-udev - userspace device management
+udev - Linux userspace device management
-For more information see the files in the docs/ directory.
+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.
-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.
+The upstream udev project's set of default rules may require a most recent
+kernel release to work properly. This is currently version 2.6.31.
+
+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
+just go away in the next release. Access to udev information is only offered
+by udevadm and libudev. Tools and rules in /lib/udev, and the entire content of
+the /dev/.udev directory is private to udev and does change whenever needed.
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.
+ - Version 2.6.27 of the Linux kernel with sysfs, procfs, signalfd, inotify,
+ unix domain sockets, networking and hotplug enabled:
+ CONFIG_HOTPLUG=y
+ CONFIG_UEVENT_HELPER_PATH=""
+ CONFIG_NET=y
+ CONFIG_UNIX=y
+ CONFIG_SYSFS=y
+ CONFIG_SYSFS_DEPRECATED*=n
+ CONFIG_PROC_FS=y
+ CONFIG_TMPFS=y
+ CONFIG_INOTIFY_USER=y
+ CONFIG_SIGNALFD=y
+ CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
+ CONFIG_BLK_DEV_BSG=y (SCSI devices)
- - 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.)
+ - Udev will not work with the CONFIG_SYSFS_DEPRECATED* option.
- - The proc filesystem must be mounted on /proc/, the sysfs filesystem must
- be mounted at /sys/. No other locations are supported by udev.
+ - 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.
-Operation:
- Udev creates and removes device nodes in /dev/, based on events the kernel
- sends out on device discovery or removal.
+ - The proc filesystem must be mounted on /proc, the sysfs filesystem must
+ be mounted at /sys. No other locations are supported by a standard
+ udev installation.
- - 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 system must have the following group names resolvable at udev startup:
+ disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, kmem.
+ Especially in LDAP setups, it is required, that getgrnam() is able to resolve
+ these group names with only the rootfs mounted, and while no network is
+ available.
- - 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.
+ - To build all 'udev extras', libacl, libglib2, libusb, usbutils, pciutils,
+ gperf are needed. These dependencies can be disabled with the
+ --disable-extras configure option.
- - 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.
+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 udevd daemon must be started on bootup to receive netlink uevents
- from the kernel driver core.
+ - 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
- - 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.
+ - Restarting the daemon does never apply any rules to existing devices.
-Please direct any comment/question/concern to the linux-hotplug mailing list at:
- linux-hotplug@vger.kernel.org
+ - 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