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.
Before you can use the images generated by dracut, your kernel must include initramfs support. The ebuild will warn you if your kernel is missing the required options:
The behavior of dracut is controlled by dracut modules (not to be confused with kernel modules). Each dracut module you include in your initramfs produces certain additional behavior at boot time. By default, sys-kernel/dracut installs a set of base modules that are most commonly needed for systems where the rootfs is found on a simple local block device. If you need additional features, such as RAID, LVM, or NFS mounted root support, you can install optional modules by listing them in the DRACUT_MODULES expansion variable in /etc/portage/make.conf:
Certain modules also require additional USE flags to be set (see #Optional_Modules for details). In general, you should not enable any of the USE flags for sys-kernel/dracut unless Portage asks you to. It will tell you if there are any missing REQUIRED_USE constraints when you attempt to merge:
Since sys-kernel/dracut is still unstable, if you are not already running ~arch, you will need to keyword it:
Building An initramfs Image
Once you have installed dracut, you can use it to build an initramfs image. The simplest way to do this is to just run
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:
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.
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 output will be written into your /boot folder (which you should already have mounted), 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:
When installed without any use flags, dracut installs a basic set of modules that can boot a system using a locally attached block device for its rootfs. 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.
|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 |
|resume||allows initramfs to resume from low-power state||always|
|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|
- 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.
The table above indicates when a particular module will be included in your initramfs image by default. 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:
dracutmodules (or the
-m parameter) will override the entire default list, and install only those modules you specified. This can help cut down significantly on your image size (for example, if you don't need any kernel modules, or i18n support, etc.) If you aren't 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 mossing) 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.
Adding additional modules to your installation involves setting the appropriate values for your DRACUT_MODULES variable, as described in the #Installation section. Each additional module you include brings with it additional prerequisite packages, and may require USE flag changes to either sys-kernel/dracut or other modules. In most cases, you should already have the additional packages installed. If dracut tries to bring in a lot of additional packages you should confirm that you are asking for the correct module.
The optional modules included as of sys-kernel/dracut-016 are listed below, with their additional requirements.
|Module||Purpose||Enabled?||USE Flag||Additional Packages|
|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||always|| sys-libs/libcap (also replaces |
|crypt||supports an encrypted rootfs||host-only: rootfs||
|crypt-gpg|| use gpg for crypt support (requires
|dmraid||support rootfs on FakeRAID||host-only: rootfs||
|dmsquash-live||supports LiveCD rootfs||never (host-only: refused)||
|gensplash||include a static splash screen||never||
|iscsi||supports rootfs on iscsi devices||host-only: rootfs||
|livenet|| retrieve rootfs over HTTP (requires
|lvm||supports rootfs on LVM device||host-only: rootfs||
|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||
|nfs||supports NFS-mounted rootfs||
||host-only: rootfs||net-fs/nfs-utils net-nds/rpcbind|
|plymouth||includes boot animation||always||
|ssh-client||includes ssh and scp clients||never||
|syslog||includes remote logging support||never||app-admin/syslog-ng or rsyslog|
|debug||includes useful troubleshooting tools||never|| installed when
|dm||includes device-mapper||never|| installed when
||sys-fs/device-mapper or sys-fs/lvm2|
|ifcfg||generates network config at runtime||never|| installed when
|network||brings up network for net booting||always|| installed when
|selinux||arranges for the selinux policy to be loaded||never|| installed when
|url-lib||includes cUrl and SSL certs||never|| installed when
- sys-boot/plymouth requires that x11-libs/libdrm be built with the
- The additional packages require that one or more dependency libraries be built with the
- 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.
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:
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
- In host-only mode, the running hosts filesystems are always installed, regardless of the
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. You can alter the behavior of the initramfs with the
fstab-sys module, and the following options:
- Create a file called /etc/fstab.sys, which will be copied to the initramfs and mounted.
--use-fstab), which will use your system's /etc/fstab instead of /proc/self/mountinfo
--add_fstab filename, which will add the contents of filename to the initramfs's /etc/fstab.
--mount "fstab mount spec", which adds individual entries into your initramfs's /etc/fstab.
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.
For booting an ext(2,3,4) filesystem you don't need any of the optional dracut modules.
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:
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:
For an NFS boot, you need only 3 modules and the nfs dracut module.
|net||No||Yes||Set of helper modules providing networking support.|
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>
For an NBD boot, you need only 3 modules and the nbd dracut module.
|net||No||Yes||Set of helper modules providing networking support.|
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>
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.
|debug||No||Yes||Module installing additional tools like strace, file editor, ssh and more.|
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.
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.
A Quick Dracut Module
See Rich0's Gentoo Blog for now.