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 <kernel-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 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. The usage of the --hostonly option is only for experts and you will have to keep the broken pieces. At least keep a copy of a general purpose image (and corresponding kernel) as a fallback to rescue your system.
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
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 module initramfs-module.img
To see a list of available dracut modules, use the --list-modules option:
# dracut --list-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
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 dracut.conf(5)):
# dracut --add-drivers mymod initramfs-with-mymod.img
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.
Generic initrd’s 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 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
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>:
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.
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.
Sometimes it is required to prevent the automatic kernel module loading of a specific kernel module. To do this, just add rd.driver.blacklist=<kernel module name>, with <kernel module name> 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.
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.
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.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.
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 can’t pass a kernel command line, then you can inject /etc/cmdline.d/mycmdline.conf, with a method described in Injecting custom Files.
To reduce the size of the initramfs, you should create it with by omitting 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:
# 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.
If the boot process does not succeed, you have several options to debug the situation.
-
Remove ''rhgb'' and ''quiet'' from the kernel command line
-
Add ''rd.shell'' to the kernel command line. This will present a shell should dracut be unable to locate your root device
-
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
-
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.
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
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 <interfacename> # ip addr show
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.
-
First, enable serial console output for both the kernel and the bootloader.
-
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
-
Also in /boot/grub2/grub.cfg, add the following boot arguments to the ''kernel'' line:
console=tty0 console=ttyS0,9600
-
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
-
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.
-
Redirecting non-interactive output
NoteYou 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 </dev/console
dracut offers a shell for interactive debugging in the event dracut fails to locate your root filesystem. To enable the shell:
-
Add the boot parameter ''rd.shell'' to your bootloader configuration file (e.g. /boot/grub2/grub.cfg)
-
Remove the boot arguments ''rhgb'' and ''quiet''
A sample /boot/grub2/grub.cfg bootloader configuration file is listed 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 rd.shell initrd /dracut-2.6.29.5-191.fc11.x86_64.img
-
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. #
-
Use this shell prompt to gather the information requested above (see 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:
-
A block device (e.g. /dev/sda7)
-
A LVM logical volume (e.g. /dev/VolGroup00/LogVol00)
-
An encrypted device (e.g. /dev/mapper/luks-4d5972ea-901c-4584-bd75-1da802417d83)
-
A network attached device (e.g. netroot=iscsi:@192.168.0.4::3260::iqn.2009-02.org.example:for.all)
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.
-
Inspect your partitions using parted
# 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
-
You recall that your root volume was a LVM logical volume. Scan and activate any logical volumes.
# lvm vgscan # lvm vgchange -ay
-
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="ext4" /dev/mapper/linux-swap: UUID="47b4d329-975c-4c08-b218-f9c9bf3635f1" TYPE="swap"
-
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.
-
Next, make a symbolic link to the unlocked root volume
# ln -s /dev/mapper/luks-$UUID /dev/root
-
With the root volume available, you may continue booting the system by exiting the dracut shell
# exit
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:
# mkdir -p /run/initramfs/etc/cmdline.d # echo "rd.debug rd.break=pre-shutdown rd.break=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.