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eea1fd84 | 1 | udev - Linux userspace device management |
c2df8b5f | 2 | |
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3 | Integrating udev in the system has complex dependencies and may differ from |
4 | distribution to distribution. A system may not be able to boot up or work | |
5 | reliably without a properly installed udev version. The upstream udev project | |
a2ddee73 | 6 | does not recommend replacing a distro's udev installation with the upstream |
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7 | version. |
8 | ||
9 | The upstream udev project's set of default rules may require a most recent | |
57c6f8ae | 10 | kernel release to work properly. |
c2df8b5f | 11 | |
0f50d0ef | 12 | Tools and rules shipped by udev are not public API and may change at any time. |
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13 | Never call any private tool in /usr/lib/udev from any external application; it |
14 | might just go away in the next release. Access to udev information is only offered | |
15 | by udevadm and libudev. Tools and rules in /usr/lib/udev and the entire contents | |
16 | of the /run/udev directory are private to udev and do change whenever needed. | |
c2df8b5f | 17 | |
c249f66a | 18 | Requirements: |
e3c14a7f | 19 | - Version 2.6.34 of the Linux kernel with sysfs, procfs, signalfd, inotify, |
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20 | unix domain sockets, networking and hotplug enabled |
21 | ||
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22 | - Some architectures might need a later kernel, that supports accept4(), |
23 | or need to backport the accept4() syscall wiring in the kernel. | |
67a77c8b | 24 | |
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25 | - These options are required: |
26 | CONFIG_DEVTMPFS=y | |
262d5cca | 27 | CONFIG_HOTPLUG=y |
57c6f8ae | 28 | CONFIG_INOTIFY_USER=y |
262d5cca | 29 | CONFIG_NET=y |
262d5cca | 30 | CONFIG_PROC_FS=y |
98ad80f8 | 31 | CONFIG_SIGNALFD=y |
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32 | CONFIG_SYSFS=y |
33 | CONFIG_SYSFS_DEPRECATED*=n | |
34 | CONFIG_UEVENT_HELPER_PATH="" | |
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35 | |
36 | - These options might be needed: | |
a6c5b514 | 37 | CONFIG_BLK_DEV_BSG=y (SCSI devices) |
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38 | CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes) |
39 | ||
40 | - The /dev directory needs the 'devtmpfs' filesystem mounted. | |
41 | Udev only manages the permissions and ownership of the | |
42 | kernel-provided device nodes, and possibly creates additional symlinks. | |
c2df8b5f | 43 | |
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44 | - Udev requires /run to be writable, which is usually done by mounting a |
45 | 'tmpfs' filesystem. | |
1e03b754 | 46 | |
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47 | - This version of udev does not work properly with the CONFIG_SYSFS_DEPRECATED* |
48 | option enabled. | |
c2df8b5f | 49 | |
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50 | - The deprecated hotplug helper /sbin/hotplug should be disabled in the |
51 | kernel configuration, it is not needed today, and may render the system | |
52 | unusable because the kernel may create too many processes in parallel | |
53 | so that the system runs out-of-memory. | |
54 | ||
bde1af68 | 55 | - The proc filesystem must be mounted on /proc, and the sysfs filesystem must |
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56 | be mounted at /sys. No other locations are supported by a standard |
57 | udev installation. | |
c2df8b5f | 58 | |
67a77c8b | 59 | - The default rule sset requires the following group names resolvable at udev startup: |
bde1af68 | 60 | disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, and kmem. |
47d9121c MW |
61 | Especially in LDAP setups, it is required that getgrnam() be able to resolve |
62 | these group names with only the rootfs mounted and while no network is | |
67c89548 | 63 | available. |
c249f66a | 64 | |
d5d6a7f3 | 65 | - Some udev extras have external dependencies like: |
57c6f8ae | 66 | libglib2, usbutils, pciutils, and gperf. |
d5d6a7f3 | 67 | All these extras can be disabled with configure options. |
c2df8b5f | 68 | |
eea1fd84 | 69 | Setup: |
eea1fd84 | 70 | - The udev daemon should be started to handle device events sent by the kernel. |
afb9771b | 71 | During bootup, the events for already existing devices can be replayed, so |
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72 | that they are configured by udev. The systemd service files contain the |
73 | needed commands to start the udev daemon and the coldplug sequence. | |
eea1fd84 | 74 | |
cb936a39 | 75 | - Restarting the daemon never applies any rules to existing devices. |
c2df8b5f | 76 | |
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77 | - New/changed rule files are picked up automatically; there is usually no |
78 | daemon restart or signal needed. | |
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79 | |
80 | Operation: | |
fd752623 | 81 | - Based on events the kernel sends out on device creation/removal, udev |
afb9771b | 82 | creates/removes device nodes and symlinks in the /dev directory. |
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83 | |
84 | - All kernel events are matched against a set of specified rules, which | |
85 | possibly hook into the event processing and load required kernel | |
49500e9b | 86 | modules to set up devices. For all devices, the kernel exports a major/minor |
3fe44a29 | 87 | number; if needed, udev creates a device node with the default kernel |
afb9771b | 88 | device name. If specified, udev applies permissions/ownership to the device |
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89 | node, creates additional symlinks pointing to the node, and executes |
90 | programs to handle the device. | |
91 | ||
92 | - The events udev handles, and the information udev merges into its device | |
93 | database, can be accessed with libudev: | |
94 | http://www.kernel.org/pub/linux/utils/kernel/hotplug/libudev/ | |
95 | http://www.kernel.org/pub/linux/utils/kernel/hotplug/gudev/ | |
96 | ||
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97 | For more details about udev and udev rules, see the udev man pages: |
98 | http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev/ | |
c2df8b5f | 99 | |
a6c5b514 | 100 | Please direct any comment/question to the linux-hotplug mailing list at: |
98520be7 | 101 | linux-hotplug@vger.kernel.org |