|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | DESCRIPTION | BOOT PROCESS STAGES | NETWORK INFRASTRUCTURE | WRITING A MODULE | AUTHOR | SEE ALSO |
|
|
|
DRACUT.MODULES(7) dracut DRACUT.MODULES(7)
dracut.modules - dracut modules
dracut uses a modular system to build and extend the initramfs
image. All modules are located in /usr/lib/dracut/modules.d or in
<git-src>/modules.d. The most basic dracut module is 80base. In
80base the initial shell script init is defined, which gets run by
the kernel after initramfs loading. Although you can replace init
with your own version of 80base, this is not encouraged. Instead
you should use, if possible, the hooks of dracut. All hooks, and
the point of time in which they are executed, are described in
BOOT PROCESS STAGES.
The main script, which creates the initramfs is dracut itself. It
parses all arguments and sets up the directory, in which
everything is installed. It then executes all check, install,
installkernel scripts found in the modules, which are to be
processed. After everything is installed, the install directory is
archived and compressed to the final initramfs image. All helper
functions used by check, install and installkernel are found in
the file dracut-functions. These shell functions are available to
all module installer (install, installkernel) scripts, without the
need to source dracut-functions.
A module can check the preconditions for install and installkernel
with the check script. Also dependencies can be expressed with
check. If a module passed check, install and installkernel will be
called to install all of the necessary files for the module. To
split between kernel and non-kernel parts of the installation, all
kernel module related parts have to be in installkernel. All other
files found in a module directory are module specific and mostly
are hook scripts and udev rules.
dracut modules can insert custom script at various points, to
control the boot process. These hooks are plain directories
containing shell scripts ending with ".sh", which are sourced by
init. Common used functions are in dracut-lib.sh, which can be
sourced by any script.
Hook: cmdline
The cmdline hook is a place to insert scripts to parse the kernel
command line and prepare the later actions, like setting up udev
rules and configuration files.
In this hook the most important environment variable is defined:
root. The second one is rootok, which indicates, that a module
claimed to be able to parse the root defined. So for example,
root=iscsi:.... will be claimed by the iscsi dracut module, which
then sets rootok.
Hook: pre-udev
This hook is executed right after the cmdline hook and a check if
root and rootok were set. Here modules can take action with the
final root, and before udev has been run.
Start Udev
Now udev is started and the logging for udev is setup.
Hook: pre-trigger
In this hook, you can set udev environment variables with udevadm
control --property=KEY=value or control the further execution of
udev with udevadm.
Trigger Udev
udev is triggered by calling udevadm trigger, which sends add
events for all devices and subsystems.
Main Loop
In the main loop of dracut loops until udev has settled and all
scripts in initqueue/finished returned true. In this loop there
are three hooks, where scripts can be inserted by calling
/sbin/initqueue.
Initqueue
This hook gets executed every time a script is inserted here,
regardless of the udev state.
Initqueue settled
This hook (initqueue/settled) gets executed every time udev has
settled.
Initqueue timeout
This hook (initqueue/timeout) gets executed, when the main loop
counter becomes half of the rd.retry counter.
Initqueue online
This hook (initqueue/online) gets executed whenever a network
interface comes online (that is, once it is up and configured by
the configured network module).
Initqueue finished
This hook (initqueue/finished) is called after udev has settled
and if all scripts herein return 0 the main loop will be ended.
Arbitrary scripts can be added here, to loop in the initqueue
until something happens, which a dracut module wants to wait for.
Hook: pre-mount
Before the root device is mounted all scripts in the hook
pre-mount are executed. In some cases (e.g. NFS) the real root
device is already mounted, though.
Hook: mount
This hook is mainly to mount the real root device.
Hook: pre-pivot
This hook is called before cleanup hook, This is a good place for
actions other than cleanups which need to be called before pivot.
Hook: cleanup
This hook is the last hook and is called before init finally
switches root to the real root device. This is a good place to
clean up and kill processes not needed anymore.
Cleanup and switch_root
Init (or systemd) kills all udev processes, cleans up the
environment, sets up the arguments for the real init process and
finally calls switch_root. switch_root removes the whole
filesystem hierarchy of the initramfs, chroot()s to the real root
device and calls /sbin/init with the specified arguments.
To ensure all files in the initramfs hierarchy can be removed, all
processes still running from the initramfs should not have any
open file descriptors left.
FIXME
A simple example module is 90kernel-modules, which modprobes a
kernel module after udev has settled and the basic device drivers
have been loaded.
All module installation information is in the file
module-setup.sh.
First we create a check() function, which just exits with 0
indicating that this module should be included by default.
check():
return 0
Then we create the install() function, which installs a cmdline
hook with priority number 20 called parse-insmodpost.sh. It also
installs the insmodpost.sh script in /sbin.
install():
inst_hook cmdline 20 "$moddir/parse-insmodpost.sh"
inst_simple "$moddir/insmodpost.sh" /sbin/insmodpost.sh
The parse-instmodpost.sh parses the kernel command line for a
argument rd.driver.post, blacklists the module from being
autoloaded and installs the hook insmodpost.sh in the
initqueue/settled.
parse-insmodpost.sh:
for p in $(getargs rd.driver.post=); do
echo "blacklist $p" >> /run/modprobe.d/initramfsblacklist.conf
_do_insmodpost=1
done
[ -n "$_do_insmodpost" ] && /sbin/initqueue --settled --unique --onetime /sbin/insmodpost.sh
unset _do_insmodpost
insmodpost.sh, which is called in the initqueue/settled hook will
just modprobe the kernel modules specified in all rd.driver.post
kernel command line parameters. It runs after udev has settled and
is only called once (--onetime).
insmodpost.sh:
. /lib/dracut-lib.sh
for p in $(getargs rd.driver.post=); do
modprobe $p
done
module-setup.sh: check()
check() is called by dracut to evaluate the inclusion of a dracut
module in the initramfs.
$hostonly
If the $hostonly variable is set, then the module check()
function should be in "hostonly" mode, which means, that the
check() should only return 0, if the module is really needed
to boot this specific host.
check() should return with:
0
Include the dracut module in the initramfs.
1
Do not include the dracut module. The requirements are not
fulfilled (missing tools, etc.)
255
Only include the dracut module, if another module requires it
or if explicitly specified in the config file or on the
argument list.
module-setup.sh: depends()
The function depends() should echo all other dracut module names
the module depends on.
module-setup.sh: cmdline()
This function should print the kernel command line options needed
to boot the current machine setup. It should start with a space
and should not print a newline.
module-setup.sh: install()
The install() function is called to install everything non-kernel
related. To install binaries, scripts, and other files, you can
use the functions mentioned in [creation].
To address a file in the current module directory, use the
variable "$moddir".
module-setup.sh: installkernel()
In installkernel() all kernel related files should be installed.
You can use all of the functions mentioned in [creation] to
install files.
Creation Functions
inst_dir <dir>
installs a directory <dir> (but not its content) to the same place
in the initramfs image.
inst [-H] <src> [<dst>]
inst_binary <src> [<dst>]
inst_script <src> [<dst>]
installs one file <src> either to the same place in the initramfs
or to an optional <dst>, and also its dependencies and .hmac file
(if it exists). inst with more than two arguments is treated the
same as inst_multiple, all arguments are treated as files to
install and none as install destinations.
options:
-H
log all installed files to /lib/dracut/hostonly-files, so they
can be removed if rd.hostonly is passed on the kernel command
line.
inst_simple [-H] <src> [<dst>]
installs one file <src> either to the same place in the initramfs
or to an optional <dst>, but without installing its dependencies
or .hmac file.
options:
-H
log all installed files to /lib/dracut/hostonly-files, so they
can be removed if rd.hostonly is passed on the kernel command
line.
inst_multiple [-H] [-o] <file> [ <file> ...]
installs multiple binaries and files. If executables are specified
without a path, dracut will search the path
PATH=/usr/sbin:/sbin:/usr/bin:/bin for the binary.
options:
-H
log all installed files to /lib/dracut/hostonly-files, so they
can be removed if rd.hostonly is passed on the kernel command
line.
-o
optional, a missing file does not lead to an error.
inst_hook <hookdir> <prio> <src>
installs an executable/script <src> in the dracut hook <hookdir>
with priority <prio>.
inst_rules <udevrule> [ <udevrule> ...]
installs one or more udev rules. Non-existent udev rules are
reported, but do not let dracut fail.
inst_libdir_dir <dir> [<dir>...]
installs multiple directories (but not their content) located on a
library directory to the initramfs image.
inst_libdir_file [-n <pattern>] [-o] <file> [<file>...]
installs multiple files located on a library directory to the
initramfs image.
options:
-n <pattern>
install all library files matching a pattern.
-o
optional, a missing library does not lead to an error.
instmods [-c] [-s] <kernelmodule> [ <kernelmodule> ... ]
instmods should be used only in the installkernel() function.
instmods installs one or more kernel modules in the initramfs.
<kernelmodule> can also be a whole subsystem, if prefixed with a
"=", like "=drivers/net/team".
instmods will not install the kernel module, if $hostonly is set
and the kernel module is not currently needed by any
/sys/.../uevent MODALIAS. To install a kernel module regardless of
the hostonly mode use the form:
hostonly='' instmods <kernelmodule>
a common pattern to call instmods is to use the following form:
`hostonly=$(optional_hostonly) instmods <kernelmodule>`
This statement installs the kernel module in either of the
following conditions: * in non-hostonly mode (regardless of the
host state) * in sloppy hostonly mode (regardless of the host
state) * in hostonly mode if the kernel module is already loaded
in the host
options:
-c
check that kernel modules exists and can be installed (i.e.,
not optional).
-s
reduce verbosity.
Initramfs Functions
FIXME
Network Modules
FIXME
Harald Hoyer
man:dracut[8].SH COLOPHON This page is part of the dracut (event
driven initramfs infrastructure) project. Information about the
project can be found at ⟨https://github.com/dracut-ng//dracut-ng⟩.
If you have a bug report for this manual page, see
⟨https://github.com/dracut-ng/dracut-ng/issues⟩. This page was
obtained from the project's upstream Git repository
⟨https://github.com/dracut-ng/dracut-ng.git⟩ on 2025-08-11. (At
that time, the date of the most recent commit that was found in
the repository was 2025-08-09.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
dracut 2025-08-09 DRACUT.MODULES(7)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|