dm-crypt/System configuration - ArchWiki

# dm-crypt/System configuration

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< Dm-crypt

This article or section needs expansion.

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.

## 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.
`keymap` | `sd-vconsole` | Provides support for non-US keymaps for typing encryption passwords; it must come before the `encrypt` hook. Set your keymap in `/etc/vconsole.conf`, see Keyboard configuration in console#Persistent configuration.
`consolefont` | 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 sd-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: This parameter is not needed to be specified manually when using GRUB. Executing grub-mkconfig is meant to determine the correct UUID of the decrypted root filesystem and specify it in the generated `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 has some limitations. It 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).

#### 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

- `device` is the path to the device backing the encrypted device. Usage of Persistent block device naming is advisable.
- `dmname` is the device-mapper name given to the device after decryption, which will be available as `/dev/mapper/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`.

#### 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.
- `fstype` is the filesystem type of `device` (or auto).
- `path` is the absolute path of the keyfile within the device.

Example: `cryptkey=/dev/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. Example: `cryptkey=/dev/sdZ: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]

Note: If USB hard drive initialization fails on boot, add `usb_storage` to the `MODULES` array of mkinitcpio [3].

See also dm-crypt/Device encryption#Keyfiles.

#### 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

In all of the following `rd.luks` can be replaced with `luks`. `rd.luks` parameters are only honored by the initrd. `luks` parameters are honored by both the main system and initrd. Unless you want to control devices which get unlocked after boot from kernel command line, use `rd.luks`. See systemd-cryptsetup-generator(8) for more options and more details.

Tip:

- If the file `/etc/crypttab.initramfs` exists, mkinitcpio will add it to the initramfs as `/etc/crypttab`, you can specify devices that need to be unlocked at boot there. Syntax is documented in #crypttab and crypttab(5).
- `/etc/crypttab.initramfs` is not limited to using only UUID like `rd.luks`. You can use any of the persistent block device naming methods.

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 detectable LUKS devices. To unlock a plain dm-crypt device or a LUKS device with a detached header, 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.`. 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=UUID

Specify the UUID of the device to be decrypted on boot with this flag. If the UUID is in `/etc/crypttab.initramfs`, the options listed there will be used. For `luks.uuid` options from `/etc/crypttab.initramfs` or `/etc/crypttab` will be used.

By default the mapped device will be located at `/dev/mapper/luks-UUID` where UUID is the UUID of the LUKS partition.

#### rd.luks.name

    rd.luks.name=UUID=name

Specify the name of the mapped device after the LUKS partition is open. For example, specifying `UUID=cryptroot` causes the unlocked device to be located at `/dev/mapper/cryptroot`. If this is not specified the mapped device will be located at `/dev/mapper/luks-UUID` where UUID is the UUID of the LUKS partition. When using this parameter you can omit `rd.luks.uuid`.

This is equivalent to the second parameter of `encrypt`'s `cryptdevice`.

#### rd.luks.options

    rd.luks.options=UUID=options

or

    rd.luks.options=options

Specify options for the device listed after `UUID` or, if not specified, for all UUIDs not specified elsewhere (e.g., crypttab).

This is roughly equivalent to the third parameter of `encrypt`'s `cryptdevice`.

Follows a similar format to options in crypttab - options are separated by commas, options with values are specified using `option=value`.

For example:

    rd.luks.options=timeout=10s,swap,cipher=aes-cbc-essiv:sha256,size=256

#### rd.luks.key

Note: `sd-encrypt` hook only supports keyfiles that are embedded in the initramfs (i.e. specified in the `FILES` array in `/etc/mkinitcpio.conf`). See systemd issue 9181.

    rd.luks.key=UUID=mykeyfile

or

    rd.luks.key=mykeyfile

Specify the location of a password file used to decrypt the device specified in `rd.luks.UUID`. There is no default location like there is with the `encrypt` hook parameter `cryptkey`.

#### 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

## 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.

Note: When using systemd-boot and the `sd-encrypt` hook, if a non-root partition's passphrase is the same as root's, there is no need to put that non-root partition in crypttab due to passphrase caching. See this forum thread for more information.
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.
- There are issues with systemd when processing `crypttab` entries for dm-crypt plain mode (`--type plain`) devices:
  - For `--type plain` devices with a keyfile, it is necessary to add the `hash=plain` option to crypttab due to a systemd incompatibility. Do not use `systemd-cryptsetup` manually for device creation to work around it.
  - It may be further required to add the `plain` option explicitly to force `systemd-cryptsetup` to recognize a `--type plain`) device at boot. 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

### 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    luks,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.

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.

A keyfile can also be set up and referenced instead of `none`. This results in an automatic unlocking, if the keyfile is accessible during boot. Since LUKS offers the option to have multiple keys, the chosen option can also be changed later.

Use the device mapper's name you have defined in `/etc/crypttab` in `/etc/fstab` as follows:

    /etc/fstab

    /dev/mapper/externaldrive      /mnt/backup               ext4    defaults,errors=remount-ro  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

You can start `systemd-cryptsetup@externaldrive.service` instead of using

    # cryptsetup luksOpen UUID=... externaldrive

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.

Category:

- Disk encryption

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