INITRD(4)                 (2019-03-06)                  INITRD(4)

          initrd - boot loader initialized RAM disk

          /dev/initrd is a read-only block device assigned major num-
          ber 1 and minor number 250.  Typically /dev/initrd is owned
          by root:disk with mode 0400 (read access by root only).  If
          the Linux system does not have /dev/initrd already created,
          it can be created with the following commands:

              mknod -m 400 /dev/initrd b 1 250
              chown root:disk /dev/initrd

          Also, support for both "RAM disk" and "Initial RAM disk"
          (e.g., CONFIG_BLK_DEV_RAM=y and CONFIG_BLK_DEV_INITRD=y)
          must be compiled directly into the Linux kernel to use
          /dev/initrd. When using /dev/initrd, the RAM disk driver
          cannot be loaded as a module.

          The special file /dev/initrd is a read-only block device.
          This device is a RAM disk that is initialized (e.g., loaded)
          by the boot loader before the kernel is started.  The kernel
          then can use /dev/initrd's contents for a two-phase system

          In the first boot-up phase, the kernel starts up and mounts
          an initial root filesystem from the contents of /dev/initrd
          (e.g., RAM disk initialized by the boot loader).  In the
          second phase, additional drivers or other modules are loaded
          from the initial root device's contents.  After loading the
          additional modules, a new root filesystem (i.e., the normal
          root filesystem) is mounted from a different device.

        Boot-up operation
          When booting up with initrd, the system boots as follows:

          1. The boot loader loads the kernel program and
             /dev/initrd's contents into memory.

          2. On kernel startup, the kernel uncompresses and copies the
             contents of the device /dev/initrd onto device /dev/ram0
             and then frees the memory used by /dev/initrd.

          3. The kernel then read-write mounts the device /dev/ram0 as
             the initial root filesystem.

          4. If the indicated normal root filesystem is also the ini-
             tial root filesystem (e.g., /dev/ram0) then the kernel
             skips to the last step for the usual boot sequence.

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          5. If the executable file /linuxrc is present in the initial
             root filesystem, /linuxrc is executed with UID 0.  (The
             file /linuxrc must have executable permission.  The file
             /linuxrc can be any valid executable, including a shell

          6. If /linuxrc is not executed or when /linuxrc terminates,
             the normal root filesystem is mounted.  (If /linuxrc
             exits with any filesystems mounted on the initial root
             filesystem, then the behavior of the kernel is
             UNSPECIFIED.  See the NOTES section for the current ker-
             nel behavior.)

          7. If the normal root filesystem has a directory /initrd,
             the device /dev/ram0 is moved from / to /initrd. Other-
             wise, if the directory /initrd does not exist, the device
             /dev/ram0 is unmounted.  (When moved from / to /initrd,
             /dev/ram0 is not unmounted and therefore processes can
             remain running from /dev/ram0. If directory /initrd does
             not exist on the normal root filesystem and any processes
             remain running from /dev/ram0 when /linuxrc exits, the
             behavior of the kernel is UNSPECIFIED.  See the NOTES
             section for the current kernel behavior.)

          8. The usual boot sequence (e.g., invocation of /sbin/init)
             is performed on the normal root filesystem.

          The following boot loader options, when used with initrd,
          affect the kernel's boot-up operation:

               Specifies the file to load as the contents of
               /dev/initrd. For LOADLIN this is a command-line option.
               For LILO you have to use this command in the LILO con-
               figuration file /etc/lilo.config. The filename speci-
               fied with this option will typically be a gzipped
               filesystem image.

               This boot option disables the two-phase boot-up opera-
               tion.  The kernel performs the usual boot sequence as
               if /dev/initrd was not initialized.  With this option,
               any contents of /dev/initrd loaded into memory by the
               boot loader contents are preserved.  This option per-
               mits the contents of /dev/initrd to be any data and
               need not be limited to a filesystem image.  However,
               device /dev/initrd is read-only and can be read only
               one time after system startup.

               Specifies the device to be used as the normal root

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               filesystem.  For LOADLIN this is a command-line option.
               For LILO this is a boot time option or can be used as
               an option line in the LILO configuration file
               /etc/lilo.config. The device specified by this option
               must be a mountable device having a suitable root

        Changing the normal root filesystem
          By default, the kernel's settings (e.g., set in the kernel
          file with rdev(8) or compiled into the kernel file), or the
          boot loader option setting is used for the normal root
          filesystems.  For an NFS-mounted normal root filesystem, one
          has to use the nfs_root_name and nfs_root_addrs boot options
          to give the NFS settings.  For more information on NFS-
          mounted root see the kernel documentation file
          Documentation/filesystems/nfs/nfsroot.txt (or
          Documentation/filesystems/nfsroot.txt before Linux 2.6.33).
          For more information on setting the root filesystem see also
          the LILO and LOADLIN documentation.

          It is also possible for the /linuxrc executable to change
          the normal root device.  For /linuxrc to change the normal
          root device, /proc must be mounted.  After mounting /proc,
          /linuxrc changes the normal root device by writing into the
          proc files /proc/sys/kernel/real-root-dev,
          /proc/sys/kernel/nfs-root-name, and /proc/sys/kernel/nfs-
          root-addrs. For a physical root device, the root device is
          changed by having /linuxrc write the new root filesystem
          device number into /proc/sys/kernel/real-root-dev. For an
          NFS root filesystem, the root device is changed by having
          /linuxrc write the NFS setting into files
          /proc/sys/kernel/nfs-root-name and /proc/sys/kernel/nfs-
          root-addrs and then writing 0xff (e.g., the pseudo-NFS-
          device number) into file /proc/sys/kernel/real-root-dev. For
          example, the following shell command line would change the
          normal root device to /dev/hdb1:

              echo 0x365 >/proc/sys/kernel/real-root-dev

          For an NFS example, the following shell command lines would
          change the normal root device to the NFS directory
          /var/nfsroot on a local networked NFS server with IP number
 for a system with IP number and
          named "idefix":

              echo /var/nfsroot >/proc/sys/kernel/nfs-root-name
              echo \
              echo 255 >/proc/sys/kernel/real-root-dev

          Note: The use of /proc/sys/kernel/real-root-dev to change
          the root filesystem is obsolete.  See the Linux kernel

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          source file Documentation/admin-guide/initrd.rst (or
          Documentation/initrd.txt before Linux 4.10) as well as
          pivot_root(2) and pivot_root(8) for information on the mod-
          ern method of changing the root filesystem.

          The main motivation for implementing initrd was to allow for
          modular kernel configuration at system installation.

          A possible system installation scenario is as follows:

          1. The loader program boots from floppy or other media with
             a minimal kernel (e.g., support for /dev/ram,
             /dev/initrd, and the ext2 filesystem) and loads
             /dev/initrd with a gzipped version of the initial

          2. The executable /linuxrc determines what is needed to (1)
             mount the normal root filesystem (i.e., device type,
             device drivers, filesystem) and (2) the distribution
             media (e.g., CD-ROM, network, tape, ...).  This can be
             done by asking the user, by auto-probing, or by using a
             hybrid approach.

          3. The executable /linuxrc loads the necessary modules from
             the initial root filesystem.

          4. The executable /linuxrc creates and populates the root
             filesystem.  (At this stage the normal root filesystem
             does not have to be a completed system yet.)

          5. The executable /linuxrc sets /proc/sys/kernel/real-root-
             dev, unmounts /proc, the normal root filesystem and any
             other filesystems it has mounted, and then terminates.

          6. The kernel then mounts the normal root filesystem.

          7. Now that the filesystem is accessible and intact, the
             boot loader can be installed.

          8. The boot loader is configured to load into /dev/initrd a
             filesystem with the set of modules that was used to bring
             up the system.  (e.g., device /dev/ram0 can be modified,
             then unmounted, and finally, the image is written from
             /dev/ram0 to a file.)

          9. The system is now bootable and additional installation
             tasks can be performed.

          The key role of /dev/initrd in the above is to reuse the
          configuration data during normal system operation without
          requiring initial kernel selection, a large generic kernel

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          or, recompiling the kernel.

          A second scenario is for installations where Linux runs on
          systems with different hardware configurations in a single
          administrative network.  In such cases, it may be desirable
          to use only a small set of kernels (ideally only one) and to
          keep the system-specific part of configuration information
          as small as possible.  In this case, create a common file
          with all needed modules.  Then, only the /linuxrc file or a
          file executed by /linuxrc would be different.

          A third scenario is more convenient recovery disks.  Because
          information like the location of the root filesystem parti-
          tion is not needed at boot time, the system loaded from
          /dev/initrd can use a dialog and/or auto-detection followed
          by a possible sanity check.

          Last but not least, Linux distributions on CD-ROM may use
          initrd for easy installation from the CD-ROM.  The distribu-
          tion can use LOADLIN to directly load /dev/initrd from CD-
          ROM without the need of any floppies.  The distribution
          could also use a LILO boot floppy and then bootstrap a big-
          ger RAM disk via /dev/initrd from the CD-ROM.


          1. With the current kernel, any filesystems that remain
             mounted when /dev/ram0 is moved from / to /initrd con-
             tinue to be accessible.  However, the /proc/mounts
             entries are not updated.

          2. With the current kernel, if directory /initrd does not
             exist, then /dev/ram0 will not be fully unmounted if
             /dev/ram0 is used by any process or has any filesystem
             mounted on it.  If /dev/ram0 is not fully unmounted, then
             /dev/ram0 will remain in memory.

          3. Users of /dev/initrd should not depend on the behavior
             given in the above notes.  The behavior may change in
             future versions of the Linux kernel.

          chown(1), mknod(1), ram(4), freeramdisk(8), rdev(8)

          Documentation/admin-guide/initrd.rst (or
          Documentation/initrd.txt before Linux 4.10) in the Linux
          kernel source tree, the LILO documentation, the LOADLIN

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          documentation, the SYSLINUX documentation

          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at

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