udev - userspace device management

For more information see the files in the docs/ directory.

Important Note:

Integrating udev in the system has complex dependencies and differs from distro

to distro. All major distros depend on udev these days and the system may not

work without a properly installed version. The upstream udev project does not

recommend to replace a distro's udev installation with the upstream version.

Requirements:

- Version 2.6.18 of the Linux kernel for reliable operation of this release of

udev. The kernel may have a requirement on udev too, see Documentation/Changes

in the kernel source tree for the actual dependency.

- The kernel must have sysfs, unix domain sockets and networking enabled.

(unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,

but it does not make any sense - don't complain if anything goes wrong.)

- The proc filesystem must be mounted on /proc/, the sysfs filesystem must

be mounted at /sys/. No other locations are supported by udev.

Operation:

Udev creates and removes device nodes in /dev/, based on events the kernel

sends out on device discovery or removal.

- Very early in the boot process, the /dev/ directory should get a 'tmpfs'

filesystem mounted, which is populated from scratch by udev. Created nodes

or changed permissions will not survive a reboot, which is intentional.

- The content of /lib/udev/devices/ directory which contains the nodes,

symlinks and directories, which are always expected to be in /dev, should

be copied over to the tmpfs mounted /dev, to provide the required nodes

to initialize udev and continue booting.

- The old hotplug helper /sbin/hotplug should be disabled on bootup, before

actions like loading kernel modules are taken, which may cause a lot of

events.

- The udevd daemon must be started on bootup to receive netlink uevents

from the kernel driver core.

- All kernel events are matched against a set of specified rules in

/lib/udev/rules.d/ which make it possible to hook into the event

processing to load required kernel modules and setup devices. For all

devices the kernel exports a major/minor number, udev will create a

device node with the default kernel name, or the one specified by a

matching udev rule.

Please direct any comment/question/concern to the linux-hotplug mailing list at:

linux-hotplug@vger.kernel.org