dracut
dracut creates an initial image used by the kernel for preloading the block device modules (such as IDE, SCSI or RAID) which are needed to access the root filesystem. Upon installing linux, you can choose between mkinitcpio and dracut. dracut is used by Fedora, RHEL, Gentoo, and Debian, among others. Arch uses mkinitcpio by default.
You can read the full project documentation for dracut in the documentation.
Installation
Install the dracut package, or dracut-gitAUR for the latest development version.
Usage
dracut is easy to use and typically does not require user configuration, even when using non-standard setups, like LVM on LUKS.
To generate an initramfs for the running kernel:
# dracut --hostonly --no-hostonly-cmdline /boot/initramfs-linux.img
To generate a fallback initramfs run:
# dracut /boot/initramfs-linux-fallback.img
/boot/initramfs-linux.img refers to the output image file. If you are using the non-regular kernel, consider changing the file name. For example, for the linux-lts kernel, the output file should be named /boot/initramfs-linux-lts.img. However, you can name these files whatever you wish as long as your boot loader configuration uses the same file names.
Additional options
The --force flag overwrites the image file if it is already present.
The --kver option specifies which kernel to use. The argument to this option must match the name of a directory present in /usr/lib/modules.
More flags can be found with dracut(8).
Advanced configuration
dracut can be configured by directly passing arguments on the command line (see dracut(8) § OPTIONS). If you wish to always execute dracut with a certain set of flags, you can save a specified configuration in a .conf file in /etc/dracut.conf.d/. For example:
/etc/dracut.conf.d/myflags.conf
hostonly="yes" compress="lz4" add_drivers+=" i915 " omit_dracutmodules+=" network iscsi "
You can see more configuration options with dracut.conf(5). Fuller descriptions of each option can be found with dracut(8). We will describe a few common options in what follows.
Dracut modules
dracut uses a modular approach to build the initramfs (see dracut.modules(7)). All of dracut 's builtin modules are located in /lib/dracut/modules.d and can be listed with dracut --list-modules. Extra modules can be provided by external packages e.g. dracut-sshd-gitAUR. dracut 's built-in modules unfortunately lack documentation, although their names can be self-explanatory.
Some of the modules are active/inactive by default, and can be activated/deactivated with --add/--omit command line argument or with the add_dracutmodules+=""/omit_dracutmodules+="" persistent config entry lines.
/etc/dracut.conf.d/myflags.conf
# ... add_dracutmodules+=" <dracut modules to activate> " omit_dracutmodules+=" <dracut modules to deactivate> " # ...
TPM2
To make use of systemd 's unlocking of luks2 encrypted volumes using TPM2 through systemd-cryptenroll, install tpm2-tools package and enable the tpm2-tss dracut module.
Early kernel module loading
Dracut enables early loading (at the initramfs stage, via modprobe) through it's --force_drivers command or force_drivers+="" config entry line. For example:
/etc/dracut.conf.d/myflags.conf
# ... force_drivers+=" nvidia nvidia_modeset nvidia_uvm nvidia_drm " # ...
Kernel command line options
Kernel command line options can be placed in a .conf file in /etc/dracut.conf.d/, and set via the kernel_cmdline= flag. Dracut will automatically source this file and create a 01-default.conf file and place it inside the initramfs directory /etc/cmdline.d/. For example, your kernel command line options file could look like:
/etc/dracut.conf.d/cmdline.conf
kernel_cmdline="rd.luks.uuid=luks-f6c738f3-ee64-4633-b6b0-eceddb1bb010 rd.lvm.lv=arch/root rd.lvm.lv=arch/swap root=/dev/arch/root rootfstype=ext4 rootflags=rw,relatime"
Miscellaneous notes
It is not necessary to specify the root block device for dracut. From dracut.cmdline(7):
- The root device used by the kernel is specified in the boot configuration file on the kernel command line, as always.
However, it may be useful to set some parameters early, and you can enable additional features like prompting for additional command line parameters. See dracut.cmdline(7) for all options. Here are some example configuration options:
- Resume from a swap partition:
resume=UUID=80895b78-7312-45bc-afe5-58eb4b579422 - Prompt for additional kernel command line parameters:
rd.cmdline=askWarning:rd.cmdline=askmay cause an issue where the keyboard is not yet initialized prior to its prompt forcing a hard reset. See dracut issue 728. - Print informational output even if
quietis set:rd.info
Unified kernel image
dracut can produce unified kernel images with the --uefi command line option or with the uefi="yes" configuration option.
Tips and tricks
View information about generated image
You can view information about a generated initramfs image, which you may wish to view in a pager:
# lsinitrd /path/to/initramfs_or_uefi_image | less
This command will list the arguments passed to dracut when the image was created, the list of included dracut modules, and the list of all included files.
Change compression program
To reduce the amount of time spent compressing the final image, you may change the compression program used.
- Make sure your kernel has your chosen decompression support compiled in, otherwise you will not be able to boot. You must also have the chosen compression program package installed.
- The upstream Linux kernel does not support zstd compressed initramfs before version 5.9. linux-zen supports it since version 5.8
Simply add any one of the following lines (not multiple) to your dracut configuration:
compress="cat" compress="gzip" compress="bzip2" compress="lzma" compress="xz" compress="lzo" compress="lz4" compress="zstd"
gzip is the default compression program used. compress="cat" will make the initramfs with no compression.
You can also use a non-officially-supported compression program:
compress="program"
Generate a new initramfs on kernel upgrade
It is possible to automatically generate new initramfs images upon each kernel upgrade. The instructions here are for the default linux kernel, but it should be easy to add extra hooks for other kernels.
esp/EFI/Linux/linux-kernel-machine_id-build_id.efi). EFI binaries in this directory are automatically detected by systemd-boot and therefore do not need an entry in /boot/loader/loader.conf.As the command to figure out the kernel version is somewhat complex, it will not work by itself in a pacman hook. So create a script anywhere on your system. For this example it will be created in /usr/local/bin/.
The script will also copy the new vmlinuz kernel file to /boot/, since the kernel packages do not place files in /boot/ anymore.[2]
/usr/local/bin/dracut-install.sh
#!/usr/bin/env bash
args=('--force' '--no-hostonly-cmdline')
while read -r line; do
if [[ "$line" == 'usr/lib/modules/'+([^/])'/pkgbase' ]]; then
read -r pkgbase < "/${line}"
kver="${line#'usr/lib/modules/'}"
kver="${kver%'/pkgbase'}"
install -Dm0644 "/${line%'/pkgbase'}/vmlinuz" "/boot/vmlinuz-${pkgbase}"
dracut "${args[@]}" --hostonly "/boot/initramfs-${pkgbase}.img" --kver "$kver"
dracut "${args[@]}" --no-hostonly "/boot/initramfs-${pkgbase}-fallback.img" --kver "$kver"
fi
done
/usr/local/bin/dracut-remove.sh
#!/usr/bin/env bash
while read -r line; do
if [[ "$line" == 'usr/lib/modules/'+([^/])'/pkgbase' ]]; then
read -r pkgbase < "/${line}"
rm -f "/boot/vmlinuz-${pkgbase}" "/boot/initramfs-${pkgbase}.img" "/boot/initramfs-${pkgbase}-fallback.img"
fi
done
You need to make the scripts executable. If you wish to add or remove flags, you should add them to your dracut configuration.
The next step is creating pacman hooks:
/etc/pacman.d/hooks/90-dracut-install.hook
[Trigger] Type = Path Operation = Install Operation = Upgrade Target = usr/lib/modules/*/pkgbase [Action] Description = Updating linux initcpios (with dracut!)... When = PostTransaction Exec = /usr/local/bin/dracut-install.sh Depends = dracut NeedsTargets
/etc/pacman.d/hooks/60-dracut-remove.hook
[Trigger] Type = Path Operation = Remove Target = usr/lib/modules/*/pkgbase [Action] Description = Removing linux initcpios... When = PreTransaction Exec = /usr/local/bin/dracut-remove.sh NeedsTargets
You should stop mkinitcpio from creating and removing initramfs images as well, either by removing mkinitcpio or with the following commands:
# ln -sf /dev/null /etc/pacman.d/hooks/90-mkinitcpio-install.hook # ln -sf /dev/null /etc/pacman.d/hooks/60-mkinitcpio-remove.hook
Bluetooth keyboard support
Dracut will enable the bluetooth module automatically when a bluetooth keyboard is detected. However it is required that dracut is in hostonly mode for this. This seems to not be the default. To enable hostonly mode you can add the following to your dracut configuration:
hostonly=yes
Troubleshooting
Hibernation
If resuming from hibernation does not work, you may need to configure dracut to include the resume module. You will need to add a configuration file:
/etc/dracut.conf.d/resume-from-hibernate.conf
add_dracutmodules+=" resume "
LVM / software RAID / LUKS
If the kernel has issues auto discovering and mounting LVM / software RAID / LUKS blocks. You can retry generating an initramfs with the following kernel command line options:
rd.auto rd.lvm=1 rd.dm=1 rd.md=1 rd.luks=1
A stop job is running for "brltty"
If you have issues booting or very long shutdown processes while the system waits for brllty, add the following to the dracut configuration line:
omit_dracutmodules+=" brltty "
Alternatively, uninstall brltty if it is not needed.
No usable keyslot is available
Cannot use whirlpool hash for keyslot encryption. Keyslot open failed. No usable keyslot is available.
A failure to boot with a message similar to the above typically will only require the user to include the crypt module via add_dracutmodules. However, if you continue to get the message after adding it, you may need to adjust your configuration with the following.
install_items+=" /usr/lib/ossl-modules/legacy.so "
This is because OpenSSL deprecated the whirlpool hash and moved it to that library. Dracut does not automatically gather it when using Arch.