systemd/README

udev - 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.

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
/dev/.udev/ is private to udev.

Requirements:
  - Version 2.6.25 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=y
      CONFIG_SIGNALFD=y
      CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
      CONFIG_BLK_DEV_BSG=y (SCSI tape devices)

  - For reliable operation, the kernel must not use the CONFIG_SYSFS_DEPRECATED*
    option.

  - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module is not
    supported.

  - The proc filesystem must be mounted on /proc/, the sysfs filesystem must
    be mounted at /sys/. No other locations are supported by udev.

  - 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.

  - 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.

  - 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.

  - 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 in the kernel
    configuration, it is not needed, and may render the system unusable
    because of a fork-bombing behavior.

  - 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.

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