3 Harald Hoyer <harald@redhat.com>
9 This section is a modified version of
10 http://en.wikipedia.org/wiki/Initrd which is licensed under the
11 Creative Commons Attribution/Share-Alike License.
14 An _initial ramdisk_ is a temporary file system used in the boot process of the
15 Linux kernel. _initrd_ and _initramfs_ refer to slightly different schemes for
16 loading this file system into memory. Both are commonly used to make
17 preparations before the real root file system can be mounted.
20 Many Linux distributions ship a single, generic kernel image that is intended to
21 boot as wide a variety of hardware as possible. The device drivers for this
22 generic kernel image are included as loadable modules, as it is not possible to
23 statically compile them all into the one kernel without making it too large to
24 boot from computers with limited memory or from lower-capacity media like floppy
27 This then raises the problem of detecting and loading the modules necessary to
28 mount the root file system at boot time (or, for that matter, deducing where or
29 what the root file system is).
31 To further complicate matters, the root file system may be on a software RAID
32 volume, LVM, NFS (on diskless workstations), or on an encrypted partition. All
33 of these require special preparations to mount.
35 Another complication is kernel support for hibernation, which suspends the
36 computer to disk by dumping an image of the entire system to a swap partition or
37 a regular file, then powering off. On next boot, this image has to be made
38 accessible before it can be loaded back into memory.
40 To avoid having to hardcode handling for so many special cases into the kernel,
41 an initial boot stage with a temporary root file system
42 —now dubbed early user space— is used. This root file system would contain
43 user-space helpers that would do the hardware detection, module loading and
44 device discovery necessary to get the real root file system mounted.
47 An image of this initial root file system (along with the kernel image) must be
48 stored somewhere accessible by the Linux bootloader or the boot firmware of the
49 computer. This can be:
51 * The root file system itself
52 * A boot image on an optical disc
53 * A small ext2/ext3 or FAT-formatted partition on a local disk
55 * A TFTP server (on systems that can boot from Ethernet)
57 The bootloader will load the kernel and initial root file system image into
58 memory and then start the kernel, passing in the memory address of the image.
60 Depending on which algorithms were compiled statically into it, the kernel can
61 currently unpack initrd/initramfs images compressed with gzip, bzip2 and LZMA.
64 dracut can generate a customized initrams image which contains only whatever is
65 necessary to boot some particular computer, such as ATA, SCSI and filesystem
66 kernel modules (host-only mode).
68 dracut can also generate a more generic initramfs image (default mode).
70 dracut's initramfs starts only with the device name of the root file system (or
71 its UUID) and must discover everything else at boot time. A complex cascade of
72 tasks must be performed to get the root file system mounted:
74 * Any hardware drivers that the boot process depends on must be loaded. All
75 kernel modules for common storage devices are packed onto the initramfs and then
76 udev pulls in modules matching the computer's detected hardware.
78 * On systems which display a boot rd.splash screen, the video hardware must be
79 initialized and a user-space helper started to paint animations onto the display
80 in lockstep with the boot process.
82 * If the root file system is on NFS, dracut does then:
83 ** Bring up the primary network interface.
84 ** Invoke a DHCP client, with which it can obtain a DHCP lease.
85 ** Extract the name of the NFS share and the address of the NFS server from the
87 ** Mount the NFS share.
89 * If the root file system appears to be on a software RAID device, there is no
90 way of knowing which devices the RAID volume spans; the standard MD utilities
91 must be invoked to scan all available block devices with a raid signature and
92 bring the required ones online.
94 * If the root file system appears to be on a logical volume, the LVM utilities
95 must be invoked to scan for and activate the volume group containing it.
97 * If the root file system is on an encrypted block device:
98 ** Invoke a helper script to prompt the user to type in a passphrase and/or
99 insert a hardware token (such as a smart card or a USB security dongle).
101 * Create a decryption target with the device mapper.
103 dracut uses udev, an event-driven hotplug agent, which invokes helper programs
104 as hardware devices, disk partitions and storage volumes matching certain rules
105 come online. This allows discovery to run in parallel, and to progressively
106 cascade into arbitrary nestings of LVM, RAID or encryption to get at the root
109 When the root file system finally becomes visible:
111 * Any maintenance tasks which cannot run on a mounted root file system
113 * The root file system is mounted read-only.
114 * Any processes which must continue running (such as the rd.splash screen helper
115 and its command FIFO) are hoisted into the newly-mounted root file system.
117 The final root file system cannot simply be mounted over /, since that would
118 make the scripts and tools on the initial root file system inaccessible for any
119 final cleanup tasks. On an initramfs, the initial root file system cannot be
120 rotated away. Instead, it is simply emptied and the final root file system
121 mounted over the top.
123 If the systemd module is used in the initramfs, the ordering of the services
124 started looks like <<dracutbootup7>>.
126 == Dracut on shutdown
128 On a systemd driven system, the dracut initramfs is also used for the shutdown procedure.
130 The following steps are executed during a shutdown:
132 * systemd switches to the shutdown.target
133 * systemd starts /lib/systemd/system/shutdown.target.wants/dracut-shutdown.service
134 * dracut-shutdown.service executes /usr/lib/dracut/dracut-initramfs-restore
135 which unpacks the initramfs to /run/initramfs
136 * systemd finishes shutdown.target
137 * systemd kills all processes
138 * systemd tries to unmount everything and mounts the remaining read-only
139 * systemd checks, if there is a /run/initramfs/shutdown executable
140 * if yes, it does a pivot_root to /run/initramfs and executes ./shutdown.
141 The old root is then mounted on /oldroot. /usr/lib/dracut/modules.d/99shutdown/shutdown.sh is the shutdown executable.
142 * shutdown will try to umount every /oldroot mount and calls the various shutdown hooks from the dracut modules
144 This ensures, that all devices are disassembled and unmounted cleanly.
148 == Creating an initramfs Image
149 To create a initramfs image, the most simple command is:
154 This will generate a general purpose initramfs image, with all possible
155 functionality resulting of the combination of the installed dracut modules and
156 system tools. The image is /boot/initramfs-_++<kernel version>++_.img and
157 contains the kernel modules of the currently active kernel with version
158 _++<kernel version>++_.
160 If the initramfs image already exists, dracut will display an error message, and
161 to overwrite the existing image, you have to use the --force option.
166 If you want to specify another filename for the resulting image you would issue
172 To generate an image for a specific kernel version, the command would be:
174 # dracut foobar.img 2.6.40-1.rc5.f20
177 A shortcut to generate the image at the default location for a specific kernel
180 # dracut --kver 2.6.40-1.rc5.f20
183 If you want to create lighter, smaller initramfs images, you may want to specify
184 the --host-only or -H option. Using this option, the resulting image will
185 contain only those dracut modules, kernel modules and filesystems, which are
186 needed to boot this specific machine. This has the drawback, that you can't put
187 the disk on another controller or machine, and that you can't switch to another
188 root filesystem, without recreating the initramfs image. The usage of the
189 --host-only option is only for experts and you will have to keep the broken
190 pieces. At least keep a copy of a general purpose image (and corresponding
191 kernel) as a fallback to rescue your system.
193 === Inspecting the Contents
194 To see the contents of the image created by dracut, you can use the lsinitrd tool.
196 # lsinitrd /boot/initramfs-$(uname -r).img | less
199 To display the contents of a file in the initramfs also use the lsinitrd tool:
201 # lsinitrd /boot/initramfs-$(uname -r).img /etc/ld.so.conf
202 include ld.so.conf.d/*.conf
205 === Adding dracut Modules
206 Some dracut modules are turned off by default and have to be activated manually.
207 You can do this by adding the dracut modules to the configuration file
208 _/etc/dracut.conf_ or _/etc/dracut.conf.d/myconf.conf_. See <<dracutconf5>>.
209 You can also add dracut modules on the command line
210 by using the -a or --add option:
212 # dracut --add bootchart initramfs-bootchart.img
215 To see a list of available dracut modules, use the --list-modules option:
217 # dracut --list-modules
220 or, if you have a dracut version earlier than +008+, issue the command:
222 # for mod in /usr/lib/dracut/modules.d/*; do echo ${mod##*/??}; done
225 === Omitting dracut Modules
226 Sometimes you don't want a dracut module to be included for reasons of speed,
227 size or functionality. To do this, either specify the omit_dracutmodules
228 variable in the _dracut.conf_ or _/etc/dracut.conf.d/myconf.conf_ configuration
229 file (see <<dracutconf5>>), or use the -o or --omit option
232 # dracut -o "multipath lvm" no-multipath-lvm.img
235 === Adding Kernel Modules
236 If you need a special kernel module in the initramfs, which is not
237 automatically picked up by dracut, you have the use the --add-drivers option
238 on the command line or the drivers vaiable in the _/etc/dracut.conf_
239 or _/etc/dracut.conf.d/myconf.conf_ configuration file (see <<dracutconf5>>):
241 # dracut --add-drivers mymod initramfs-with-mymod.img
245 The generated initramfs.img file normally does not contain any system
246 configuration files (except for some special exceptions), so the configuration
247 has to be done on the kernel command line. With this flexibility, you can easily
248 boot from a changed root partition, without the need to recompile the initramfs
249 image. So, you could completly change your root partition (move it inside a md
250 raid with encryption and LVM on top), as long as you specify the correct
251 filesystem LABEL or UUID on the kernel command line for your root device, dracut
252 will find it and boot from it.
254 The kernel command line usually can be configured in _/boot/grub/grub.conf_, if
255 grub is your bootloader and it also can be edited in the real boot process in
258 The kernel command line can also be provided by the dhcp server with the
259 root-path option. See <<NetworkBoot>>.
261 For a full reference of all kernel command line parameters, see <<dracut8>>.
263 === Specifying the root Device
264 This is the only option dracut really needs to boot from your root partition.
265 Because your root partition can live in various environments, there are a lot of
266 formats for the root= option. The most basic one is root=_++<path to device
272 Because device node names can change, dependent on the drive ordering, you are
273 encouraged to use the filesystem identifier (UUID) or filesystem label (LABEL)
274 to specify your root partition:
276 root=UUID=19e9dda3-5a38-484d-a9b0-fa6b067d0331
282 root=LABEL=myrootpartitionlabel
285 To see all UUIDs or LABELs on your system, do:
287 # ls -l /dev/disk/by-uuid
293 # ls -l /dev/disk/by-label
296 If your root partition is on the network see <<NetworkBoot>>.
298 === Keyboard Settings
299 If you have to input passwords for encrypted disk volumes, you might want to set
300 the keyboard layout and specify a display font.
302 A typical german kernel command would contain:
304 vconsole.font=latarcyrheb-sun16 vconsole.keymap=de-latin1-nodeadkeys locale.LANG=de_DE.UTF-8
307 Setting these options can override the setting stored on your system, if you use
308 a modern init system, like systemd.
310 For dracut versions prior to version +008+ the line would look like:
312 LANG=de_DE.UTF-8 SYSFONT=latarcyrheb-sun16 KEYBOARDTYPE=pc KEYTABLE=de-latin1-nodeadkeys
315 === Blacklisting Kernel Modules
316 Sometimes it is required to prevent the automatic kernel module loading of a
317 specific kernel module. To do this, just add rd.blacklist=_++<kernel module
318 name>++_, with _++<kernel module name>++_ not containing the _.ko_
319 suffix, to the kernel command line. For example:
321 rd.driver.blacklist=mptsas rd.driver.blacklist=nouveau
324 The option can be specified multiple times on the kernel command line.
326 === Speeding up the Boot Process
327 If you want to speed up the boot process, you can specify as much information
328 for dracut on the kernel command as possible. For example, you can tell dracut,
329 that you root partition is not on a LVM volume or not on a raid partition, or
330 that it lives inside a specific crypto LUKS encrypted volume. By default, dracut
331 searches everywhere. A typical dracut kernel command line for a plain primary or
332 logical partition would contain:
334 rd.luks=0 rd.lvm=0 rd.md=0 rd.dm=0
337 On systems with dracut version prior to +008+ the line would look like:
339 rd_NO_LUKS rd_NO_LVM rd_NO_MD rd_NO_DM
342 This turns off every automatic assembly of LVM, MD raids, DM raids and crypto LUKS.
344 Of course, you could also omit the dracut modules in the initramfs creation
345 process, but then you would lose the posibility to turn it on on demand.
349 === Injecting custom Files
350 To add your own files to the initramfs image, you have several possibilities.
352 The --include option let you specify a source path and a target path. For example
354 # dracut --include cmdline-preset /etc/cmdline.d/mycmdline.conf initramfs-cmdline-pre.img
356 will create an initramfs image, where the file cmdline-preset will be copied
357 inside the initramfs to _/etc/cmdline.d/mycmdline.conf_. --include can only be specified once.
361 # mkdir rd.live.overlay
362 # mkdir rd.live.overlay/etc
363 # mkdir rd.live.overlay/etc/conf.d
364 # mkdir rd.live.overlay/etc/cmdline.d
365 # echo "ip=auto" >> rd.live.overlay/etc/cmdline.d/mycmdline.conf
366 # echo export FOO=testtest >> rd.live.overlay/etc/conf.d/testvar.conf
367 # echo export BAR=testtest >> rd.live.overlay/etc/conf.d/testvar.conf
368 # tree rd.live.overlay/
376 # dracut --include rd.live.overlay / initramfs-rd.live.overlay.img
379 This will put the contents of the rd.live.overlay directory into the root of the
382 The --install option let you specify several files, which will get installed in
383 the initramfs image at the same location, as they are present on initramfs
388 # dracut --install 'strace fsck.ext3 ssh' initramfs-dbg.img
391 This will create an initramfs with the strace, fsck.ext3 and ssh executables,
392 together with the libraries needed to start those. The --install option can be
393 specified multiple times.
399 If your root partition is on a network drive, you have to have the network
400 dracut modules installed to create a network aware initramfs image.
402 On a Red Hat Enterprise Linux or Fedora system, this means, you have to install
403 the _dracut-network_ rpm package:
407 # yum install dracut-network
410 The resulting initramfs image can be served by a boot manager residing on your
411 local hard drive or it can be served by a PXE/TFTP server.
413 How to setup your PXE/TFTP server can be found in the
414 http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Storage_Administration_Guide/[Red
415 Hat Enterprise Linux Storage Administration Guide].
417 If you specify ip=auto on the kernel command line, then dracut asks a dhcp
418 server about the ip adress for the machine. The dhcp server can also serve an
419 additional root-path, which will set the root device for dracut. With this
420 mechanism, you have static configuration on your client machine and a
421 centralized boot configuration on your TFTP/DHCP server. If you can't pass a
422 kernel command line, then you can inject _/etc/cmdline.d/mycmdline.conf_, with a method described
428 === Reducing the Image Size
430 To reduce the size of the initramfs, you should create it with by ommitting all
431 dracut modules, which you know, you don't need to boot the machine.
433 You can also specify the exact dracut and kernel modules to produce a very tiny
436 For example for a NFS image, you would do:
440 # dracut -m "nfs network base" initramfs-nfs-only.img
443 Then you would boot from this image with your target machine and reduce the size
444 once more by creating it on the target machine with the --host-only option:
448 # dracut -m "nfs network base" --host-only initramfs-nfs-host-only.img
451 This will reduce the size of the initramfs image significantly.
459 === iSCSI Root Device
469 If the boot process does not succeed, you have several options to debug the
470 situation. Some of the basic operations are covered here. For more information
471 you should also visit:
472 http://fedoraproject.org/wiki/How_to_debug_Dracut_problems
475 [[identifying-your-problem-area]]
476 === Identifying your problem area
477 . Remove ''rhgb'' and ''quiet'' from the kernel command line
478 . Add ''rd.shell'' to the kernel command line. This will present a shell should
479 dracut be unable to locate your root device
480 . Add ''rd.shell rd.debug log_buf_len=1M'' to the kernel command line so that
481 dracut shell commands are printed as they are executed
482 . With dracut >= 002-11, you can inspect the rd.debug output with:
485 # less /run/initramfs/init.log
488 . With dracut >= 022 and systemd, you can inspect the rd.debug output with:
492 . With dracut >= 025 the file /run/initramfs/rdsosreport.txt is generated, which contains all the logs and the output of all significant tools, which are mentioned later.
494 If you want to save that output, simply mount /boot by hand or insert an USB stick and mount that.
495 Then you can store the output for later inspection.
497 [[information-to-include-in-your-report]]
498 === Information to include in your report
502 In all cases, the following should be mentioned and attached to your bug report:
504 * The exact kernel command-line used. Typically from the bootloader
505 configuration file (e.g. _/etc/grub.conf_) or from _/proc/cmdline_.
506 * A copy of your disk partition information from _/etc/fstab_, which might be
507 obtained booting an old working initramfs or a rescue medium.
508 * A device listing from device-mapper. This can be obtained by running the
515 * A list of block device attributes. This can be obtained by running the commands:
521 * Turn on dracut debugging (see _the 'debugging dracut' section_), and attach
522 all relevant information from the boot log. This can be obtained by running the
529 * If you use a dracut configuration file, please include _/etc/dracut.conf_ and
530 all files in _/etc/dracut.conf.d/*.conf_
532 [[logical-volume-management-related-problems]]
533 ==== Logical Volume Management related problems
534 As well as the information from <<all-bug-reports>> include the following
537 * Include physical volume information by running the command:
543 * Include volume group information by running the command:
549 * Include logical volume information by running the command:
555 [[software-raid-related-problems]]
556 ==== Software RAID related problems
557 As well as the information from <<all-bug-reports>>, include the following
560 * If using software RAID disk partitions, please include the output of
566 [[network-root-device-related-problems]]
567 ==== Network root device related problems
568 This section details information to include when experiencing problems on a
569 system whose root device is located on a network attached volume (e.g. iSCSI,
570 NFS or NBD). As well as the information from <<all-bug-reports>>, include the
571 following information:
574 * Please include the output of
577 # /sbin/ifup <interfacename>
585 [[configure-a-serial-console]]
586 ==== Configure a serial console
588 Successfully debugging dracut will require some form of console
589 logging during the system boot. This section documents configuring a
590 serial console connection to record boot messages.
592 . First, enable serial console output for both the kernel and the bootloader.
593 . Open the file _/etc/grub.conf_ for editing. Below the line ''timeout=5'', add
597 serial --unit=0 --speed=9600
598 terminal --timeout=5 serial console
601 . Also in _/etc/grub.conf_, add the following boot arguemnts to the ''kernel''
605 console=tty0 console=ttyS0,9600
608 . When finished, the _/etc/grub.conf_ file should look similar to the example
614 serial --unit=0 --speed=9600
615 terminal --timeout=5 serial console
616 title Fedora (2.6.29.5-191.fc11.x86_64)
618 kernel /vmlinuz-2.6.29.5-191.fc11.x86_64 ro root=/dev/mapper/vg_uc1-lv_root console=tty0 console=ttyS0,9600
619 initrd /dracut-2.6.29.5-191.fc11.x86_64.img
622 . More detailed information on how to configure the kernel for console output
624 http://www.faqs.org/docs/Linux-HOWTO/Remote-Serial-Console-HOWTO.html#CONFIGURE-KERNEL.
625 . Redirecting non-interactive output
628 NOTE: You can redirect all non-interactive output to _/dev/kmsg_ and the kernel
629 will put it out on the console when it reaches the kernel buffer by doing
632 # exec >/dev/kmsg 2>&1 </dev/console
636 [[using-the-dracut-shell]]
637 ==== Using the dracut shell
639 dracut offers a shell for interactive debugging in the event dracut fails to
640 locate your root filesystem. To enable the shell:
642 . Add the boot parameter ''rd.shell'' to your bootloader configuration file
643 (e.g. _/etc/grub.conf_)
644 . Remove the boot arguments ''rhgb'' and ''quiet''
646 A sample _/etc/grub.conf_ bootloader configuration file is listed below.
651 serial --unit=0 --speed=9600
652 terminal --timeout=5 serial console
653 title Fedora (2.6.29.5-191.fc11.x86_64)
655 kernel /vmlinuz-2.6.29.5-191.fc11.x86_64 ro root=/dev/mapper/vg_uc1-lv_root console=tty0 rd.shell
656 initrd /dracut-2.6.29.5-191.fc11.x86_64.img
659 . If system boot fails, you will be dropped into a shell as seen in the example below.
663 Dropping to debug shell.
668 . Use this shell prompt to gather the information requested above (see <<all-bug-reports>>).
670 [[accessing-the-root-volume-from-the-dracut-shell]]
671 ==== Accessing the root volume from the dracut shell
672 From the dracut debug shell, you can manually perform the task of locating and
673 preparing your root volume for boot. The required steps will depend on how your
674 root volume is configured. Common scenarios include:
676 * A block device (e.g. _/dev/sda7_)
677 * A LVM logical volume (e.g. _/dev/VolGroup00/LogVol00_)
678 * An encrypted device (e.g. _/dev/mapper/luks-4d5972ea-901c-4584-bd75-1da802417d83_)
679 * A network attached device (e.g. netroot=iscsi:@192.168.0.4::3260::iqn.2009-02.org.fedoraproject:for.all)
681 The exact method for locating and preparing will vary. However, to continue with
682 a successful boot, the objective is to locate your root volume and create a
683 symlink _/dev/root_ which points to the file system. For example, the following
684 example demonstrates accessing and booting a root volume that is an encrypted
687 . Inspect your partitions using parted
690 # parted /dev/sda -s p
691 Model: ATA HTS541060G9AT00 (scsi)
692 Disk /dev/sda: 60.0GB
693 Sector size (logical/physical): 512B/512B
694 Partition Table: msdos
695 Number Start End Size Type File system Flags
696 1 32.3kB 10.8GB 107MB primary ext4 boot
697 2 10.8GB 55.6GB 44.7GB logical lvm
700 . You recall that your root volume was a LVM logical volume. Scan and activate
708 . You should see any logical volumes now using the command blkid:
712 /dev/sda1: UUID="3de247f3-5de4-4a44-afc5-1fe179750cf7" TYPE="ext4"
713 /dev/sda2: UUID="Ek4dQw-cOtq-5MJu-OGRF-xz5k-O2l8-wdDj0I" TYPE="LVM2_member"
714 /dev/mapper/linux-root: UUID="def0269e-424b-4752-acf3-1077bf96ad2c" TYPE="crypto_LUKS"
715 /dev/mapper/linux-home: UUID="c69127c1-f153-4ea2-b58e-4cbfa9257c5e" TYPE="ext3"
716 /dev/mapper/linux-swap: UUID="47b4d329-975c-4c08-b218-f9c9bf3635f1" TYPE="swap"
719 . From the output above, you recall that your root volume exists on an encrypted
720 block device. Following the guidance disk encryption guidance from the
721 Installation Guide, you unlock your encrypted root volume.
724 # UUID=$(cryptsetup luksUUID /dev/mapper/linux-root)
725 # cryptsetup luksOpen /dev/mapper/linux-root luks-$UUID
726 Enter passphrase for /dev/mapper/linux-root:
730 . Next, make a symbolic link to the unlocked root volume
733 # ln -s /dev/mapper/luks-$UUID /dev/root
736 . With the root volume available, you may continue booting the system by exiting
743 [[additional-dracut-boot-parameters]]
744 ==== Additional dracut boot parameters
745 For more debugging options, see <<dracutkerneldebug>> in <<dracutcmdline7>>.
751 dracut uses a modular system to build and extend the initramfs image. All
752 modules are located in _/usr/lib/dracut/modules.d_ or in _<git-src>/modules.d_.
753 The most basic dracut module is _99base_. In _99base_ the initial shell script
754 init is defined, which gets run by the kernel after initramfs loading. Although
755 you can replace init with your own version of _99base_, this is not encouraged.
756 Instead you should use, if possible, the hooks of dracut. All hooks, and the
757 point of time in which they are executed, are described in <<stages>>.
759 The main script, which creates the initramfs is dracut itsself. It parses all
760 arguments and sets up the directory, in which everything is installed. It then
761 executes all check, install, installkernel scripts found in the modules, which
762 are to be processed. After everything is installed, the install directory is
763 archived and compressed to the final initramfs image. All helper functions used
764 by check, install and installkernel are found in in the file _dracut-functions_.
765 These shell functions are available to all module installer (install,
766 installkernel) scripts, without the need to source _dracut-functions_.
768 A module can check the preconditions for install and installkernel with the
769 check script. Also dependencies can be expressed with check. If a module passed
770 check, install and installkernel will be called to install all of the necessary
771 files for the module. To split between kernel and non-kernel parts of the
772 installation, all kernel module related parts have to be in installkernel. All
773 other files found in a module directory are module specific and mostly are hook
774 scripts and udev rules.
778 == Boot Process Stages
780 The init script in _99base_ is the main script, which prepares the root file
781 system for usage, runs udev, mounts the real root device, kills the remaining
782 processes, and switches to the real root device for further booting. dracut
783 modules can insert custom script at various points, to control the boot process.
784 These hooks are plain directories containing shell scripts ending with ".sh",
785 which are sourced by init.
786 Common used functions are in _dracut-lib.sh_, which can be sourced by any script.
792 The first thing init does, is to mount _/proc_ and _/sys_ and manually create
793 the basic device nodes and symbolic links in _/dev_ needed to execute basic
794 commands. Then logging is setup according to kernel command line arguments.
795 _/dev/pts_ and _/dev/shm_ are mounted and the first hook is sourced.
801 The _cmdline_ hook is a place to insert scripts to parse the kernel command line
802 and prepare the later actions, like setting up udev rules and configuration
805 In this hook the most important environment variable is defined: root. The
806 second one is rootok, which indicates, that a module claimed to be able to parse
807 the root defined. So for example, **root=**__iscsi:....__ will be claimed by the
808 iscsi dracut module, which then sets rootok.
812 This hook is executed right after the cmdline hook and a check if root and
813 rootok were set. Here modules can take action with the final root, and before
820 Now udev is started and the logging for udev is setup.
824 === Hook: pre-trigger
826 In this hook, you can set udev environment variables with **udevadm control
827 --property=KEY=_value_** or control the further execution of udev with
834 udev is triggered by calling udevadm trigger, which sends add events for all
835 devices and subsystems.
841 Now the main loop of 99base/init begins. Here we loop until udev has settled and
842 all scripts in _initqueue/finished_ returned true. In this loop there are three
843 hooks, where scripts can be inserted by calling /sbin/initqueue.
849 This hook gets executed every time a script is inserted here, regardless of the
854 ==== Initqueue settled
856 This hooks gets executed every time udev has settled.
860 ==== Initqueue timeout
862 This hooks gets executed, when the main loop counter becomes half of the
867 ==== Initqueue finished
869 This hook is called after udev has settled and if all scripts herein return 0
870 the main loop will be ended.
876 Before the root device is mounted all scripts in the hook pre-mount are
877 executed. In some cases (e.g. NFS) the real root device is already mounted,
884 This hook is mainly to mount the real root device.
890 This hook is called before cleanup hook, This is a good place for
891 actions other than cleanups which need to be called before pivot.
896 This hook is the last hook and is called before init finally switches root to
897 the real root device. This is a good place to clean up and kill processes not
901 === Cleanup and switch_root
903 Init kills all udev processes, cleans up the environment, sets up the arguments
904 for the real init process and finally calls switch_root. switch_root removes the
905 whole filesystem hierarchy of the initramfs, chroot()s to the real root device
906 and calls /sbin/init with the specified arguments.
908 To ensure all files in the initramfs hierarchy can be removed, all processes
909 still running from the initramfs should not have any open file descriptors left.
913 == Network Infrastructure
920 A simple example module is _96insmodpost_, which modprobes a kernel module after
921 udev has settled and the basic device drivers have been loaded.
923 All module installation information is in the file module-setup.sh.
925 First we create a check() function, which just exits with 0 indicating that this
926 module should be included by default.
935 The we create the install() function, which installs a cmdline hook with
936 priority number 20 called _parse-insmodpost.sh_. It also installs the
937 _insmodpost.sh_ script in _/sbin_.
943 inst_hook cmdline 20 "$moddir/parse-insmodpost.sh"
944 inst_simple "$moddir/insmodpost.sh" /sbin/insmodpost.sh
947 The _pase-instmodpost.sh_ parses the kernel command line for a argument
948 rd.driver.post, blacklists the module from being autoloaded and installs the
949 hook _insmodpost.sh_ in the _initqueue/settled_.
951 _parse-insmodpost.sh_:
955 for p in $(getargs rd.driver.post=); do
956 echo "blacklist $p" >> /etc/modprobe.d/initramfsblacklist.conf
960 [ -n "$_do_insmodpost" ] && /sbin/initqueue --settled --unique --onetime /sbin/insmodpost.sh
965 _insmodpost.sh_, which is called in the _initqueue/settled_ hook will just
966 modprobe the kernel modules specified in all rd.driver.post kernel command line
967 parameters. It runs after udev has settled and is only called once (--onetime).
975 for p in $(getargs rd.driver.post=); do
985 _check()_ is called by dracut to evaluate the inclusion of a dracut module in
988 $hostonly:: If the $hostonly variable is set, then the module check() function
989 should be in "hostonly" mode, which means, that the check() should only return
990 0, if the module is really needed to boot this specific host.
992 check() should return with:
994 0:: Include the dracut module in the initramfs.
996 1:: Do not include the dracut module. The requirements are not fullfilled
997 (missing tools, etc.)
999 255:: Only include the dracut module, if another module requires it or if
1000 explicitly specified in the config file or on the argument list.
1006 The function depends() should echo all other dracut module names the module
1031 === Creation Functions
1036 === Initramfs Functions
1047 include::dracut.bootup.7.asc[]
1051 include::dracut.8.asc[]
1054 include::dracut.conf.5.asc[]
1057 include::dracut.cmdline.7.asc[]
1060 include::lsinitrd.1.asc[]
1063 include::mkinitrd.8.asc[]
1069 This work is licensed under the Creative Commons Attribution/Share-Alike
1070 License. To view a copy of this license, visit
1071 http://creativecommons.org/licenses/by-sa/3.0/ or send a letter to Creative
1072 Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.