dm-crypt/System configuration
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mkinitcpio
When encrypting a system it is necessary to regenerate the initial ramdisk after properly configuring mkinitcpio. Depending on the particular scenarios, a subset of the following hooks will have to be enabled:
-
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. -
shutdown
: recommended before mkinitcpio 0.16 to ensure controlled unmounting during system shutdown. It is still functional, but not deemed necessary anymore. -
keymap
: provides support for foreign keymaps for typing encryption passwords; it must come before theencrypt
hook. -
keyboard
: needed to make USB keyboards work in early userspace.-
usbinput
: deprecated, but can be given a try in casekeyboard
does not work.
-
Other hooks needed should be clear from other manual steps followed during the installation of the system.
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 using GRUB the relevant parameters are best added to /etc/default/grub
before generating the boot configuration. See also GRUB#Warning when installing in chroot as another point to be aware of when installing the GRUB loader.
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 raw 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
.
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/mapper/volumegroup-logicalvolume
.
resume
resume=device
-
device
is the device file of the decrypted (swap) filesystem used for suspend2disk. If swap is on a separate partition, it will be in the form of/dev/mapper/swap
. See also Dm-crypt/Swap encryption.
cryptkey
This parameter is required by the encrypt hook for reading a keyfile to unlock the cryptdevice. It can have two parameter sets, depending on whether the keyfile exists as a file or a bitstream starting on a specific location.
For a file the format is:
cryptkey=device:fstype:path
-
device
is the raw block device where the key exists. -
fstype
is the filesystem type ofdevice
(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
Example: cryptkey=/dev/sdZ:0:512
reads a 512 bit keyfile starting at the beginning of the device.
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:
crypttab
The /etc/crypttab
(or, encrypted device table) file contains a list of encrypted devices that are to be unlocked when the system boots, similar to fstab. This file can be used for automatically mounting encrypted swap devices or secondary filesystems.
It is read before fstab, so that dm-crypt containers can be unlocked before the filesystem inside is mounted. Note that crypttab is read after the system has booted, so it is not a replacement for unlocking via mkinitcpio hooks and boot loader options in the case of an encrypted root scenario. The boot time processing of crypttab is done by the systemd-cryptsetup-generator
automatically, i. e. there is no need to activate it.
See the crypttab man page for details, below for further examples and #Mounting at boot time for setup steps using a device's UUID.
/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, just enter the device's UUID in /etc/crypttab
. You get the UUID (partition) by using the command lsblk -f
and adding it to
/etc/crypttab
externaldrive UUID=2f9a8428-ac69-478a-88a2-4aa458565431 none luks,timeout=180
The first parameter is your preferred device mapper's name for your encrypted drive. The option none
will trigger a prompt during boot to type the passphrase for unlocking the partition. The timeout
option defines the timeout in seconds for entering the decryption password while booting.
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've defined in /etc/crypttab
in /etc/fstab
as shown here:
/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.