-udev - userspace device management
+udev - Linux userspace device management
-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.
+Integrating udev in the system has complex dependencies and may differ from
+distribution to distribution. A system may not be able to boot up or work
+reliably without a properly installed udev version. The upstream udev project
+does not recommend to replace a distro's udev installation with the upstream
+version.
+
+The upstream udev project's set of default rules may require a most recent
+kernel release to work properly.
Tools and rules shipped by udev are not public API and may change at any time.
Never call any private tool in /lib/udev from any external application, it might
CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
CONFIG_BLK_DEV_BSG=y (SCSI devices)
- - For reliable operations, the kernel must not use the CONFIG_SYSFS_DEPRECATED*
- option.
+ - Udev will not work with the CONFIG_SYSFS_DEPRECATED* option.
- Unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
but it is not supported.
+ - The deprecated hotplug helper /sbin/hotplug should be disabled in the
+ kernel configuration, it is not needed today, and may render the system
+ unusable because the kernel may create too many processes in parallel
+ so that the system runs out-of-memory.
+
- The proc filesystem must be mounted on /proc, the sysfs filesystem must
- be mounted at /sys. No other locations are supported by udev.
+ be mounted at /sys. No other locations are supported by a standard
+ udev installation.
- The system must have the following group names resolvable at udev startup:
disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, kmem.
these group names with only the rootfs mounted, and while no network is
available.
- - To build all udev extras, libacl, libglib2, libusb, usbutils, pciutils,
+ - To build all 'udev extras', libacl, libglib2, libusb, usbutils, pciutils,
gperf are needed. These dependencies can be disabled with the
- --disable-extras option.
-
-Operation:
- Udev creates and removes device nodes in /dev, based on events the kernel
- sends out on device discovery or removal.
+ --disable-extras configure option.
- - Early in the boot process, the /dev directory should get a 'tmpfs'
- filesystem mounted, which is maintained by udev. Created nodes or changed
- permissions will not survive a reboot, which is intentional.
+Setup:
+ - At bootup, the /dev directory should get the 'devtmpfs' filesystem
+ mounted. Udev will manage permissions and ownership of the kernel-created
+ device nodes, and possibly create additional symlinks. If needed, udev also
+ works on an empty 'tmpfs' filesystem, but some static device nodes like
+ /dev/null, /dev/console, /dev/kmsg are needed to be able to start udev itself.
- - The content of /lib/udev/devices directory which contains the nodes,
+ - The content of /lib/udev/devices directory which contains static content like
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.
+ be copied over to the mounted /dev directory:
+ cp -axT --remove-destination /lib/udev/devices /dev
- - The deprecated hotplug helper /sbin/hotplug should be disabled in the
- kernel configuration, it is not needed today, and may render the system
- unusable because the kernel may create too many processes in parallel
- so that the system runs out-of-memory.
+ - The udev daemon should be started to handle device events sent by the kernel.
+ During bootup, the kernel can be asked to send events for all already existing
+ devices, to apply the configuration to these devices. This is usually done by:
+ /sbin/udevadm trigger --type=subsystems
+ /sbin/udevadm trigger --type=devices
+
+ - Restarting the daemon does never apply any rules to existing devices.
- - 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.
+ - New/changed rule files are picked up automatically, there is no daemon
+ restart or signal needed.
+
+Operation:
+ - Udev creates/removes device nodes in /dev, based on events the kernel
+ sends out on device creation/removal.
+
+ - All kernel events are matched against a set of specified rules, which
+ possibly hook into the event processing and load required kernel
+ modules to setup devices. For all devices the kernel exports a major/minor
+ number, if needed, udev will create a device node with the default kernel
+ name. If specified, udev applies permissions/ownership to the device
+ node, creates additional symlinks pointing to the node, and executes
+ programs to handle the device.
+
+ - The events udev handles, and the information udev merges into its device
+ database, can be accessed with libudev:
+ http://www.kernel.org/pub/linux/utils/kernel/hotplug/libudev/
+ http://www.kernel.org/pub/linux/utils/kernel/hotplug/gudev/
+
+For more details about udev and udev rules see the udev(7) man page.
Please direct any comment/question to the linux-hotplug mailing list at:
linux-hotplug@vger.kernel.org
-
projects / systemd / commitdiff