dracut (8) - Linux Manuals
dracut: low-level tool for generating an initramfs image
NAME
dracut - low-level tool for generating an initramfs image
SYNOPSIS
dracut [OPTION...] [<image> [<kernel version>]]
DESCRIPTION
Create an initramfs <image> for the kernel with the version <kernel version>. If <kernel version> is omitted, then the version of the actual running kernel is used. If <image> is omitted or empty, then the default location /boot/initramfs-<kernel version>.img is used.
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.
For a complete list of kernel command line options see 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 safed 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.
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 is /boot/initramfs-<kernel version>.img and contains the kernel modules of the currently active kernel with version <kernel version>.
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 can't put the disk on another controller or machine, and that you can't switch to another root filesystem, without recreating the initramfs image.
-
Note
On RHEL-7 the hostonly mode is the default mode. Generic "non-hostonly" images are created, if the dracut-config-generic rpm is installed. The rescue kernel entry in the bootloader menu is also a generic image.
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 dracut.conf(5). You can also add dracut modules on the command line by using the -a or --add option:
-
# dracut --add bootchart initramfs-bootchart.img
To see a list of available dracut modules, use the --list-modules option:
-
# dracut --list-modules
Omitting dracut Modules
Sometimes you don't 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 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 vaiable in the /etc/dracut.conf or /etc/dracut.conf.d/myconf.conf configuration file (see 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 completly 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.
The kernel command line usually can be configured in /boot/grub2/grub.cfg (or /boot/efi/EFI/redhat/grub.cfg for EFI systems), if grub2 is your bootloader and it also can be edited in the real boot process in the grub menu.
The kernel command line can also be provided by the dhcp server with the root-path option. See the section called "Network Boot".
For a full reference of all kernel command line parameters, see dracut.cmdline(5).
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
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=<path to device node>:
Specifying the root Device
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 the section called "Network Boot".
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 would contain:
Keyboard Settings
rd.vconsole.font=latarcyrheb-sun16 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.
Sometimes it is required to prevent the automatic kernel module loading of a specific kernel module. To do this, just add rd.blacklist=<kernel module name>, with <kernel module name> not containing the .ko suffix, to the kernel command line. For example:
Blacklisting Kernel Modules
rd.driver.blacklist=mptsas rd.driver.blacklist=nouveau
The option can be specified multiple times on the kernel command line.
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:
Speeding up the Boot Process
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 posibility 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/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/mycmdline.conf. --include can only be specified once.
-
# mkdir -p rd.live.overlay/etc/cmdline.d # mkdir -p rd.live.overlay/etc/conf.d # echo "ip=dhcp" >> rd.live.overlay/etc/cmdline.d/mycmdline.conf # echo export FOO=testtest >> rd.live.overlay/etc/conf.d/testvar.conf # echo export BAR=testtest >> rd.live.overlay/etc/conf.d/testvar.conf # tree rd.live.overlay/ rd.live.overlay/ `-- etc |-- cmdline.d | `-- mycmdline.conf `-- conf.d `-- testvar.conf # dracut --include rd.live.overlay / initramfs-rd.live.overlay.img
This will put the contents of the rd.live.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.ext3 ssh' initramfs-dbg.img
This will create an initramfs with the strace, fsck.ext3 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.
On a Red Hat Enterprise Linux or Fedora system, this means, you have to install the dracut-network rpm package:
-
# yum install dracut-network
The resulting initramfs image can be served by a boot manager residing on your local hard drive or it can be served by a PXE/TFTP server.
How to setup your PXE/TFTP server can be found in the m[blue]Red Hat Enterprise Linux Storage Administration Guidem[][1].
If you specify ip=dhcp on the kernel command line, then dracut asks a dhcp server about the ip adress 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 can't pass a kernel command line, then you can inject /etc/cmdline.d/mycmdline.conf, with a method described in the section called "Injecting custom Files".
To reduce the size of the initramfs, you should create it with by ommitting all dracut modules, which you know, you don't 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:
Reducing the Image Size
# 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. Some of the basic operations are covered here. For more information you should also visit: m[blue]http://fedoraproject.org/wiki/How_to_debug_Dracut_problemsm[]
Identifying your problem area
- 1. Remove 'rhgb' 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
In all cases, the following should be mentioned and attached to your bug report:
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 the section called "All bug reports", include the following information:
All bug reports
# /sbin/ifup <interfacename> # ip addr show
Debugging dracut
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.
Configure a serial console
console=tty0 console=ttyS0,9600
- 2. More detailed information on how to configure the kernel for console output can be found at m[blue]http://www.faqs.org/docs/Linux-HOWTO/Remote-Serial-Console-HOWTO.html#CONFIGURE-KERNELm[].
dracut offers a shell for interactive debugging in the event dracut fails to locate your root filesystem. To enable the shell:
A sample
grub.cfg
bootloader configuration file snippet is listed below.
Using the dracut shell
menuentry 'Red Hat Enterprise Linux Server (3.10.0-308.el7.x86_64) 7.0 (Maipo)' [...] {
[...]
linux16 /vmlinuz-3.10.0-308.el7.x86_64 ro root=/dev/mapper/vg_uc1-lv_root console=tty0 rd.shell
initrd16 /initramfs-3.10.0-308.el7.x86_64.img
[...]
}
-
3.
If system boot fails, you will be dropped into a shell as seen in the example below.
-
No root device found Dropping to debug shell. #
-
- 4. Use this shell prompt to gather the information requested above (see the section called "All bug reports").
From the dracut debug shell, you can manually perform the task of locating and preparing your root volume for boot. The required steps will depend on how your root volume is configured. Common scenarios include:
The exact method for locating and preparing will vary. However, to continue with a successful boot, the objective is to locate your root volume and create a symlink /dev/root which points to the file system. For example, the following example demonstrates accessing and booting a root volume that is an encrypted LVM Logical volume.
Accessing the root volume from the dracut shell
# parted /dev/sda -s p Model: ATA HTS541060G9AT00 (scsi) Disk /dev/sda: 60.0GB Sector size (logical/physical): 512B/512B Partition Table: msdos Number Start End Size Type File system Flags 1 32.3kB 10.8GB 107MB primary ext4 boot 2 10.8GB 55.6GB 44.7GB logical lvm
-
2.
You recall that your root volume was a LVM logical volume. Scan and activate any logical volumes.
-
# lvm vgscan # lvm vgchange -ay
-
-
3.
You should see any logical volumes now using the command blkid:
-
# blkid /dev/sda1: UUID="3de247f3-5de4-4a44-afc5-1fe179750cf7" TYPE="ext4" /dev/sda2: UUID="Ek4dQw-cOtq-5MJu-OGRF-xz5k-O2l8-wdDj0I" TYPE="LVM2_member" /dev/mapper/linux-root: UUID="def0269e-424b-4752-acf3-1077bf96ad2c" TYPE="crypto_LUKS" /dev/mapper/linux-home: UUID="c69127c1-f153-4ea2-b58e-4cbfa9257c5e" TYPE="ext3" /dev/mapper/linux-swap: UUID="47b4d329-975c-4c08-b218-f9c9bf3635f1" TYPE="swap"
-
-
4.
From the output above, you recall that your root volume exists on an encrypted block device. Following the guidance disk encryption guidance from the Installation Guide, you unlock your encrypted root volume.
-
# UUID=$(cryptsetup luksUUID /dev/mapper/linux-root) # cryptsetup luksOpen /dev/mapper/linux-root luks-$UUID Enter passphrase for /dev/mapper/linux-root: Key slot 0 unlocked.
-
-
5.
Next, make a symbolic link to the unlocked root volume
-
# ln -s /dev/mapper/luks-$UUID /dev/root
-
-
6.
With the root volume available, you may continue booting the system by exiting the dracut shell
-
# exit
-
For more debugging options, see dracut.cmdline(7).
To debug the shutdown sequence on systemd systems, you can rd.break on pre-shutdown or shutdown.
To do this from an already booted system:
Additional dracut boot parameters
# mkdir -p /run/initramfs/etc/cmdline.d # echo "rd.break=pre-shutdown" > /run/initramfs/etc/cmdline.d/debug.conf # touch /run/initramfs/.need_shutdown
This will give you a dracut shell after the system pivot'ed back in the initramfs.
OPTIONS
--kver <kernel version>
- 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.
-m, --modules <list of dracut modules>
-
specify a space-separated list of dracut modules to call when building the initramfs. Modules are located in
/usr/lib/dracut/modules.d. This parameter can be specified multiple times.
-
Note
If [LIST] has multiple arguments, then you have to put these in quotes. For example:-
# dracut --modules "module1 module2" ...
-
-
-o, --omit
-a, --add
--force-add
-d, --drivers
--add-drivers
--omit-drivers
--filesystems
-k, --kmoddir
--fwdir
--kernel-cmdline <parameters>
--kernel-only
--no-kernel
--early-microcode
--no-early-microcode
--print-cmdline
--mdadmconf
--nomdadmconf
--lvmconf
--nolvmconf
--fscks [LIST]
--nofscks
--strip
--nostrip
--prelink
--noprelink
--hardlink
--nohardlink
--prefix
--noprefix
-h, --help
--debug
-v, --verbose
-q, --quiet
-c, --conf
Default:
/etc/dracut.conf
--confdir
Default:
/etc/dracut.conf.d
--tmpdir
Default:
/var/tmp
--sshkey
--logfile
Default:
/var/log/dracut.log
-l, --local
-H, --hostonly
-N, --no-hostonly
--hostonly-cmdline: Store kernel command line arguments needed in the initramfs
--no-hostonly-cmdline: Do not store kernel command line arguments needed in the initramfs
--hostonly-i18n: Install only needed keyboard and font files according to the host configuration (default).
--no-hostonly-i18n: Install all keyboard and font files available.
--persistent-policy <policy>
--fstab
--add-fstab <filename>
--mount
--add-device <device>
-i, --include <SOURCE> <TARGET>
-I, --install <file list>
--install-optional <file list>
--gzip
--bzip2
--lzma
--xz
--lzo
Make sure your kernel has lzo decompression support compiled in, otherwise you will not be able to boot.
--lz4
Make sure your kernel has lz4 decompression support compiled in, otherwise you will not be able to boot.
--compress
--no-compress
--list-modules
-M, --show-modules
--keep
--printsize
--profile: Output profile information of the build process
--ro-mnt: Mount / and /usr read-only by default.
-L, --stdlog
--regenerate-all
/var/log/dracut.log
/tmp/dracut.log
/etc/dracut.conf
/etc/dracut.conf.d/*.conf
/usr/lib/dracut/dracut.conf.d/*.conf
/etc/conf.d/
/etc/cmdline
/etc/cmdline.d/*.conf
The dracut command is part of the dracut package and is available from m[blue]https://dracut.wiki.kernel.orgm[]
Harald Hoyer
Victor Lowther
Philippe Seewer
Warren Togami
Amadeusz Żołnowski
Jeremy Katz
David Dillow
dracut.cmdline(7) dracut.conf(5)
Note
If [LIST] has multiple arguments, then you have to put these in quotes. For example:
# dracut --omit "module1 module2" ...
Note
If [LIST] has multiple arguments, then you have to put these in quotes. For example:
# dracut --add "module1 module2" ...
Note
If [LIST] has multiple arguments, then you have to put these in quotes. For example:
# dracut --force-add "module1 module2" ...
Note
If [LIST] has multiple arguments, then you have to put these in quotes. For example:
# dracut --drivers "kmodule1 kmodule2" ...
Note
If [LIST] has multiple arguments, then you have to put these in quotes. For example:
# dracut --add-drivers "kmodule1 kmodule2" ...
Note
If [LIST] has multiple arguments, then you have to put these in quotes. For example:
# dracut --omit-drivers "kmodule1 kmodule2" ...
Note
If [LIST] has multiple arguments, then you have to put these in quotes. For example:
# dracut --filesystems "filesystem1 filesystem2" ...
Note
If [LIST] has multiple arguments, then you have to put these in quotes. For example:
# dracut --fscks "fsck.foo barfsck" ...
Warning
If chrooted to another root other than the real root device, use "--fstab" and provide a valid
/etc/fstab.
Note
If [LIST] has multiple arguments, then you have to put these in quotes. For example:
# dracut --install "/bin/foo /sbin/bar" ...
Warning
Make sure your kernel has bzip2 decompression support compiled in, otherwise you will not be able to boot. Equivalent to "--compress=bzip2"
Warning
Make sure your kernel has lzma decompression support compiled in, otherwise you will not be able to boot. Equivalent to "lzma --compress=lzma -9"
Warning
Make sure your kernel has xz decompression support compiled in, otherwise you will not be able to boot. Equivalent to "lzma --compress=xz --check=crc32 --lzma2=dict=1MiB"
Warning
Warning
0 - suppress any messages
1 - only fatal errors
2 - all errors
3 - warnings
4 - info
5 - debug info (here starts lots of output)
6 - trace info (and even more)
FILES
Configuration in the initramfs
AVAILABILITY
AUTHORS
NOTES
SEE ALSO