dm-crypt/System configuration

Tip:

If in need to remotely unlock root or other early-boot filesystems (headless machine, distant servers...), follow the specific instructions from dm-crypt/Specialties#Remote unlocking of the root (or other) partition.
To ease inputting UUIDs, PARTUUIDs, etc. in configuration files, you can install and use a text editor that supports inserting command output (e.g. nano with Ctrl+t, Vim or Neovim with :read or mcedit with Alt+u) and pass it the apropriate lsblk or blkid command. Alternatively, you can install a terminal multiplexer and use its copy and paste functionality.

Unlocking in early userspace

Booting an encrypted root volume requires that the initramfs contains the necessary tools for early userspace to unlock the volume. The instructions on what to unlock are topically passed via kernel parameters.

The following sections describe how to configure mkinitcpio and list which kernel parameters are required.

mkinitcpio

Depending on the particular scenarios, a subset of the following mkinitcpio hooks will have to be enabled:

busybox	systemd	Use case
`encrypt`	`sd-encrypt`	Needed when the root partition is encrypted or when an encrypted partition needs to be mounted before the root partition. Not needed in other cases as system initialization scripts like `/etc/crypttab` already take care of unlocking other non-root partitions. This hook must be placed after the `udev` or `systemd` hook.
`keyboard`		Needed to make keyboards work in early userspace. Tip: For systems that are booted with different hardware configurations (e.g. laptops with external keyboard vs. internal keyboard or headless systems), it is helpful to place this hook before `autodetect` in order to always include all keyboard drivers. Otherwise the external keyboard only works in early userspace if it was connected when creating the image.
`keymap`		Provides support for non-US keymaps for typing encryption passwords; it must come before the `encrypt` hook, otherwise you will need to enter your encryption password using the default US keymap. Set your keymap in , see Keyboard configuration in console#Persistent configuration.
		Loads an alternative console font in early userspace. Set your font in , see Linux console#Persistent configuration.

Other hooks needed should be clear from other manual steps followed during the installation of the system.

Examples

A typical /etc/mkinitcpio.conf configuration using encrypt hook:

/etc/mkinitcpio.conf

...
HOOKS=(base '''udev''' autodetect modconf kms keyboard '''keymap''' '''consolefont''' block '''encrypt''' lvm2 filesystems fsck)
...

A configuration with systemd-based initramfs using sd-encrypt hook:

Kernel parameters

The kernel parameters you need to specify depend on whether the encrypt hook or the sd-encrypt hook is being used. and are specified the same way for both.

For example, if using GRUB, the relevant parameters are added to before generating the main configuration file. See also GRUB#Warning when installing in chroot as another point to be aware of when installing the GRUB loader.

root

The root= parameter specifies the of the actual (decrypted) root file system:

root=device

If the file system is formatted directly on the decrypted device file this will be .
If a LVM gets activated first and contains an encrypted logical rootvolume, the above form applies as well.
If the root file system is contained in a logical volume of a fully encrypted LVM, the device mapper for it will be in the general form of .

resume

resume=device

is the device file of the decrypted (swap) filesystem used for suspend to disk. If swap is on a separate partition, it will be in the form of /dev/mapper/swap. See also dm-crypt/Swap encryption.

cryptdevice

This parameter will make the system prompt for the passphrase to unlock the device containing the encrypted root on a cold boot. It is parsed by the encrypt hook to identify which device contains the encrypted system:

cryptdevice=device:dmname:options

is the path to the device backing the encrypted device. Usage of persistent block device naming is strongly recommended.
is the device-mapper name given to the device after decryption, which will be available as .
(optional) are comma separated options, e.g. for TRIM support. If no options are required, omit this parameter (use ).
If a LVM contains the encrypted root, the LVM gets activated first and the volume group containing the logical volume of the encrypted root serves as device. It is then followed by the respective volume group to be mapped to root. The parameter follows the form of .

cryptkey

This parameter specifies the location of a keyfile and is required by the encrypt hook for reading such a keyfile to unlock the (unless a key is in the default location, see below). It can have three parameter sets, depending on whether the keyfile exists as a file in a particular device, a bitstream starting on a specific location, or a file in the initramfs.

For a file in a device the format is:

cryptkey=device:fstype:path

is the raw block device where the key exists. Usage of persistent block device naming is strongly recommended.
is the filesystem type of (or auto).
is the absolute path of the keyfile within the device.

Example: cryptkey=LABEL=usbstick:vfat:/secretkey

For a bitstream on a device the key's location is specified with the following:

cryptkey=device:offset:size

where the offset and size are in bytes. For example, reads a 512 byte keyfile starting at the beginning of the device.

For a file included in the initramfs the format is:

cryptkey=rootfs:path

Example:

Also note that if is not specified, it defaults to (in the initramfs).

crypto

This parameter is specific to pass dm-crypt plain mode options to the encrypt hook.

It takes the form

crypto=hash:cipher:keysize:offset:skip

The arguments relate directly to the cryptsetup options. See dm-crypt/Device encryption#Encryption options for plain mode.

For a disk encrypted with just plain default options, the arguments must be specified, but each entry can be left blank:

crypto=::::

A specific example of arguments is

crypto=sha512:twofish-xts-plain64:512:0:

Using systemd-cryptsetup-generator

systemd-cryptsetup-generator is a systemd unit generator that reads a subset of kernel parameters, and /etc/crypttab, for the purpose of unlocking encrypted devices. See the man page for more details about it and all options it supports.

systemd-cryptsetup-generator is run during the initramfs stage when using the sd-encrypt mkinitcpio hook or the systemd dracut module.

In what follows, we describe some of the kernel parameters that systemd-cryptsetup-generator interprets.

Tip:

If the file /etc/crypttab.initramfs exists, it will be added to the initramfs as /etc/crypttab, there you can specify devices that need to be unlocked at the initramfs phase. See #crypttab for the syntax. If /etc/crypttab.initramfs does not exist, there will be no /etc/crypttab in the initramfs and the unlockable devices will need to be specified via kernel parameters listed below.
/etc/crypttab.initramfs is not limited to using only UUID like rd.luks. You can use any of the persistent block device naming methods.
Passwords entered during boot are cached in the kernel keyring by systemd-cryptsetup(8), so if multiple devices can be unlocked with the same password (this includes devices in crypttab that are unlocked after boot), then you will only need to input each password once.

rd.luks.uuid

Tip: rd.luks.uuid can be omitted when using rd.luks.name.

rd.luks.uuid=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX

Specify the UUID of the device to be decrypted on boot with this flag.

By default the mapped device will be located at where XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX is the UUID of the LUKS partition.

rd.luks.name

rd.luks.name=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX=name

Specify the name of the mapped device after the LUKS partition is open, where XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX is the UUID of the LUKS partition. This is equivalent to the second parameter of encrypt's .

For example, specifying causes the unlocked LUKS device with UUID to be located at .

rd.luks.key

Specify the location of a password file used to decrypt the device specified by its UUID. There is no default location like there is with the encrypt hook parameter .

If the keyfile is included in the initramfs:

rd.luks.key=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX=/path/to/keyfile

or

rd.luks.key=/path/to/keyfile

If the keyfile is on another device:

rd.luks.key=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX=/path/to/keyfile:UUID=ZZZZZZZZ-ZZZZ-ZZZZ-ZZZZ-ZZZZZZZZZZZZ

Replace UUID=ZZZZZZZZ-ZZZZ-ZZZZ-ZZZZ-ZZZZZZZZZZZZ with the identifier of the device on which the keyfile is located.

rd.luks.options

rd.luks.options=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX=options

or

rd.luks.options=options

Set options for the device specified by it UUID or, if not specified, for all UUIDs not specified elsewhere (e.g., crypttab).

This parameter is the analogue of crypttab's options field. The format is the same—options are separated by commas, options with values are specified using . This is roughly equivalent to the third parameter of encrypt's .

For example:

rd.luks.options=timeout=10s,discard,password-echo=no,tries=1

Timeout

There are two options that affect the timeout for entering the password during boot:

specifies the timeout for querying for a password
specifies how long systemd should wait for the rootfs device to show up before giving up (defaults to 90 seconds)

If you want to disable the timeout altogether, then set both timeouts to zero:

rd.luks.options=timeout=0 rootflags=x-systemd.device-timeout=0

Password echo

When the user is typing the password, systemd-cryptsetup by default outputs asterisks (*) for each typed character. This is unlike the encrypt hook, which does not output anything. To silence the output, set the option:

rd.luks.options=password-echo=no

Trusted Platform Module

If a TPM2 chip is available in your system, you can use it to automatically unlock your volume instead of using a password or a keyfile. Further information and detailed instructions on doing so can be found at Trusted Platform Module#Data-at-rest encryption with LUKS.

rd.luks.data

When using a detached LUKS header, specify the block device with the encrypted data. Must be used together with to specify the header file location.

See dm-crypt/Specialties#Encrypted system using a detached LUKS header for details and instructions.

Unlocking in late userspace

crypttab

The /etc/crypttab (encrypted device table) file is similar to the fstab file and contains a list of encrypted devices to be unlocked during system boot up. This file can be used for automatically mounting encrypted swap devices or secondary file systems.

is read before , so that dm-crypt containers can be unlocked before the file system inside is mounted. Note that  is read after the system has booted up, therefore it is not a replacement for unlocking encrypted partitions by using mkinitcpio hooks and configuring them by using kernel parameters as in the case of encrypting the root partition.  processing at boot time is made by the systemd-cryptsetup-generator automatically.

See for details, read below for some examples, and the #Mounting at boot time section for instructions on how to use UUIDs to mount an encrypted device.

To test your crypttab immediately after editing it, reload the systemd manager configuration with a daemon-reload and start the newly generated .

# cryptsetup status ''name''

/dev/mapper/''name'' is active.
  type:    ...
  cipher:  ...
  keysize: ... bits
  key location: ...
  device:  /dev/sdxN
  sector size:  ...
  offset:  ... sectors
  size:    ... sectors
  mode:    ...
  flags:   ...

For more on , see #Mounting on demand.

Mounting at boot time

If you want to mount an encrypted drive at boot time, enter the device's UUID in /etc/crypttab. You get the UUID (partition) by using the command and adding it to in the form:

The first parameter is your preferred device mapper's name for the encrypted drive. The option none will trigger a prompt during boot to type the passphrase for unlocking the partition. The option defines a timeout in seconds for entering the decryption password during boot.

Unlocking with a keyfile

If the keyfile for a secondary file system is itself stored inside an encrypted root, it is safe while the system is powered off and can be sourced to automatically unlock the mount during with boot via crypttab. For example, unlock a crypt specified by UUID:

Then use the device mapper's name (defined in /etc/crypttab) to make an entry in :

Since /dev/mapper/externaldrive already is the result of a unique partition mapping, there is no need to specify an UUID for it. In any case, the mapper with the filesystem will have a different UUID than the partition it is encrypted in.

Mounting a stacked blockdevice

The systemd generators also automatically process stacked block devices at boot.

For example, you can create a RAID setup, use cryptsetup on it and create an LVM logical volume with respective filesystem inside the encrypted block device. A resulting:

will ask for the passphrase and mount automatically at boot.

Given you specify the correct corresponding crypttab (e.g. UUID for the device) and fstab () entries, there is no need to add additional mkinitcpio hooks/configuration, because /etc/crypttab processing applies to non-root mounts only. One exception is when the mdadm_udev hook is used already (e.g. for the root device). In this case and the initramfs need updating to achieve the correct root raid is picked first.

Mounting on demand

Instead of using

# cryptsetup open UUID=... externaldrive

you can start when you have an entry as follows in your /etc/crypttab:

That way you do not need to remember the exact crypttab options. It will prompt you for the passphrase if needed.

The corresponding unit file is generated automatically by . You can list all generated unit files using:

$ systemctl list-unit-files | grep systemd-cryptsetup

Troubleshooting

System stuck on boot/password prompt does not show

If you are using Plymouth, make sure to use the correct modules (see Plymouth#mkinitcpio) or disable it. Otherwise Plymouth will swallow the password prompt, making a system boot impossible.

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