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 99base. In 99base
   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 99base, 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 the section called "BOOT

   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 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.

       This hook gets executed every time a script is inserted here,
       regardless of the udev state.

   Initqueue settled
       This hooks (initqueue/settled) gets executed every time udev has

   Initqueue timeout
       This hooks (initqueue/timeout) gets executed, when the main loop
       counter becomes half of the rd.retry counter.

   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.
       Abritary 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.




   A simple example module is 96insmodpost, which modprobes a kernel
   module after udev has settled and the basic device drivers have been

   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.


       return 0

   The 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.


       inst_hook cmdline 20 "$moddir/parse-insmodpost.sh"
       inst_simple "$moddir/insmodpost.sh" /sbin/insmodpost.sh

   The pase-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.


       for p in $(getargs rd.driver.post=); do
           echo "blacklist $p" >> /etc/modprobe.d/initramfsblacklist.conf

       [ -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).


       . /lib/dracut-lib.sh

       for p in $(getargs rd.driver.post=); do
           modprobe $p

   module-setup.sh: check()
   check() is called by dracut to evaluate the inclusion of a dracut
   module in the initramfs.

       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

   check() should return with:

       Include the dracut module in the initramfs.

       Do not include the dracut module. The requirements are not
       fullfilled (missing tools, etc.)

       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

   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_multiple [-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. If the option
       "-o" is given as the first parameter, a missing file does not lead
       to an error.

   inst <src> [<dst>]
       installs one file <src> either to the same place in the initramfs
       or to an optional <dst>.

   inst_hook <hookdir> <prio> <src>
       installs an executable/script <src> in the dracut hook <hookdir>
       with priority <prio>.

   inst_rules <udevrule> [ <udevrule> ...]
       installs one ore more udev rules. Non-existant udev rules are
       reported, but do not let dracut fail.

   instmods <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>

   Initramfs Functions

   Network Modules


   Harald Hoyer




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