Map scancodes to keycodes

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Note: This page assumes that you have read Extra keyboard keys, which provides wider context to the problem.

Mapping scancodes to keycodes is universal and not specific to Linux console or Xorg, which means that changes to this mapping will be effective in both.

There are two ways of mapping scancodes to keycodes:

  • Using udev
  • Using setkeycodes

The preferred method is to use udev because it uses hardware information (which is a quite reliable source) to choose the keyboard model in a database. It means that if your keyboard model has been found in the database, your keys are recognized out of the box.

Identifying scancodes

You need to know the scancodes of keys you wish to remap. See Extra keyboard keys#Scancodes for details.

Using udev

udev provides a builtin function called hwdb to maintain the hardware database index in /etc/udev/hwdb.bin. The database is compiled from files with .hwdb extension located in directories /usr/lib/udev/hwdb.d/, /run/udev/hwdb.d/ and /etc/udev/hwdb.d/. The default scancodes-to-keycodes mapping file is /usr/lib/udev/hwdb.d/60-keyboard.hwdb. See man udev for details.

Note: From systemd 220 the udev ABI changed. Users using custom udev hwdb rules should update them according to the new ABI

The .hwdb file can contain multiple blocks of mappings for different keyboards, or one block can be applied to multiple keyboards. The evdev: prefix is used to match a block against a hardware, the following hardware matches are supported:

  • Generic input devices (also USB keyboards) identified by the usb kernel modalias:
    where <vendor_id>, <product_id> and <version_id> are the 4-digit hex uppercase vendor, product and version IDs (you can find those by running the lsusb command) and <modalias> is an arbitrary length input-modalias describing the device capabilities. <bus_id> is the 4-digit hex bus id and should be 0003 for usb devices. The possible <bus_id> values are defined in /usr/include/linux/input.h (you can use the following command to get a list awk '/BUS_/ {print $2, $3}' /usr/include/linux/input.h).
  • AT keyboard DMI data matches:
    where <vendor> and <product> are the firmware-provided strings exported by the kernel DMI modalias.
  • Input driver device name and DMI data match:
    evdev:name:<input device name>:dmi:bvn*:bvr*:bd*:svn<vendor>:pn*
    where <input_device_name> is the name device specified by the driver and <vendor> is the firmware-provided string exported by the kernel DMI modalias.

The format of each line in the block body is KEYBOARD_KEY_<scancode>=<keycode>. The value of <scancode> is hexadecimal, but without the leading 0x (i.e. specify a0 instead of 0xa0), whereas the value of <keycode> is the lower-case keycode name string as listed in /usr/include/linux/input-event-codes.h (see the KEY_<KEYCODE> variables), a sorted list is available at [1]. It is not possible to specify decimal value in <keycode>.

Example for custom hwdb

The example hwdb file will match all AT keyboards:


Here is an example of rebinding modifiers on a laptop and USB keyboard:

evdev:input:b0003v05AFp8277* # was tested on Kensington Slim Type USB (with old ABI)
 KEYBOARD_KEY_70039=leftalt  # bind capslock to leftalt
 KEYBOARD_KEY_700e2=leftctrl # bind leftalt to leftctrl

evdev:atkbd:dmi:*            # built-in keyboard: match all AT keyboards for now
 KEYBOARD_KEY_3a=leftalt     # bind capslock to leftalt
 KEYBOARD_KEY_38=leftctrl    # bind leftalt to leftctrl

Updating the Hardware Database Index

After changing the configuration files, the hardware database index, hwdb.bin, needs to be rebuilt.

  • Update hwdb.bin manually by running
# udevadm hwdb --update

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: Don't edit in /usr/lib/, use systemctl edit. (Discuss in Talk:Map scancodes to keycodes#)
  • Update automatically on each reboot by commenting out ConditionNeedsUpdate in systemd-udev-hwdb-update.service.
#  This file is part of systemd.

After systemd-udev-hwdb-update.service finished loading systemd-udev-trigger.service will reload the changes from hwdb.bin.

  • Automatically after Systemd upgrade.

On each upgrade of Systemd, the installation script rebuilds hwdb.bin by running # udevadm hwdb --update so we do not need to care about it.

Reloading the Hardware Database Index

The kernel loads hwdb.bin as part of the boot process, rebooting the system will promise the loading of the updated hwdb.bin.

With udevadm it is possible to load new key mapping from the updated hwdb.bin by running

# udevadm trigger

Be aware that with udevadm only added or changed key mapping are loaded so if we delete a mapping from the config file, rebuild hwdb.bin and run # udevadm trigger then the deleted mapping still kept by the kernel, at least until a reboot.

Querying the database

You can check that your configuration was loaded either by pressing keys, or by running udevadm info. For the USB keyboard in the above example, this outputs the mapping we configured as follows:

# udevadm info /dev/input/by-path/*-usb-*-kbd | grep KEYBOARD_KEY
E: KEYBOARD_KEY_70039=leftalt
E: KEYBOARD_KEY_700e2=leftctrl

Using setkeycodes

setkeycodes is a tool to load scancodes-to-keycodes mapping table into Linux kernel. Its usage is:

# setkeycodes scancode keycode ...

It is possible to specify multiple pairs at once. Scancodes are given in hexadecimal, keycodes in decimal.

Note: Apparently setkeycodes does not work with USB keyboards (Linux 3.14.44-1-lts):
# setkeycodes 45 30     # bind NumLock (0x45) to KEY_A (30) on AT keyboard
# setkeycodes 70053 30  # bind NumLock (0x70053) to KEY_A (30) on USB keyboard
KDSETKEYCODE: Invalid argument
failed to set scancode 620d3 to keycode 31