DRACUT(8) dracut DRACUT(8)
NAME
dracut - low-level tool for generating an initramfs/initrd image
SYNOPSIS
dracut [OPTION...] [ []]
DESCRIPTION
Create an initramfs for the kernel with the version . If is
omitted, then the version of the actual running kernel is used. If is omitted or empty, the
default location will be determined by the local configuration or Linux distribution policy.
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, mounting the root filesystem and
booting into the real system.
At boot time, the kernel unpacks that archive into RAM disk, mounts and uses it as initial root file
system. All finding of the root device happens in this early userspace.
Initramfs images are also called "initrd".
For a complete list of kernel command line options see man:dracut.cmdline[7].
If you are dropped to an emergency shell, while booting your initramfs, the file
/run/initramfs/rdsosreport.txt is created, which can be saved to a (to be mounted by hand) partition
(usually /boot) or a USB stick. Additional debugging info can be produced by adding rd.debug to the
kernel command line. /run/initramfs/rdsosreport.txt contains all logs and the output of some tools.
It should be attached to any report about dracut problems.
Dracut is expected to run as the root user to have unrestricted access to the root filesystem during
initramfs generation. Running dracut as any user other than root supports only a limited set of
functionalities. As an example, to facilitate secure sulogin in the emergency console with the host
password requires read access to /etc/shadow on the host, which is usually only allowed for the root
user.
USAGE
To create a initramfs image, the most simple command is:
# dracut
This will generate a general purpose initramfs image, with all possible functionality resulting of
the combination of the installed dracut modules and system tools. The image contains the kernel
modules of the currently active kernel with version . The default location of the
image is determined by the local configuration or Linux distribution policy.
If the initramfs image already exists, dracut will display an error message, and to overwrite the
existing image, you have to use the --force option.
# dracut --force
If you want to specify another filename for the resulting image you would issue a command like:
# dracut foobar.img
To generate an image for a specific kernel version, the command would be:
# dracut foobar.img 2.6.40-1.rc5.f20
A shortcut to generate the image at the default location for a specific kernel version is:
# dracut --kver 2.6.40-1.rc5.f20
If you want to create lighter, smaller initramfs images, you may want to specify the --hostonly or -H
option. Using this option, the resulting image will contain only those dracut modules, kernel modules
and filesystems, which are needed to boot this specific machine. This has the drawback, that you
canât put the disk on another controller or machine, and that you canât switch to another root
filesystem, without recreating the initramfs image. It is recommended to keep a copy of a general
purpose image (and corresponding kernel) as a fallback to rescue your system.
INSPECTING THE CONTENTS
To see the contents of the image created by dracut, you can use the lsinitrd tool.
# lsinitrd | less
To display the contents of a file in the initramfs also use the lsinitrd tool:
# lsinitrd -f /etc/ld.so.conf
include ld.so.conf.d/*.conf
ADDING DRACUT MODULES
Some dracut modules are turned off by default and have to be activated manually. You can do this by
adding the dracut modules to the configuration file /etc/dracut.conf or
/etc/dracut.conf.d/myconf.conf. See man:dracut.conf[5] You can also add dracut modules on the command
line by using the -a or --add option:
# dracut --add module initramfs-module.img
To see a list of available dracut modules, use the --list-modules option:
# dracut --list-modules
Tip
Omitted modules will override added modules. --force-add can be used to add a module in host-only
mode if it is specified as omitted (usually via a distribution configuration file).
OMITTING DRACUT MODULES
Sometimes you donât want a dracut module to be included for reasons of speed, size or functionality.
To do this, either specify the omit_dracutmodules variable in the dracut.conf or
/etc/dracut.conf.d/myconf.conf configuration file (see man:dracut.conf[5]) or use the -o or --omit
option on the command line:
# dracut -o "multipath lvm" no-multipath-lvm.img
ADDING KERNEL MODULES
If you need a special kernel module in the initramfs, which is not automatically picked up by dracut,
you have the use the --add-drivers option on the command line or the drivers variable in the
/etc/dracut.conf or /etc/dracut.conf.d/myconf.conf configuration file (see man:dracut.conf[5]):
# dracut --add-drivers mymod initramfs-with-mymod.img
BOOT PARAMETERS
An initramfs generated without the "hostonly" mode, does not contain any system configuration files
(except for some special exceptions), so the configuration has to be done on the kernel command line.
With this flexibility, you can easily boot from a changed root partition, without the need to
recompile the initramfs image. So, you could completely change your root partition (move it inside a
md raid with encryption and LVM on top), as long as you specify the correct filesystem LABEL or UUID
on the kernel command line for your root device, dracut will find it and boot from it.
Files from the host root filesystem under the /etc, /var, or /run directories should only be copied
over to the generated initramfs in "hostonly" mode, as these host directories are meant to be
customized by users for each host. Only host files from the /usr directory in the host root
filesystem are meant to be included in the generated initramfs in both "hostonly" and "non-hostonly"
mode. As an example /etc/fstab and /etc/cryptab files should be only consulted when dracut is run in
"hostonly" mode.
The only exception to this rule is the /etc/dracut.conf file and /etc/dracut.conf.d/ directory that
is considered both in "hostonly" and "non-hostonly" modes.
Some command line arguments (e.g. --include) can override the default "hostonly"/"non-hostonly"
modes.
Generic initrdâs are larger, but should be able to automatically boot any bootable configuration with
appropriate boot flags (root device, network configuration information, etc.)
The kernel command line can also be provided by the dhcp server with the root-path option. See
NETWORK BOOT.
For a full reference of all kernel command line parameters, see man:dracut.cmdline[7].
To get a quick start for the suitable kernel command line on your system, use the --print-cmdline
option:
# dracut --print-cmdline
root=UUID=8b8b6f91-95c7-4da2-831b-171e12179081 rootflags=rw,relatime,discard,data=ordered rootfstype=ext4
Specifying the root Device
This is the only option dracut really needs to boot from your root partition. Because your root
partition can live in various environments, there are a lot of formats for the root= option. The most
basic one is root=:
root=/dev/sda2
Because device node names can change, dependent on the drive ordering, you are encouraged to use the
filesystem identifier (UUID) or filesystem label (LABEL) to specify your root partition:
root=UUID=19e9dda3-5a38-484d-a9b0-fa6b067d0331
or
root=LABEL=myrootpartitionlabel
To see all UUIDs or LABELs on your system, do:
# ls -l /dev/disk/by-uuid
or
# ls -l /dev/disk/by-label
If your root partition is on the network see NETWORK BOOT.
Keyboard Settings
If you have to input passwords for encrypted disk volumes, you might want to set the keyboard layout
and specify a display font.
A typical german kernel command line would contain:
rd.vconsole.font=eurlatgr rd.vconsole.keymap=de-latin1-nodeadkeys rd.locale.LANG=de_DE.UTF-8
Setting these options can override the setting stored on your system, if you use a modern init
system, like systemd.
Blacklisting Kernel Modules
Sometimes it is required to prevent the automatic kernel module loading of a specific kernel module.
To do this, just add rd.driver.blacklist=, with not
containing the .ko suffix, to the kernel command line. For example:
rd.driver.blacklist=mptsas rd.driver.blacklist=nouveau
The option can be specified multiple times on the kernel command line.
Speeding up the Boot Process
If you want to speed up the boot process, you can specify as much information for dracut on the
kernel command as possible. For example, you can tell dracut, that you root partition is not on a LVM
volume or not on a raid partition, or that it lives inside a specific crypto LUKS encrypted volume.
By default, dracut searches everywhere. A typical dracut kernel command line for a plain primary or
logical partition would contain:
rd.luks=0 rd.lvm=0 rd.md=0 rd.dm=0
This turns off every automatic assembly of LVM, MD raids, DM raids and crypto LUKS.
Of course, you could also omit the dracut modules in the initramfs creation process, but then you
would lose the possibility to turn it on on demand.
INJECTING CUSTOM FILES
To add your own files to the initramfs image, you have several possibilities.
The --include option let you specify a source path and a target path. For example
# dracut --include cmdline-preset /etc/cmdline.d/50-mycmdline.conf initramfs-cmdline-pre.img
will create an initramfs image, where the file cmdline-preset will be copied inside the initramfs to
/etc/cmdline.d/50-mycmdline.conf.
# mkdir -p rd.overlay/etc/cmdline.d
# mkdir -p rd.overlay/etc/conf.d
# echo "ip=dhcp" >> rd.overlay/etc/cmdline.d/50-mycmdline.conf
# echo export FOO=testtest >> rd.overlay/etc/conf.d/testvar.conf
# echo export BAR=testtest >> rd.overlay/etc/conf.d/testvar.conf
# tree rd.overlay/
rd.overlay/
`-- etc
|-- cmdline.d
| `-- 50-mycmdline.conf
`-- conf.d
`-- testvar.conf
# dracut --include rd.overlay / initramfs-rd.overlay.img
This will put the contents of the rd.overlay directory into the root of the initramfs image.
The --install option let you specify several files, which will get installed in the initramfs image
at the same location, as they are present on initramfs creation time.
# dracut --install 'strace fsck.ext4 ssh' initramfs-dbg.img
This will create an initramfs with the strace, fsck.ext4 and ssh executables, together with the
libraries needed to start those. The --install option can be specified multiple times.
NETWORK BOOT
If your root partition is on a network drive, you have to have the network dracut modules installed
to create a network aware initramfs image.
If you specify ip=dhcp on the kernel command line, then dracut asks a dhcp server about the ip
address for the machine. The dhcp server can also serve an additional root-path, which will set the
root device for dracut. With this mechanism, you have static configuration on your client machine and
a centralized boot configuration on your TFTP/DHCP server. If you canât pass a kernel command line,
then you can inject /etc/cmdline.d/50-mycmdline.conf, with a method described in INJECTING CUSTOM
FILES.
Reducing the Image Size
To reduce the size of the initramfs, you should create it with by omitting all dracut modules, which
you know, you donât need to boot the machine.
You can also specify the exact dracut and kernel modules to produce a very tiny initramfs image.
For example for a NFS image, you would do:
# dracut -m "nfs network base" initramfs-nfs-only.img
Then you would boot from this image with your target machine and reduce the size once more by
creating it on the target machine with the --host-only option:
# dracut -m "nfs network base" --host-only initramfs-nfs-host-only.img
This will reduce the size of the initramfs image significantly.
TROUBLESHOOTING
If the boot process does not succeed, you have several options to debug the situation.
IDENTIFYING YOUR PROBLEM AREA
1. Remove 'splash' and 'quiet' from the kernel command line
2. Add 'rd.shell' to the kernel command line. This will present a shell should dracut be unable to
locate your root device
3. Add 'rd.shell rd.debug log_buf_len=1M' to the kernel command line so that dracut shell commands
are printed as they are executed
4. The file /run/initramfs/rdsosreport.txt is generated, which contains all the logs and the output
of all significant tools, which are mentioned later.
If you want to save that output, simply mount /boot by hand or insert an USB stick and mount that.
Then you can store the output for later inspection.
INFORMATION TO INCLUDE IN YOUR REPORT
All bug reports
In all cases, the following should be mentioned and attached to your bug report:
⢠The exact kernel command-line used. Typically from the bootloader configuration file (e.g.
/boot/grub2/grub.cfg) or from /proc/cmdline.
⢠A copy of your disk partition information from /etc/fstab, which might be obtained booting an old
working initramfs or a rescue medium.
⢠Turn on dracut debugging (see the debugging dracut section), and attach the file
/run/initramfs/rdsosreport.txt.
⢠If you use a dracut configuration file, please include /etc/dracut.conf and all files in
/etc/dracut.conf.d/*.conf
Network root device related problems
This section details information to include when experiencing problems on a system whose root device
is located on a network attached volume (e.g. iSCSI, NFS or NBD). As well as the information from All
bug reports, include the following information:
⢠Please include the output of
# /sbin/ifup
# ip addr show
DEBUGGING DRACUT
Configure a serial console
Successfully debugging dracut will require some form of console logging during the system boot. This
section documents configuring a serial console connection to record boot messages.
1. First, enable serial console output for both the kernel and the bootloader.
2. Open the file /boot/grub2/grub.cfg for editing. Below the line 'timeout=5', add the following:
serial --unit=0 --speed=9600
terminal --timeout=5 serial console
3. Also in /boot/grub2/grub.cfg, add the following boot arguments to the 'kernel' line:
console=tty0 console=ttyS0,9600
4. When finished, the /boot/grub2/grub.cfg file should look similar to the example below.
default=0
timeout=5
serial --unit=0 --speed=9600
terminal --timeout=5 serial console
title Fedora (2.6.29.5-191.fc11.x86_64)
root (hd0,0)
kernel /vmlinuz-2.6.29.5-191.fc11.x86_64 ro root=/dev/mapper/vg_uc1-lv_root console=tty0 console=ttyS0,9600
initrd /dracut-2.6.29.5-191.fc11.x86_64.img
5. More detailed information on how to configure the kernel for console output can be found at
http://www.faqs.org/docs/Linux-HOWTO/Remote-Serial-Console-HOWTO.html#CONFIGURE-KERNEL.
6. Redirecting non-interactive output
Note
You can redirect all non-interactive output to /dev/kmsg and the kernel will put it out on
the console when it reaches the kernel buffer by doing
# exec >/dev/kmsg 2>&1 /run/initramfs/etc/cmdline.d/debug.conf
# touch /run/initramfs/.need_shutdown
This will give you a dracut shell after the system pivotâed back in the initramfs.
OPTIONS
--kver
Set the kernel version. This enables to specify the kernel version, without specifying the
location of the initramfs image. For example:
# dracut --kver 3.5.0-0.rc7.git1.2.fc18.x86_64
-f, --force
Overwrite existing initramfs file.