dm-crypt/System configuration

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Reason: Aggregate here all the generic information on system configuration from the other sub-articles of dm-crypt. (Discuss in Talk: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.

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`	Always needed when encrypting the root partition, or a partition that needs to be mounted before root. It is not needed in all the other cases, as system initialization scripts like `/etc/crypttab` take care of unlocking other encrypted 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`	`sd-vconsole`	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 `/etc/vconsole.conf`, see Keyboard configuration in console#Persistent configuration.
`consolefont`	`sd-vconsole`	Loads an alternative console font in early userspace. Set your font in `/etc/vconsole.conf`, see Linux console#Persistent configuration.

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

Remember to regenerate the initramfs after saving the changes.

Examples

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

/etc/mkinitcpio.conf

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

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

/etc/mkinitcpio.conf

...
HOOKS=(base systemd autodetect keyboard sd-vconsole modconf block sd-encrypt lvm2 filesystems fsck)
...

Boot loader

In order to enable booting an encrypted root partition, a subset of the following kernel parameters need to be set. See kernel parameters for instructions specific to your boot loader.

For example, if using GRUB, the relevant parameters are added to /etc/default/grub 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.

The kernel parameters you need to specify depend on whether or not you are using the encrypt hook or the sd-encrypt hook.

Kernel parameters

Kernel parameters like root and resume are specified the same way for both encrypt and sd-encrypt hooks.

root

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

root=device

If the file system is formatted directly on the decrypted device file this will be /dev/mapper/dmname.
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 root=/dev/volumegroup/logicalvolume.

Tip: When using GRUB and generating grub.cfg with grub-mkconfig, this parameter does not need to be specified manually. grub-mkconfig will determine the correct UUID of the decrypted root filesystem and add it to grub.cfg automatically.

resume

resume=device

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.

Using encrypt hook

Note: Compared to the sd-encrypt hook, the encrypt hook does not support:

Unlocking multiple encrypted disks (FS#23182). Only one device can be unlocked in the initramfs.
Using a detached LUKS header (FS#42851).
Setting additional options that are supported by crypttab, e.g. sector-size (FS#72119).

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

device is the path to the device backing the encrypted device. Usage of persistent block device naming is strongly recommended.
dmname is the device-mapper name given to the device after decryption, which will be available as /dev/mapper/dmname.
options (optional) are comma separated options, e.g. for TRIM support. If no options are required, omit this parameter (use cryptdevice=device:dmname).
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 cryptdevice=/dev/vgname/lvname:dmname.

Tip: One may want to enable Discard/TRIM support for solid state drives (SSD).

cryptkey

This parameter specifies the location of a keyfile and is required by the encrypt hook for reading such a keyfile to unlock the cryptdevice (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

device is the raw block device where the key exists. Usage of persistent block device naming is strongly recommended.
fstype is the filesystem type of device (or auto).
path 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, cryptkey=UUID=ZZZZZZZZ-ZZZZ-ZZZZ-ZZZZ-ZZZZZZZZZZZZ:0:512 reads a 512 byte keyfile starting at the beginning of the device.

Tip: If the device path you want to access contains the character :, you have to escape it with a backslash \. In that case the cryptkey parameter would be as follow: cryptkey=/dev/disk/by-id/usb-123456-0\:0:0:512 for a usb key with the id usb-123456-0:0.

For a file included in the initramfs the format is[1]:

cryptkey=rootfs:path

Example: cryptkey=rootfs:/secretkey

Also note that if cryptkey is not specified, it defaults to /crypto_keyfile.bin (in the initramfs).[2]

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 crypto 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 sd-encrypt hook

The sd-encrypt hook relies on systemd-cryptsetup-generator to unlock encrypted devices in initramfs. See the systemd-cryptsetup-generator(8) man page for more details about it and all options it supports.

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.
/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.

Note:

All of the rd.luks parameters can be specified multiple times to unlock multiple LUKS encrypted volumes.
The rd.luks parameters only support unlocking LUKS devices. To unlock a plain dm-crypt device, you must specify it in /etc/crypttab.initramfs. See #crypttab for the syntax.

Warning: If you are using /etc/crypttab or /etc/crypttab.initramfs together with luks.* or rd.luks.* parameters, only those devices specified on the kernel command line will be activated and you will see Not creating device 'devicename' because it was not specified on the kernel command line.. This is because the luks.* or rd.luks.* parameters control which devices from the crypttab get activated. To activate all devices in /etc/crypttab do not specify any luks.* parameters and use rd.luks.*. To activate all devices in /etc/crypttab.initramfs do not specify any luks.* or rd.luks.* parameters.

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 /dev/mapper/luks-XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX where XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX is the UUID of the LUKS partition.

rd.luks.name

Tip: When using this parameter you can omit rd.luks.uuid.

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 cryptdevice.

For example, specifying rd.luks.name=12345678-9abc-def0-1234-56789abcdef0=cryptroot causes the unlocked LUKS device with UUID 12345678-9ABC-DEF0-1234-56789ABCDEF0 to be located at /dev/mapper/cryptroot.

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 cryptkey.

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

Tip: The whole rd.luks.key parameter can be omitted if the keyfile is included as /etc/cryptsetup-keys.d/name.key.

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.

Warning:

If the type of file system is different than your root file system, you must include the kernel module for it in the initramfs.
rd.luks.key with a keyfile on another device by default does not fallback to asking for a password if the device is not available. To fallback to a password prompt, specify the keyfile-timeout= option in rd.luks.options. E.g. for a 10 second timeout:
```
rd.luks.options=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX=keyfile-timeout=10s
```

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 option=value. This is roughly equivalent to the third parameter of encrypt's cryptdevice.

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:

rd.luks.options=timeout=mytimeout specifies the timeout for querying for a password
rootflags=x-systemd.device-timeout=mytimeout 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 password-echo=no 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 rd.luks.options to specify the header file location.

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

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.

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

See crypttab(5) 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.

Warning:

If the nofail option is specified, the password entry screen may disappear while typing the password. nofail should therefore only be used together with keyfiles.
For dm-crypt plain mode devices, the plain option must be explicitly set to force systemd-cryptsetup to recognize them. See systemd issue 442.

/etc/crypttab

# Example crypttab file. Fields are: name, underlying device, passphrase, cryptsetup options.

# Mount /dev/lvm/swap re-encrypting it with a fresh key each reboot
 swap	/dev/lvm/swap	/dev/urandom	swap,cipher=aes-xts-plain64,size=256

# Mount /dev/lvm/tmp as /dev/mapper/tmp using plain dm-crypt with a random passphrase, making its contents unrecoverable after it is dismounted.
tmp	/dev/lvm/tmp	/dev/urandom	tmp,cipher=aes-xts-plain64,size=256 

# Mount /dev/lvm/home as /dev/mapper/home using LUKS, and prompt for the passphrase at boot time.
home   /dev/lvm/home

# Mount /dev/sdb1 as /dev/mapper/backup using LUKS, with a passphrase stored in a file.
backup /dev/sdb1       /home/alice/backup.key

To test your crypttab immediately after editing it, reload the systemd manager configuration with:

# systemctl daemon-reload

and start the newly generated systemd-cryptsetup@name.service.

# 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 systemd-cryptsetup@name.service, see #Mounting on demand.

Tip: If you use GPT and specific partition type UUIDs, you can avoid using crypttab and fstab for some mount points with systemd. For more information, see systemd#GPT partition automounting.

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 lsblk -f and adding it to crypttab in the form:

/etc/crypttab

externaldrive         UUID=2f9a8428-ac69-478a-88a2-4aa458565431        none    timeout=180

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 timeout option defines a timeout in seconds for entering the decryption password during boot.

Tip: Passwords entered in the password prompt are cached in the kernel keyring by systemd-cryptsetup(8) (when using the sd-encrypt hook, this also includes passwords entered in the initramfs stage). If a device in crypttab uses a previously entered password, the third parameter can be set to none and the cached password will be automatically used.

Note: Keep in mind that the timeout option in crypttab only determines the amount of time allowed for entering the password of the encrypted device. In addition, systemd also has a default timeout which determines the amount of time allowed for the device to be available (defaulting to 90 seconds), which is independent of the password timer. In consequence, even when the timeout option in crypttab is set to a value larger than 90 seconds (or it is at its default value of 0, meaning unlimited time), systemd will still only wait a maximum of 90 seconds for the device to be unlocked. In order to change the time systemd will wait for a device to be available, the option x-systemd.device-timeout (see systemd.mount(5)) can be set in fstab for said device. It is probably desired, then, that the amount of time of the timeout option in crypttab is equal to the amount of time of the x-systemd.device-timeout option in fstab for each device mounted at boot time.

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:

/etc/crypttab

home-crypt    UUID=UUID-identifier    /etc/cryptsetup-keys.d/home-crypt.key

Tip:

If a keyfile is not specified, systemd-cryptsetup(8) will automatically try to load it from /etc/cryptsetup-keys.d/name.key and /run/cryptsetup-keys.d/name.key.[3]
If you prefer to use a --plain mode blockdevice, the encryption options necessary to unlock it are specified in /etc/crypttab. Take care to apply the systemd workaround mentioned in crypttab in this case.

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

/etc/fstab

/dev/mapper/home-crypt        /home   ext4        defaults        0       2

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:

$ lsblk -f

─sdXX                  linux_raid_member    
│ └─md0                 crypto_LUKS   
│   └─cryptedbackup     LVM2_member 
│     └─vgraid-lvraid   ext4              /mnt/backup
└─sdYY                  linux_raid_member    
  └─md0                 crypto_LUKS       
    └─cryptedbackup     LVM2_member 
      └─vgraid-lvraid   ext4              /mnt/backup

will ask for the passphrase and mount automatically at boot.

Given you specify the correct corresponding crypttab (e.g. UUID for the crypto_LUKS device) and fstab (/dev/vgraid/lvraid) 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 /etc/madadm.conf 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 systemd-cryptsetup@externaldrive.service when you have an entry as follows in your /etc/crypttab:

/etc/crypttab

externaldrive UUID=... none noauto

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 systemd-cryptsetup-generator(8). 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#The plymouth hook) or disable it. Otherwise Plymouth will swallow the password prompt, making a system boot impossible.