Dracut

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This article has some todo items:

• Update regarding the removal of v86d

Dracut is an initramfs infrastructure and aims to have as little as possible hard-coded into the initramfs. It originated from the Fedora Project and was ported to Gentoo in the 2010 Google Summer of Code. For more detailed information, refer to the documentation at kernel.org.

Installation

Kernel

Before you can use the images generated by dracut, the Linux kernel must include initramfs support. The ebuild will provide a warning post-installation if the kernel is missing the required options:

KERNEL Enabling the initramfs (CONFIG_BLK_DEV_INITRD and CONFIG_DEVTMPFS respectively)

General setup  --->
      [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support
   Device Drivers --->
      Generic Driver Options --->
         [*] Maintain a devtmpfs filesystem to mount at /dev

Certain dracut modules also require additional dependencies to function. A list of optional dependencies is listed at the end of the installation. The DRACUT_MODULES variable in make.conf is no longer used.

Emerge

root #emerge --ask sys-kernel/dracut

Building an initramfs image

Default images

Once you have installed dracut, you can use it to build an initramfs image. The simplest way to do this is to just run

root #mount /boot

root #dracut

The initramfs image created this way is a generic image, which will include all installed modules and system tools that it can find, suitable for booting a variety of machines. If you are producing an initramfs for just a single machine, you can specify that dracut should go into host-only mode:

root #dracut --hostonly

The produced image will contain only those elements used by the local machine for booting. Certain modules behave differently in host-only mode. The kernel-modules module, for example, will only include the tools and modules used by the current rootfs file system, and the i18n module will install the fonts and keymaps for the local machine. Otherwise, every available file system, font, etc. would be included. You can set hostonly mode as the default in /etc/dracut.conf.

By default, dracut will produce an image suitable for booting the currently active kernel; it will pull the kernel modules needed for that kernel, etc. The image will be written in a file called /boot/initramfs-<kernel version>.img. You can override both options by specifying a new file name and kernel version on the command line, in that order. Both parameters are optional, but if you want to override the kernel version and keep the default file name you need to pass a placeholder empty string as the first parameter:

root #dracut --hostonly '' 3.2.5-hardened

Modules

Dracut installs all available modules though some may need additional dependencies. The purpose of each module is to arrange for files to be included in the final initramfs image. In addition, dracut exposes hooks that run at certain points, which modules can hook into to perform required boot actions.

Adding modules

Dracut comes with a suitable set of default modules (see below). You can override this module list in two ways: command line, or configuration file. Editing your configuration file will set up dracut to be re-run easily when you change kernels or other boot-time options. Options specified in /etc/dracut.conf can be overridden by files in /etc/dracut.conf.d, which are installed by various dracut-aware packages. Both options can be overridden by command-line parameters. The following configuration options deal with the default modules list:

FILE /etc/dracut.conf

# Equivalent to -H
hostonly="yes"

# Equivalent to -m "module module module"
dracutmodules+="dash kernel-modules rootfs-block udev-rules usrmount base fs-lib shutdown"

# Equivalent to -a "module"
add_dracutmodules+="module"

# Equivalent to -o "module"
omit_dracutmodules+="module"

Specifying dracutmodules (or the -m option) will override the entire default list, and install only the modules specified. This can cut down significantly on image size (for example, if you don't need any kernel modules, or i18n support, etc.). If you are not sure which modules you need, you should build one fully-featured image and keep it in /boot while you experiment. If anything goes wrong, you can edit your grub.conf at boot to switch to that.

The other options can be used to add or remove items from the default list, including your own custom modules. If a module refuses to install (for example, its prerequisite binaries are missing) there is a --force-add command-line switch (but no matching configuration option) to add them anyway. Note that forcing a module to be added this way is dangerous, as the installed scripts will likely to run execute commands that are not present.

List of modules

An incomplete list of modules is below, with their additional requirements.

Module	Description	Enabled?	Additional packages
dash	Includes /bin/dash as /bin/sh	always
i18n	Includes keymaps, console fonts, etc.	always
rpmversion	Includes dracut version in initramfs.	when /etc/redhat-release exists
convertfs	Merges / into /usr on next boot.	never
kernel-modules	Includes the kernel modules for root filesystems and other boot-time devices.	always
fstab-sys	Arranges for arbitrary partitions to be mounted before rootfs.	when /etc/fstab.sys exists, or command line includes --fstab or --add_fstab
resume	Allows initramfs to resume from low-power state.	when swap partition exists
rootfs-block	Arranges for the block device containing the rootfs to be mounted.	always
terminfo	Includes a terminfo file.	always
udev-rules	Includes udev and some basic rules.	always
securityfs	Arranges for the securityfs to be mounted early.	never
usrmount	Arranges for /usr to be mounted.	always
base	Includes most basic utilities.	always
fs-lib	Includes filesystem tools (including mount).	always
img-lib	Includes tools to decompress images.	never
shutdown	Sets up hooks to run on shutdown.	always
biosdevname	Enables BIOS network device renaming.	always	sys-apps/biosdevname, sys-apps/pciutils
btrfs	Supports a btrfs rootfs.	host-only: rootfs	sys-fs/btrfs-progs
caps	Supports dropping capabilities before init.	systemd not in use	sys-libs/libcap (also replaces dash with bash)
crypt	Supports an encrypted rootfs.	host-only: rootfs	sys-fs/cryptsetup
crypt-gpg	Use gpg for crypt support (requires crypt module).	never (see note below)	app-crypt/gnupg
dmraid	Support rootfs on FakeRAID.	host-only: rootfs	sys-fs/multipath-tools, sys-fs/dmraid
dmsquash-live	Supports LiveCD rootfs.	never (host-only: refused)
gensplash	Include a static splash screen.	never	media-gfx/splashutils
iscsi	Supports rootfs on iscsi devices.	host-only: rootfs	sys-block/open-iscsi
livenet	Retrieve rootfs over HTTP. (requires dmsquash-live modules)	never
lvm	Supports rootfs on LVM device.	host-only: rootfs	sys-fs/lvm2
mdraid	Supports rootfs on software RAID.	host-only: rootfs	sys-fs/mdadm
multipath	Supports rootfs on multi-path device.	host-only: rootfs	sys-fs/multipath-tools
nbd	Supports rootfs on a network block device.	host-only: rootfs	sys-block/nbd
nfs	Supports NFS-mounted rootfs.	host-only: rootfs	net-fs/nfs-utils net-nds/rpcbind
plymouth	Includes boot animation.	always	sys-boot/plymouth
ssh-client	Includes ssh and scp clients.	never	dev-libs/openssl
syslog	Includes remote logging support.	never	app-admin/syslog-ng or rsyslog
debug	Includes useful troubleshooting tools.	never	installed when USE=debug
dm	Includes device-mapper.	never	sys-fs/device-mapper or sys-fs/lvm2
ifcfg	Generates network config at runtime.	never
network	Brings up network for net booting.	always	net-misc/dhcp, sys-apps/iproute2
selinux	Arranges for the selinux policy to be loaded.	installed when USE=selinux
url-lib	Includes cUrl and SSL certs.	never	net-misc/curl

• The Enabled field indicates when a particular module will be included by default into your initramfs image.
• Modules with pre-requisite packages cannot be included (even explicitly) if the required software is missing from the host.

1. sys-boot/plymouth requires that x11-libs/libdrm be built with the libkms flag
2. The additional packages require that one or more dependency libraries be built with the static-libs flag
3. The Enabled field specifies when an installed module will be included by default into a built image:
   • Modules enabled for "host-only: rootfs" are included in host-only images if the active rootfs requires that module, and included in general-purpose images "always".
   • Modules enabled "never" must be explicitly added via the command-line or configuration file, as above.
   • Modules with pre-requisite packages cannot be included, even explicitly, if the required software is missing from the dracut build host.
   • dmsquash-live (and, by extension, livenet) cannot be installed in host-only mode, as that would be silly.
4. crypt-gpg does not seem to have any effect on systemd. This is because crypto volumes are activated by systemd services. However, systemd will only ask once for the password if several volumes use the same password.

Customizing the image

Besides defining the list of modules to include, you can also customize the final initramfs in a few other ways. The install_items option in the configuration file lets you specify an arbitrary number of arbitrary items to add to your image. Other options add specific elements to the image, as described below:

Kernel modules

By default, the kernel-modules driver scans the list of available kernel modules, and installs the set of modules needed to boot a system and bring up the rootfs. This includes the hardware bus drivers (SCSI, ATA, USB, etc), keyboard drivers, block device drivers, and file system drivers. (In host-only mode, only the file systems used by the build host are included.) You can update the list of installed modules via the command-line or configuration file. When specifying a kernel module name, do not include the .ko extension.

FILE /etc/dracut.conf

# Equivalent to --drivers="module module module"
drivers+="module module module"

# Equivalent to --add-drivers
add_drivers+="module"

# Equivalent to --omit-drivers="module"
omit_drivers+="module"

# Equivalent to --filesystems="fs fs fs"
filesystems+="fs fs fs"

# Equivalent to --kmoddir="/lib/modules/fixed"
drivers_dir="/lib/modules/fixed"

# Equivalent to  --fwdir=":/lib/fw/alt:/lib/fw/alt2"
fw_dir+=":/lib/fw/alt:/lib/fw/alt2"

• In host-only mode, the running hosts filesystems are always installed, regardless of the filesystems parameter.

Filesystems and mount points

The default image will arrange for your rootfs and your /usr partition (if different) to be mounted at boot time. The build process will obtain the device and file system information it needs from /proc/self/mountinfo. This may not be what you want, or you may need to have other partitions mounted as well. To just have additional partitions mounted at boot you can add them under

FILE /etc/dracut.conf

# Bring up <device> in initramfs, <device> should be the device name
add_device+=" /dev/mapper/sysvg-home /dev/mapper/sysvg-swap /dev/mapper/hdvg-private "

To further alter the behavior of the initramfs one needs the fstab-sys module, and the following options:

• Create a file called /etc/fstab.sys, which will be copied to the initramfs and mounted.
• Specify use_fstab="yes" (or --use-fstab), which will use your system's /etc/fstab instead of /proc/self/mountinfo
• Specify add_fstab+="filename" or --add_fstab filename, which will add the contents of filename to the initramfs's /etc/fstab.
• Specify --mount fstab mount spec, which adds individual entries into your initramfs' /etc/fstab.

(Note that add_fstab does not automatically trigger fstab-sys to be included in the image; this may be a bug, as use_fstab does include the module but does not actually supply an /etc/fstab.)

Booting the initramfs

Any parameters you need to pass to the dracut modules are passed on the kernel command line. If you are net booting, the DHCP server can also provide command-line parameters. The dracut scripts parse the kernel command line very early in the boot process and use the information found there to adjust their behavior. The list of options, by module, can be found in the man dracut.cmdline page.

Tasks

ext boot

For booting an ext(2,3,4) filesystem you don't need any of the optional dracut modules.

root #dracut -m "rootfs-block base" initramfs.img

Once you have the image built, the final step is to include the image in the boot loader. To do this, simply add a line in your grub.conf:

FILE /boot/grub/grub.conf

kernel /boot/vmlinuz root=UUID=00000000-0000-0000-0000-000000000000
initrd /boot/initramfs.img

It is recommended that you use the UUID= (or LABEL=) form of the root specification; to guard against your device names changing between boots. You can find the UUID of your root device by running this command:

root #ls -alF /dev/disk/by-uuid

LVM on LUKS

To boot from a root residing on an LVM volume located inside of an encrypted LUKS container these kernel command line options can be used: root=UUID=<root volume UUID> rd.luks.uuid=<LUKS partition UUID> rd.lvm.vg=<volume group name>. rd.lvm.vg might not be need depending on specific configuration but might lead to not all LVM partitions being activated. If for example the system has the following partitions:

root #blkid

/dev/sda1: UUID="2acb7668-fff1-492d-b46e-f05ead26d153" TYPE="crypto_LUKS" PARTUUID="cbe8be3c-3bcf-0d44-9f7b-27c7ad4dd9f6"
/dev/mapper/luks-2acb7668-fff1-492d-b46e-f05ead26d153: UUID="19Mtok-zEfG-7JXI-GXvM-Neoz-JnST-vwNlO3" TYPE="LVM2_member"
/dev/mapper/Vg-root: UUID="199bb83d-c783-4254-a6eb-fdbb83c33144" TYPE="ext4"

the kernel command line options will look like root=UUID=199bb83d-c783-4254-a6eb-fdbb83c33144 rd.luks.uuid=2acb7668-fff1-492d-b46e-f05ead26d153 rd.lvm.vg=Vg.

NFS boot

Warning: Currently Dracut does not have a proper network useflag (see Bug 590566), so you need to make sure that you do not have "net-analyzer/arping" installed, but instead "net-misc/iputils[arping]".

For an NFS boot, you need only 3 modules and the nfs dracut module.

root #dracut -m "nfs network base" initramfs-nfs-only.img

Booting is done with the following line in your PXE config. The rd.ip=auto let's it auto detect the NFS settings through DHCP. The NFS boot path is provided to the client through the DHCP option "root-path" with the following value: <SERVER>:<NFS Export>

FILE /var/lib/tftpboot/pxelinux.cfg/default

...
 APPEND initrd=initramfs-nfs-only.img rd.ip=auto root=dhcp
...

For more specific problems, consult official Dracut page at Fedora Wiki:

• https://fedoraproject.org/wiki/Dracut/Options#NFS

NBD boot

For an NBD boot, you need only 3 modules and the nbd dracut module.

root #dracut -m "nbd network base" initramfs-nbd-only.img

Booting is done with the following line in your PXE config. The rd.ip=auto let's it auto detect the NBD settings through DHCP. The NBD boot path is provided to the client through the DHCP option "root-path" with the following value: nbd:<SERVER>:<NBD_PORT>:<FS>

Note
Connecting to name based exports is not documented. A code review of the module shows that specifying an export name instead of a port number is allowed and can be used to connect to name based exports.

FILE /var/lib/tftpboot/pxelinux.cfg/default

...
 APPEND initrd=initramfs-nbd-only.img rd.ip=auto root=dhcp
...

It is also possible to boot from a partitioned nbd device. To do this, the PXE config needs to include

• a kernel parameter "nbd.max_part=X" with X>0 to load the nbd module with partition support
• a root device setting specifying the partition to mount
• a netroot setting as described above for the root-path.

FILE /var/lib/tftpboot/pxelinux.cfg/default

...
 APPEND initrd=initramfs-nbd-only.img rd.ip=auto nbd.max_part=X root=/dev/nbd0pY netroot=nbd:<SERVER>:<NBD_PORT|EXPORTNAME>
...

Note
During shutdown, networking (including dhcp) and nbd-client must not be stopped, otherwise the nbd can not be unmounted cleanly. For OpenRC, this can be configured in /etc/conf.d/net for DHCP client daemon options and /etc/conf.d/killprocs to exclude nbd-client processes from being killed.

Debug

For debugging the Dracut boot process, a special module is required. Including the module in the initramfs also includes several useful tools like scp for copying logfiles. Enable the debug USE flag.

root #dracut -m "<other modules>" initramfs-with-debug-only.img

Using the rd.debug flag, all Dracut commands are shown. When you are dropped to the Dracut shell, the logfile will be available in /run/initramfs/init.log.

FILE /var/lib/tftpboot/pxelinux.cfg/default

...
 APPEND initrd=initramfs-with-debug.img rd.debug
...

Other filesystems

Other filesystems than the ones that are available through the Dracut modules can be installed using the filesystems parameter. To include kernel modules, the kernel-modules Dracut module also has to be included. This module is installed by default.

root #dracut -m "kernel-modules base" --filesystems "squashfs" initramfs-with-squashfs.img

Custom modules

A quick Dracut module

See Rich0's Gentoo Blog for now.

40uvesafb

Load the uvesafb kernel module and add the sys-apps/v86d binary to the initramfs.

FILE /usr/lib/dracut/modules.d/40uvesafb/module-setup.sh

#!/bin/bash

check() {
    [[ -x /sbin/v86d && -f "$srcmods/kernel/drivers/video/fbdev/uvesafb.ko" ]]
}

depends() {
    return 0
}

installkernel() {
   hostonly='' instmods uvesafb
}

install() {
    dracut_install /sbin/v86d
    inst_hook pre-trigger 9 "$moddir/uvesafb-pretrigger.sh"
}

FILE /usr/lib/dracut/modules.d/40uvesafb/uvesafb-pretrigger.sh

#!/bin/sh
if getargbool 1 rd.uvesafb -n rd_NO_UVESAFB; then
    [ ! -c /dev/fb0 -a -e /sbin/v86d ] && modprobe uvesafb
fi

See also

• Genkernel - Gentoo's initramfs generator.

External resources

• The Official Fedora Wiki Dracut Page