.\" This man-page is Copyright (C) 1997 John S. Kallal

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.\"  John S. Kallal -

.\"	email: <kallal@voicenet.com>

.\"	mail: 518 Kerfoot Farm RD, Wilmington, DE 19803-2444, USA

.\"	phone: (302)654-5478

.\"

.\" $Id: initrd.4,v 0.9 1997/11/07 05:05:32 kallal Exp kallal $

.TH INITRD 4 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

initrd \- boot loader initialized RAM disk

.SH CONFIGURATION

.I /dev/initrd

is a read-only block device assigned

major number 1 and minor number 250.

Typically

.I /dev/initrd

is owned by

.I root.disk

with mode 0400 (read access by root only).

If the Linux system does not have

.I /dev/initrd

already created, it can be created with the following commands:

.PP

.in +4n

.EX

mknod \-m 400 /dev/initrd b 1 250

chown root:disk /dev/initrd

.EE

.in

.PP

Also, support for both "RAM disk" and "Initial RAM disk"

(e.g.,

.BR CONFIG_BLK_DEV_RAM=y

and

.BR CONFIG_BLK_DEV_INITRD=y )

must be compiled directly into the Linux kernel to use

.IR /dev/initrd .

When using

.IR /dev/initrd ,

the RAM disk driver cannot be loaded as a module.

.\"

.\"

.\"

.SH DESCRIPTION

The special file

.I /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

.IR /dev/initrd "'s "

contents for a two-phase system boot-up.

.PP

In the first boot-up phase, the kernel starts up

and mounts an initial root filesystem from the contents of

.I /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.

.\"

.\"

.\"

.SS Boot-up operation

When booting up with

.BR initrd ,

the system boots as follows:

.IP 1. 3

The boot loader loads the kernel program and

.IR /dev/initrd 's

contents into memory.

.IP 2.

On kernel startup,

the kernel uncompresses and copies the contents of the device

.I /dev/initrd

onto device

.I /dev/ram0

and then frees the memory used by

.IR /dev/initrd .

.IP 3.

The kernel then read-write mounts the device

.I /dev/ram0

as the initial root filesystem.

.IP 4.

If the indicated normal root filesystem is also the initial

root filesystem (e.g.,

.IR /dev/ram0 )

then the kernel skips to the last step for the usual boot sequence.

.IP 5.

If the executable file

.IR /linuxrc

is present in the initial root filesystem,

.I /linuxrc

is executed with UID 0.

(The file

.I /linuxrc

must have executable permission.

The file

.I /linuxrc

can be any valid executable, including a shell script.)

.IP 6.

If

.I /linuxrc

is not executed or when

.I /linuxrc

terminates, the normal root filesystem is mounted.

(If

.I /linuxrc

exits with any filesystems mounted on the initial root

filesystem, then the behavior of the kernel is

.BR UNSPECIFIED .

See the NOTES section for the current kernel behavior.)

.IP 7.

If the normal root filesystem has a directory

.IR /initrd ,

the device

.I /dev/ram0

is moved from

.IR /

to

.IR /initrd .

Otherwise, if the directory

.IR /initrd

does not exist, the device

.I /dev/ram0

is unmounted.

(When moved from

.IR /

to

.IR /initrd ,

.I /dev/ram0

is not unmounted and therefore processes can remain running from

.IR /dev/ram0 .

If directory

.I /initrd

does not exist on the normal root filesystem

and any processes remain running from

.IR /dev/ram0

when

.I /linuxrc

exits, the behavior of the kernel is

.BR UNSPECIFIED .

See the NOTES section for the current kernel behavior.)

.IP 8.

The usual boot sequence (e.g., invocation of

.IR /sbin/init )

is performed on the normal root filesystem.

.\"

.\"

.\"

.SS Options

The following boot loader options, when used with

.BR initrd ,

affect the kernel's boot-up operation:

.TP

.BI initrd= "filename"

Specifies the file to load as the contents of

.IR /dev/initrd .

For

.B LOADLIN

this is a command-line option.

For

.B LILO

you have to use this command in the

.B LILO

configuration file

.IR /etc/lilo.config .

The filename specified with this

option will typically be a gzipped filesystem image.

.TP

.I noinitrd

This boot option disables the two-phase boot-up operation.

The kernel performs the usual boot sequence as if

.I /dev/initrd

was not initialized.

With this option, any contents of

.I /dev/initrd

loaded into memory by the boot loader contents are preserved.

This option permits the contents of

.I /dev/initrd

to be any data and need not be limited to a filesystem image.

However, device

.I /dev/initrd

is read-only and can be read only one time after system startup.

.TP

.BI root= "device-name"

Specifies the device to be used as the normal root filesystem.

For

.B LOADLIN

this is a command-line option.

For

.B LILO

this is a boot time option or

can be used as an option line in the

.B LILO

configuration file

.IR /etc/lilo.config .

The device specified by the this option must be a mountable

device having a suitable root filesystem.

.\"

.\"

.\"

.SS Changing the normal root filesystem

By default,

the kernel's settings

(e.g., set in the kernel file with

.BR 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

.B nfs_root_name

and

.B nfs_root_addrs

boot options to give the NFS settings.

For more information on NFS-mounted root see the kernel documentation file

.I Documentation/filesystems/nfs/nfsroot.txt

.\" commit dc7a08166f3a5f23e79e839a8a88849bd3397c32

(or

.I Documentation/filesystems/nfsroot.txt

before Linux 2.6.33).

For more information on setting the root filesystem see also the

.BR LILO

and

.BR LOADLIN

documentation.

.PP

It is also possible for the

.I /linuxrc

executable to change the normal root device.

For

.I /linuxrc

to change the normal root device,

.IR /proc

must be mounted.

After mounting

.IR /proc ,

.I /linuxrc

changes the normal root device by writing into the proc files

.IR /proc/sys/kernel/real-root-dev ,

.IR /proc/sys/kernel/nfs-root-name ,

and

.IR /proc/sys/kernel/nfs-root-addrs .

For a physical root device, the root device is changed by having

.I /linuxrc

write the new root filesystem device number into

.IR /proc/sys/kernel/real-root-dev .

For an NFS root filesystem, the root device is changed by having

.I /linuxrc

write the NFS setting into files

.IR /proc/sys/kernel/nfs-root-name

and

.I /proc/sys/kernel/nfs-root-addrs

and then writing 0xff (e.g., the pseudo-NFS-device number) into file

.IR /proc/sys/kernel/real-root-dev .

For example, the following shell command line would change

the normal root device to

.IR /dev/hdb1 :

.PP

.in +4n

.EX

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

.EE

.in

.PP

For an NFS example, the following shell command lines would change the

normal root device to the NFS directory

.I /var/nfsroot

on a local networked NFS server with IP number 193.8.232.7 for a system with

IP number 193.8.232.2 and named "idefix":

.PP

.in +4n

.EX

echo /var/nfsroot >/proc/sys/kernel/nfs-root-name

echo 193.8.232.2:193.8.232.7::255.255.255.0:idefix \\

    >/proc/sys/kernel/nfs-root-addrs

echo 255 >/proc/sys/kernel/real-root-dev

.EE

.in

.PP

.BR Note :

The use of

.I /proc/sys/kernel/real-root-dev

to change the root filesystem is obsolete.

See the Linux kernel source file

.I Documentation/admin\-guide/initrd.rst

.\" commit 9d85025b0418163fae079c9ba8f8445212de8568

(or

.I Documentation/initrd.txt

before Linux 4.10)

as well as

.BR pivot_root (2)

and

.BR pivot_root (8)

for information on the modern method of changing the root filesystem.

.\" FIXME . Should this manual page  describe the pivot_root mechanism?

.\"

.\"

.\"

.SS Usage

The main motivation for implementing

.B initrd

was to allow for modular kernel configuration at system installation.

.PP

A possible system installation scenario is as follows:

.IP 1. 3

The loader program boots from floppy or other media with a minimal kernel

(e.g., support for

.IR /dev/ram ,

.IR /dev/initrd ,

and the ext2 filesystem) and loads

.IR /dev/initrd

with a gzipped version of the initial filesystem.

.IP 2.

The executable

.I /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.

.IP 3.

The executable

.I /linuxrc

loads the necessary modules from the initial root filesystem.

.IP 4.

The executable

.I /linuxrc

creates and populates the root filesystem.

(At this stage the normal root filesystem does not have to be a

completed system yet.)

.IP 5.

The executable

.IR /linuxrc

sets

.IR /proc/sys/kernel/real-root-dev ,

unmount

.IR /proc ,

the normal root filesystem and any other filesystems

it has mounted, and then terminates.

.IP 6.

The kernel then mounts the normal root filesystem.

.IP 7.

Now that the filesystem is accessible and intact,

the boot loader can be installed.

.IP 8.

The boot loader is configured to load into

.I /dev/initrd

a filesystem with the set of modules that was used to bring up the system.

(e.g., Device

.I /dev/ram0

can be modified, then unmounted, and finally, the image is written from

.I /dev/ram0

to a file.)

.IP 9.

The system is now bootable and additional installation tasks can be

performed.

.PP

The key role of

.I /dev/initrd

in the above is to reuse the configuration data during normal system operation

without requiring initial kernel selection, a large generic kernel or,

recompiling the kernel.

.PP

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

.I /linuxrc

file or a file executed by

.I /linuxrc

would be different.

.PP

A third scenario is more convenient recovery disks.

Because information like the location of the root filesystem

partition is not needed at boot time, the system loaded from

.I /dev/initrd

can use a dialog and/or auto-detection followed by a

possible sanity check.

.PP

Last but not least, Linux distributions on CD-ROM may use

.B initrd

for easy installation from the CD-ROM.

The distribution can use

.B LOADLIN

to directly load

.I /dev/initrd

from CD-ROM without the need of any floppies.

The distribution could also use a

.B LILO

boot floppy and then bootstrap a bigger RAM disk via

.IR /dev/initrd

from the CD-ROM.

.\"

.\"

.\"

.SH FILES

.I /dev/initrd

.br

.I /dev/ram0

.br

.I /linuxrc

.br

.I /initrd

.\"

.\"

.\"

.SH NOTES

.IP 1. 3

With the current kernel, any filesystems that remain mounted when

.I /dev/ram0

is moved from

.I /

to

.I /initrd

continue to be accessible.

However, the

.I /proc/mounts

entries are not updated.

.IP 2.

With the current kernel, if directory

.I /initrd

does not exist, then

.I /dev/ram0

will

.B not

be fully unmounted if

.I /dev/ram0

is used by any process or has any filesystem mounted on it.

If

.IR /dev/ram0

is

.B not

fully unmounted, then

.I /dev/ram0

will remain in memory.

.IP 3.

Users of

.I /dev/initrd

should not depend on the behavior give in the above notes.

The behavior may change in future versions of the Linux kernel.

.\"

.\"

.\"

.\" .SH AUTHORS

.\" The kernel code for device

.\" .BR initrd

.\" was written by Werner Almesberger <almesber@lrc.epfl.ch> and

.\" Hans Lermen <lermen@elserv.ffm.fgan.de>.

.\" The code for

.\" .BR initrd

.\" was added to the baseline Linux kernel in development version 1.3.73.

.SH SEE ALSO

.BR chown (1),

.BR mknod (1),

.BR ram (4),

.BR freeramdisk (8),

.BR rdev (8)

.PP

.I Documentation/admin\-guide/initrd.rst

.\" commit 9d85025b0418163fae079c9ba8f8445212de8568

(or

.I Documentation/initrd.txt

before Linux 4.10)

in the Linux kernel source tree, the LILO documentation,

the LOADLIN documentation, the SYSLINUX documentation

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.