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+udev - Linux userspace device management
+
+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 replacing 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 /usr/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 /usr/lib/udev and the entire contents
+of the /run/udev directory are private to udev and do change whenever needed.
+
+Requirements:
+  - Version 2.6.34 of the Linux kernel with sysfs, procfs, signalfd, inotify,
+    unix domain sockets, networking and hotplug enabled
+
+  - Some architectures might need a later kernel, that supports accept4(),
+    or need to backport the accept4() syscall wiring in the kernel.
+
+  - These options are required:
+      CONFIG_DEVTMPFS=y
+      CONFIG_HOTPLUG=y
+      CONFIG_INOTIFY_USER=y
+      CONFIG_NET=y
+      CONFIG_PROC_FS=y
+      CONFIG_SIGNALFD=y
+      CONFIG_SYSFS=y
+      CONFIG_SYSFS_DEPRECATED*=n
+      CONFIG_UEVENT_HELPER_PATH=""
+
+  - These options might be needed:
+      CONFIG_BLK_DEV_BSG=y (SCSI devices)
+      CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
+
+  - The /dev directory needs the 'devtmpfs' filesystem mounted.
+    Udev only manages the permissions and ownership of the
+    kernel-provided device nodes, and possibly creates additional symlinks.
+
+  - Udev requires /run to be writable, which is usually done by mounting a
+    'tmpfs' filesystem.
+
+  - This version of udev does not work properly with the CONFIG_SYSFS_DEPRECATED*
+    option enabled.
+
+  - 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, and the sysfs filesystem must
+    be mounted at /sys. No other locations are supported by a standard
+    udev installation.
+
+  - The default rule sset requires the following group names resolvable at udev startup:
+      disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, and kmem.
+    Especially in LDAP setups, it is required that getgrnam() be able to resolve
+    these group names with only the rootfs mounted and while no network is
+    available.
+
+  - Some udev extras have external dependencies like:
+      libglib2, usbutils, pciutils, and gperf.
+    All these extras can be disabled with configure options.
+
+Setup:
+  - The udev daemon should be started to handle device events sent by the kernel.
+    During bootup, the events for already existing devices can be replayed, so
+    that they are configured by udev. The systemd service files contain the
+    needed commands to start the udev daemon and the coldplug sequence.
+
+  - Restarting the daemon never applies any rules to existing devices.
+
+  - New/changed rule files are picked up automatically; there is usually no
+    daemon restart or signal needed.
+
+Operation:
+  - Based on events the kernel sends out on device creation/removal, udev
+    creates/removes device nodes and symlinks in the /dev directory.
+
+  - All kernel events are matched against a set of specified rules, which
+    possibly hook into the event processing and load required kernel
+    modules to set up devices. For all devices, the kernel exports a major/minor
+    number; if needed, udev creates a device node with the default kernel
+    device 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 man pages:
+      http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev/
+
+Please direct any comment/question to the linux-hotplug mailing list at:
+  linux-hotplug@vger.kernel.org