpoint=/ may define the property org.zfsbootmenu:active=off to opt out of recognition by ZFSBootMenu.

Command-line arguments

Kernel command-line (KCL) arguments should be configured by setting the org.zfsbootmenu:commandline property for each boot environment. Do not set a root= option in this property; ZFSBootMenu will add an appropriate root= argument when it boots the environment and will actively suppress any conflicting option.

Because ZFS properties are inherited by default, it is possible to set the org.zfsbootmenu:commandline property on a common parent to apply the same KCL arguments to multiple environments. Setting the property locally on individual boot environments will override the common defaults.

As a special accommodation, the substitution keyword %{parent} in the KCL property will be recursively expanded to whatever the value of org.zfsbootmenu:commandline would be on the parent dataset. This allows, for example, mixing options common to multiple environments with those specific to each:

zfs set org.zfsbootmenu:commandline=""zfs.zfs_arc_max=8589934592"" zroot/ROOT
zfs set org.zfsbootmenu:commandline="%{parent} loglevel=4" zroot/ROOT/void.2019.11.01
zfs set org.zfsbootmenu:commandline="loglevel=7 %{parent}" zroot/ROOT/void.2019.10.04

will cause ZFSBootMenu to interpret the KCL for zroot/ROOT/void.2019.11.01 as

zfs.zfs_arc_max=8589934592 loglevel=4

while the KCL for zroot/ROOT/void.2019.10.04 would be

loglevel=7 zfs.zfs_arc_max=8589934592

EFI booting

Although ZFSBootMenu images can be booted on legacy BIOS systems or (on other platforms) alternative firmware, ZFSBootMenu integrates nicely with modern UEFI systems. ZFSBootMenu builds a custom initramfs image around a standard Linux kernel. Most distributions compile the Linux kernel with an EFI stub loader; the ZFSBootMenu kernel and initramfs pair can therefore be booted directly by most UEFI implementations or by EFI boot managers like rEFInd or gummiboot (systemd-boot).

When generating ZFSBootMenu images from a local host, it is possible to edit /etc/zfsbootmenu/config.yaml to copy the ZFSBootMenu kernel and initramfs directly to your EFI system partition. Suppose that the directory listing for your current /boot looks like

# ls /boot
config-5.3.18_1
config-5.4.6_1
efi
initramfs-5.3.18_1.img
initramfs-5.4.6_1.img
System.map-5.3.18_1
System.map-5.4.6_1
vmlinuz-5.3.18_1
vmlinuz-5.4.6_1

Typically, EFI system partitions (ESP) are mounted at /boot/efi, as is shown above. An ESP may contain a number of sub-directories, including an EFI directory that often contains multiple independent EFI executables. In this example layout, /boot/efi/EFI/zbm may hold ZFSBootMenu kernels and initramfs images. After setting the ImageDir property of the Components section of /etc/zfsbootmenu/config.yaml to /boot/efi/EFI/zbm, running generate-zbm will cause ZFSBootMenu kernel and initramfs pairs to be installed in the desired location:

# lsblk -f /dev/sda
NAME   FSTYPE LABEL UUID                                 FSAVAIL FSUSE% MOUNTPOINT
sdg
├─sda1 vfat         AFC2-35EE                               7.9G     1% /boot/efi
└─sda2 swap         412401b6-4aec-4452-a6bd-6fc20fbdc2a5                [SWAP]

# ls /boot/efi/EFI/zbm/
initramfs-1.12.0_1.img
initramfs-1.12.0_2.img
vmlinuz-1.12.0_1
vmlinuz-1.12.0_2

After the kernel and initramfs pairs are made available on the ESP, you'll need a way to boot them on your system. This can be done directly via efibootmgr or via a third-party boot manager like rEFInd.

efibootmgr

efibootmgr --disk /dev/sda \
  --part 1 \
  --create \
  --label "ZFSBootMenu" \
  --loader '\EFI\zbm\vmlinuz-1.12.0_2' \
  --unicode 'zbm.prefer=zroot ro initrd=\EFI\zbm\initramfs-1.12.0_2.img quiet' \
  --verbose

Take note to adjust the arguments to --disk and --part, the path to the kernel in --loader, and the initramfs path (initrd=) and pool preference (zbm.prefer=) to match your system configuration.

Each time ZFSBootMenu is updated, a new EFI entry will need to be manually added, unless you disable versioning in the ZFSBootMenu configuration.

rEFInd

rEFInd is considerably easier to install and manage. Refer to your distribution's packages for installation. Once rEFInd has been installed, you can create refind_linux.conf in the directory holding the ZFSBootMenu files (/boot/efi/EFI/zbm in our example):

"Boot default"  "zbm.prefer=zroot ro quiet loglevel=0 zbm.skip"
"Boot to menu"  "zbm.prefer=zroot ro quiet loglevel=0 zbm.show"

As with the efibootmgr section, the zbm.prefer= option needs to be configured to match your environment.

This file will configure rEFInd to create two entries for each kernel and initramfs pair it finds. The first will directly boot into the environment set via the bootfs pool property. The second will force ZFSBootMenu to display its interactive user interface and allow you to boot alternate environments, kernels and snapshots.

Run-time configuration of ZFSBootMenu

ZFSBootMenu may be configured via a combination of command-line parameters and ZFS properties that are described in detail in the zfsbootmenu(7) manual page.

Local image creation

bin/generate-zbm can be used to create an initramfs on your system. It ships with Void-specific defaults in etc/zfsbootmenu/config.yaml. To create an initramfs, the following additional tools/libraries will need to be available on your system:

If you want to create a unified EFI executable (which bundles the kernel, initramfs and command line), you will also need:

  • linuxx64.efi.stub (typically packaged with gummiboot or systemd-boot)

Your distribution should have packages for these already.

Image configuration

config.yaml is used to control the operation of generate-zbm.

Dealing with driver conflicts

For some combination of hardware and kernel modules, the ZFSBootMenu kernel may leave hardware in an unexpected state and prevent the boot environment from properly initializing and attaching drivers. The simplest way to avoid this issue is to disable the affected kernel modules in ZFSBootMenu, leaving all hardware initialization to the final kernel. For example, if Nvidia graphics hardware does not function as expected, a dracut configuration file can be added to /etc/zfsbootmenu/dracut.conf.d to exclude the nouveau and nvidia drivers from ZFSBootMenu. Adding the line

omit_drivers+=" nouveau nvidia "

to a file called, e.g., /etc/zfsbootmenu/dracut.conf.d/nvidia.conf should restore expected functionality to your boot environment after recreating your ZFSBootMenu image with generate-zbm.

In other cases, it is not possible to exclude drivers without depriving ZFSBootMenu of critical hardware support. For example, some XHCI USB controllers may not be properly initialized after a kexec, leaving a boot environment without USB devices like a keyboard. However, excluding XHCI drivers from ZFSBootMenu would make the same keyboard inoperable in the boot menu, making it impossible to interact with the menus. ZFSBootMenu provides "teardown hooks" that can sometimes be used to address these situations. Teardown hooks are invoked immediately before a target kernel is booted via kexec and provide an opportunity to run last-minute commands to prepare the system for the boot. Scripts may be registered as teardown hooks by adding text of the form

zfsbootmenu_teardown+=" <path to script> "

where <path to script> points to an executable script or program. A sample XHCI teardown script demonstrates the use of teardown hooks to unbind the XHCI driver from the USB controllers in the ZFSBootMenu kernel before launching the selected boot environment, allowing the next kernel to properly initialize the controller.

Native encryption

ZFSBootMenu can import pools or filesystems with native encryption enabled. If your boot environments are not encrypted but, for example, /home is, you will not receive a decryption prompt during boot. To ensure that you can decrypt your pool to load the kernel and initramfs, you'll need to you have the filesystem parameters configured correctly.

zfs get all zroot | egrep '(encryption|keylocation|keyformat)'
zroot  encryption            aes-256-gcm                -
zroot  keylocation           file:///etc/zfs/zroot.key  local
zroot  keyformat             passphrase                 -
zroot  encryptionroot        zroot                      -

It's critical that keyformat is set to passphrase, otherwise you'll be unable to enter the correct value in the boot loader. ZFS on Linux currently supports only one key, but in a way which we can exploit: if you configure the keylocation value to a file on disk, put your passphrase in that, and then include that file into the FINAL initramfs (the OS-managed one), you won't receive a second password prompt on boot. You'll still receive a password prompt in the boot loader, since we can force a prompt for passphrase input.

For Dracut-based systems, this can be done by creating a /etc/dracut.conf.d/zol.conf file with the following contents:

install_items+=" /etc/zfs/zroot.key "

It's critical that you do not include this key file into the ZFSBootMenu initramfs, since that file exists on an unencrypted volume - leaving your pool essentially wide-open.

For convenience, ZFSBootMenu recognizes the ZFS property org.zfsbootmenu:keysource as the name of a filesystem that should be searched for ZFS key files. When a boot environment specifies a file:// URI as its keylocation, ZFSBootMenu will attempt to mount a filesystem indicated by the org.zfsbootmenu:keysource property (if it exists) and search for the named keylocation therein. If found, ZFSBootMenu will copy the key into a cache within the in-memory root filesystem so that subsequent operations that require reloading the key (for example, changing the default boot environment or cloning a snapshot) will not prompt the user for passphrases.

When searching for a keylocation relative to the filesystem named by org.zfsbootmenu:keysource, ZFSBootMenu will first try to strip the mountpoint of the keysource filesystem from any keylocation URI that references the keys to map the keylocation that would be observed on a running system to the proper location in the keysource. For example, if the running system is set up so that zroot is the encryptionroot for all filesystems on a pool, running the commands

zfs create -o mountpoint=/etc/zfs/keys zroot/keystore
echo "MySecretPassphrase" > /etc/zfs/keys/zroot.key
chmod 000 /etc/zfs/keys/zroot.key
zfs set keylocation=file:///etc/zfs/keys/zroot.key zroot
zfs set org.zfsbootmenu:keysource=zroot/keystore zroot
echo install_optional_items+=" /etc/zfs/keys/zroot.key " >> /etc/dracut.conf.d/zol.conf

will cause ZFSBootMenu to attempt to cache the key file:///etc/zfs/keys/zroot.key from zroot/keystore when unlocking the zroot pool. Because zroot/keystore specifies mountpoint=/etc/zfs/keys, ZFSBootMenu will first try to strip /etc/zfs/keys from the keylocation URI, looking for the file zroot.key at the root of the filesystem zroot/keystore. If this fails, ZFSBootMenu will fall back to the full path, looking for etc/zfs/keys/zroot.key within the keysource filesystem. If either location is found, ZFSBootMenu will retain a cache of the key should it be needed to unlock the pool again.

Signature Verification and Prebuilt EFI Executables

ZFSBootMenu is now distributed as a prebuilt EFI executable alongside the source releases. For many systems, it is sufficient to drop the EFI executable on an EFI System Partition and configure your firmware to boot the file.

Each EFI executable we release is signed with signify, which provides a simple method for verifying that the contents of the file are as this project intended. Once you've installed signify (that's left as an exercise, although Void Linux provides the signify package for this purpose), just download the EFI bundle from the releases page, download the sha256.sig file alongside it, and run

signify -C -x sha256.sig

You will also need the public key used to sign ZFSBootMenu executables. The key is available at releng/keys/zfsbootmenu.pub. Install this file as /etc/signify/zfsbootmenu.pub if you like; this key can be used for all subsequent verifications. Otherwise, look at the -p command-line option for signify to provide a path to the key.

The signature file sha256.sig also includes a signature for the source tarball corresponding to the release. If this file is not present alongside the EFI bundle and the signature file, signify will complain about its signature. This error message is OK to ignore; alternatively, tell signify to verify only the EFI bundle, or download the source tarball alongside the other files.

The signify key zfsbootmenu.pub may be verified; alongside the public key is releng/keys/zfsbootmenu.pub.gpg, a GnuPG signature produced with a personal key from @ahesford, one of the members of the ZFSBootMenu project. To verify the signify key, download the key zfsbootmenu.pub and its signature file zfsbootmenu.pub.gpg, then run

gpg2 --recv-key 0x312485BE75E3D7AC
gpg2 --verify zfsbootmenu.pub.gpg

NOTE: on some distributions, gpg2 may instead by gpg.