"udev is the device manager for the Linux kernel. Primarily, it manages device nodes in Template:Filename. It is the successor of devfs and hotplug, which means that it handles the Template:Filename directory and all user space actions when adding/removing devices, including firmware load." Source: Wikipedia:Udev
udev loads kernel modules by utilizing coding parallelism to provide a potential performance advantage versus loading these modules serially. The modules are therefore loaded asynchronously. The inherent disadvantage of this method is that udev does not always load modules in the same order on each boot. If the machine has multiple block devices, this may manifest itself in the form of device nodes changing designations randomly. For example, if the machine has two hard drives, /dev/sda may randomly become /dev/sdb. See below for more info on this.
- 1 About udev rules
- 2 Tips & Tricks
- 2.1 UDisks
- 2.2 Auto mounting USB devices
- 2.2.1 Mount under Template:Filename; use partition label if present
- 2.2.2 Mount under Template:Filename; use partition label if present; supports LUKS encryption
- 2.2.3 Mount under Template:Filename; use partition label if present; support user un-mounting
- 2.2.4 Mount under Template:Filename; create symbolic link under Template:Filename
- 2.2.5 Mount under Template:Filename only if the partition has a label
- 2.2.6 Mount under Template:Filename; use partition label if present; ntfs-3g
- 2.2.7 Mount SD cards
- 2.2.8 Mount CDs
- 2.2.9 Accessing Firmware Programmers and USB Virtual Comm Devices
- 2.3 Execute on USB Insert
- 2.4 Mount internal drives as a normal user
- 2.5 Mark internal SATA-Ports as eSATA-Ports
- 3 Troubleshooting
- 3.1 Blacklisting Modules
- 3.2 udevd hangs at boot
- 3.3 Known Problems with Hardware
- 3.4 Known Problems with Auto-Loading
- 3.5 Known Problems for Custom Kernel Users
- 3.6 IDE CD/DVD-drive support
- 4 Other Resources
About udev rules
udev rules written by the administrator go in Template:Filename, their file name has to end with Template:Filename. The udev rules shipped with various packages are found in Template:Filename. If there are two files by the same name under /lib and /etc, the ones in /etc take precedence.
If you want to learn how to write udev rules see Writing udev rules.
To get a list of all the attributes of a device you can use to write rules:
# udevadm info -a -n [device name]
Replace [device name] with the device present in the system, such as '/dev/sda' or '/dev/ttyUSB0'.
Udev automatically detects changes to rule files, so changes take effect immediately without requiring udev to be restarted. However, the rules are not re-triggered automatically on already existing devices, so hotpluggable devices, such as USB devices, will probably have to be reconnected for the new rules to take effect.
Tips & Tricks
Simply install UDisks:
pacman -S udisks
and all your media should be auto mounted in GNOME and KDE SC 4.6. There is no need for any additional rules this way. As an extra bonus you can remove HAL if you were only using that for auto mounting purposes.
Automounting UDisks Wrappers
A UDisks wrapper has the advantage of being very easy to install and needing no (or minimal) configuration. The wrapper will automatically mount things like CDs and flash drives.
- devmon - devmon (AUR) is a configuration-less bash wrapper script for udisks which automounts optical discs and removable drives. It can also selectively autostart apps or execute commands after mounting, ignore specified devices and volume labels, and unmount removable drives.
- udiskie - Written in Python. Allows auto mount and unmount by any user.
- udisksevt - Written in Haskell. Allows auto mount by any user. Designed to be integrated with traydevice.
- The udisksvm bash script uses udisks and Traydevice to automount removable media and to control in GUI, with mouse clicks in systray, the un-mounting and re-mounting of disks or the ejection of optical disks.
UDisks Shell Functions
While UDisks includes a simple method of (un)mounting devices via command-line, it can be tiresome to type the commands out each time. These shell functions will generally shorten and ease command-line usage.
- udisks_functions - Written for Bash.
- bashmount - bashmount (AUR) is a menu-driven bash script with a configuration file that makes it easy to configure and extend.
Auto mounting USB devices
Mount under Template:Filename; use partition label if present
The following udev rule set automatically mounts devices/partitions that are represented by /dev/sd* (USB drives, external hard drives and sometimes SD cards). If a partition label is available, it mounts the device to /media/<label> and otherwise to /media/usbhd-sd* (ex: /media/usbhd-sdb1): Template:File
Mount under Template:Filename; use partition label if present; supports LUKS encryption
Similar to the above rule set, but if the device is a LUKS-encrypted partition it will open an xterm window to ask for the passphrase (provided that xterm is installed). Also see this post and the follow-ups.
Mount under Template:Filename; use partition label if present; support user un-mounting
This is a variation on the above rule set. It uses pmount (which will need to be installed) instead of mount, allowing a non-root user to unmount udev-mounted devices. The required username must be hard-coded in the RUN command, so this rule set may not be suitable for multi-user systems. LUKS support has also been removed from the example, but can be easily reinstated as above. You must edit the /bin/su invocation to run as the correct user for your system. Template:File
The following rule set does not make use of partition labels; instead it mounts devices as usbhd-sdXY under the /mnt directory (ex: /mnt/usbhd-sdb1) and creates a symbolic link under /media. Template:File
Mount under Template:Filename only if the partition has a label
Mount under Template:Filename; use partition label if present; ntfs-3g
Yet another example, this time making use of ntfs-3g read/write drivers for NTFS filesystems:
Mount SD cards
To auto mount a CD a simple UDisks wrapper will get the job done properly.
Accessing Firmware Programmers and USB Virtual Comm Devices
The following ruleset will allow normal users (within the "users" group) the ability to access the USBtinyISP USB programmer for AVR microcontrollers and a generic (SiLabs CP2102) USB to UART adapter. Adjust the permissions accordingly. Verified as of 2010-02-11.
Execute on USB Insert
Mount internal drives as a normal user
If you want to mount a internal drive on KDE or Gnome (maybe on other Desktop Environments too) as a normal user (without the need to type your superuser password) you just have to edit the Template:Filename (remember to backup first) and change the line <allow_active>auth_admin_keep</allow_active> on <action id="org.freedesktop.udisks.filesystem-mount-system-internal"> to <allow_active>yes</allow_active>.
Mark internal SATA-Ports as eSATA-Ports
If you connected a eSATA bay or an other eSATA adapter the system will still recognize this disk as an internal SATA drive. Gnome and KDE will ask you for your root password all the time. The following rule will mark the specified SATA-Port as an external eSATA-Port. With that, a normal Gnome user can connect their eSATA drives to that port like a USB drive, without any root password and so on.
In rare cases, udev can make mistakes and load the wrong modules. To prevent it from doing this, you can blacklist modules. Once blacklisted, udev will never load that module. See blacklisting. Not at boot-time or later on when a hotplug event is received (eg, you plug in your USB flash drive).
udevd hangs at boot
After migrating to LDAP or updating an LDAP-backed system udevd can hang at boot at the message "Starting UDev Daemon". This is usually caused by udevd trying to look up a name from LDAP but failing, because the network is not up yet. The solution is to ensure that all system group names are present locally.
Extract the group names referenced in udev rules and the group names actually present on the system:
# fgrep -r GROUP /etc/udev/rules.d/ /lib/udev/rules.d | perl -nle '/GROUP\s*=\s*"(.*?)"/ && print $1;' | sort | uniq > udev_groups # cut -f1 -d: /etc/gshadow /etc/group | sort | uniq > present_groups
To see the differences, do a side-by-side diff:
# diff -y present_groups udev_groups ... network < nobody < ntp < optical optical power | pcscd rfkill < root root scanner scanner smmsp < storage storage ...
In this case, the pcscd group is for some reason not present in the system. Add the missing groups:
# groupadd pcscd
Also, make sure local resources are looked up before resorting to LDAP. Template:Filename should contain the line
group: files ldap
Known Problems with Hardware
BusLogic devices can be broken and will cause a freeze during startup
This is a kernel bug and no fix has been provided yet.
Some devices, that should be treated as removable, are not
Create a custom udev rule, setting UDISKS_SYSTEM_INTERNAL=0. For more details, see the manpage of udisks.
Known Problems with Auto-Loading
CPU frequency modules
The current detection method for the various CPU frequency controllers is inadequate, so this has been omitted from the auto-loading process for the time being. To use CPU Frequency Scaling, load the proper module explicitly in your Template:Codeline array in Template:Filename. Further reading: rc.conf.
Sound Problems or Some Modules Not Loaded Automatically
Some users have traced this problem to old entries in Template:Filename. Try cleaning that file out and trying again.
Mixed Up Devices, Sound/Network Cards Changing Order Each Boot
Because udev loads all modules asynchronously, they are initialized in a different order. This can result in devices randomly switching names. For example, with two network cards, you may notice a switching of designations between Template:Codeline and Template:Codeline.
Arch Linux provides the advantage of specifying the module load order by listing the modules in the Template:Codeline array in Template:Filename. Modules in this array are loaded before udev begins auto-loading, so you have full control over the load order.
# Always load 8139too before e100 MODULES=(8139too e100)
Another method for network card ordering is to use the udev-sanctioned method of statically-naming each interface. Create the following file to bind the MAC address of each of your cards to a certain interface name: Template:File
A couple things to note:
- To get the MAC address of each card, use this command: Template:Codeline
- Make sure to use the lower-case hex values in your udev rules. It doesn't like upper-case.
- Some people have problems naming their interfaces after the old style: eth0, eth1, etc. Try something like "lan" or "wlan" if you experience this problem.
Don't forget to update your Template:Filename and other configuration files using the old ethX notation!
Other methods for setting network interface names are described in the Configuring Network#Interface names varying wiki entry.
Known Problems for Custom Kernel Users
Udev doesn't start at all
Make sure you have a kernel version later than or equal to 2.6.32. Earlier kernels do not have the necessary uevent stuff that udev needs for auto-loading.
IDE CD/DVD-drive support
Starting with version 170, udev doesn't support CD-ROM/DVD-ROM drives, which are loaded as traditional IDE drives with the Template:Codeline module and show up as Template:Filename. The drive remains usable for tools which access the hardware directly, like cdparanoia, but is invisible for higher userspace programs, like KDE.
A cause for the loading of the ide_cd_mod module prior to others, like sr_mod, could be e.g. that you have for some reason the module piix loaded with your initramfs. In that case you can just replace it with ata_piix in your Template:Filename.