Systemd/README

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
    /etc/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.


Compile Options:
  DESTDIR
  	Prefix of install target, used for package building.
  USE_LOG
  	If set to 'true', udev is able to pass errors or debug information
  	to syslog. This is very useful to see what udev is doing or not doing.
  	It is enabled by default, don't expect any useful answer, if you
  	need to hunt a bug, but you can't enable syslog.
  DEBUG
  	If set to 'true', very verbose debugging messages will be compiled
  	into the udev binaries. The actual level of debugging is specified
  	in the udev config file.
  USE_SELINUX
  	If set to 'true', udev will be built with SELinux support
  	enabled.  This is disabled by default.
  EXTRAS
  	list of helper programs in extras/ to build.
        make EXTRAS="extras/cdrom_id extras/scsi_id extras/volume_id"


Installation:
  - The install target intalls the udev binaries in the default locations,
    All binaries will be installed in /lib/udev or /sbin.

  - The default location for scripts and binaries that are called from
    rules is /lib/udev. Other packages who install udev rules, may use
    that directory too.

  - It is recommended to use the /lib/udev/devices/ directory to place
    device nodes and symlinks in, which are copied to /dev at every boot.
    That way, nodes for broken subsystems or devices which can't be
    detected automatically by the kernel, will always be available.

  - Default udev rules and persistent device naming rules are required by other
    software that depends on the data udev collects from the devices,
    and should be installed by default with every udev installation.

Please direct any comment/question/concern to the linux-hotplug-devel mailing list at:
  linux-hotplug@vger.kernel.org