README

   1 udev - userspace device management
   2
   3 For more information see the files in the docs/ directory.
   4
   5 Important Note:
   6   Integrating udev in the system has complex dependencies and differs from distro
   7   to distro. All major distros depend on udev these days and the system may not
   8   work without a properly installed version. The upstream udev project does not
   9   recommend to replace a distro's udev installation with the upstream version.
  10
  11 Requirements:
  12   - Version 2.6.25 of the Linux kernel with sysfs, procfs, signalfd, inotify,
  13     unix domain sockets, networking and hotplug enabled:
  14       CONFIG_HOTPLUG=y
  15       CONFIG_UEVENT_HELPER_PATH=""
  16       CONFIG_NET=y
  17       CONFIG_UNIX=y
  18       CONFIG_SYSFS=y
  19       CONFIG_SYSFS_DEPRECATED*=n
  20       CONFIG_PROC_FS=y
  21       CONFIG_TMPFS=y
  22       CONFIG_TMPFS_POSIX_ACL=y
  23
  24   - For reliable operation, the kernel must not use the CONFIG_SYSFS_DEPRECATED*
  25     option.
  26
  27   - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module is not
  28     supported.
  29
  30   - The proc filesystem must be mounted on /proc/, the sysfs filesystem must
  31     be mounted at /sys/. No other locations are supported by udev.
  32
  33   - The system must have the following group names resolvable at udev startup:
  34       disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, kmem.
  35     Especially in LDAP setups, it is required, that getgrnam() is able to resolve
  36     these group names with only the rootfs mounted, and while no network is
  37     available.
  38
  39 Operation:
  40   Udev creates and removes device nodes in /dev/, based on events the kernel
  41   sends out on device discovery or removal.
  42
  43   - Early in the boot process, the /dev/ directory should get a 'tmpfs'
  44     filesystem mounted, which is maintained by udev. Created nodes or changed
  45     permissions will not survive a reboot, which is intentional.
  46
  47   - The content of /lib/udev/devices/ directory which contains the nodes,
  48     symlinks and directories, which are always expected to be in /dev, should
  49     be copied over to the tmpfs mounted /dev, to provide the required nodes
  50     to initialize udev and continue booting.
  51
  52   - The old hotplug helper /sbin/hotplug should be disabled in the kernel
  53     configuration, it is not needed, and may render the system unusable
  54     because of a fork-bombing behavior.
  55
  56   - All kernel events are matched against a set of specified rules in
  57     /lib/udev/rules.d/ which make it possible to hook into the event
  58     processing to load required kernel modules and setup devices. For all
  59     devices the kernel exports a major/minor number, udev will create a
  60     device node with the default kernel name, or the one specified by a
  61     matching udev rule.
  62
  63 Please direct any comment/question/concern to the linux-hotplug mailing list at:
  64   linux-hotplug@vger.kernel.org
  65