CRYPTTAB man page on Oracle

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CRYPTTAB(5)			   crypttab			   CRYPTTAB(5)

NAME
       crypttab - Configuration for encrypted block devices

SYNOPSIS
       /etc/crypttab

DESCRIPTION
       The /etc/crypttab file describes encrypted block devices that are set
       up during system boot.

       Empty lines and lines starting with the "#" character are ignored. Each
       of the remaining lines describes one encrypted block device, fields on
       the line are delimited by white space. The first two fields are
       mandatory, the remaining two are optional.

       Setting up encrypted block devices using this file supports three
       encryption modes: LUKS, TrueCrypt and plain. See cryptsetup(8) for more
       information about each mode. When no mode is specified in the options
       field and the block device contains a LUKS signature, it is opened as a
       LUKS device; otherwise, it is assumed to be in raw dm-crypt (plain
       mode) format.

       The first field contains the name of the resulting encrypted block
       device; the device is set up within /dev/mapper/.

       The second field contains a path to the underlying block device or
       file, or a specification of a block device via "UUID=" followed by the
       UUID.

       The third field specifies the encryption password. If the field is not
       present or the password is set to "none" or "-", the password has to be
       manually entered during system boot. Otherwise, the field is
       interpreted as a absolute path to a file containing the encryption
       password. For swap encryption, /dev/urandom or the hardware device
       /dev/hw_random can be used as the password file; using /dev/random may
       prevent boot completion if the system does not have enough entropy to
       generate a truly random encryption key.

       The fourth field, if present, is a comma-delimited list of options. The
       following options are recognized:

       discard
	   Allow discard requests to be passed through the encrypted block
	   device. This improves performance on SSD storage but has security
	   implications.

       cipher=
	   Specifies the cipher to use. See cryptsetup(8) for possible values
	   and the default value of this option. A cipher with unpredictable
	   IV values, such as "aes-cbc-essiv:sha256", is recommended.

       hash=
	   Specifies the hash to use for password hashing. See cryptsetup(8)
	   for possible values and the default value of this option.

       keyfile-offset=
	   Specifies the number of bytes to skip at the start of the key file.
	   See cryptsetup(8) for possible values and the default value of this
	   option.

       keyfile-size=
	   Specifies the maximum number of bytes to read from the key file.
	   See cryptsetup(8) for possible values and the default value of this
	   option. This option is ignored in plain encryption mode, as the key
	   file size is then given by the key size.

       luks
	   Force LUKS mode. When this mode is used, the following options are
	   ignored since they are provided by the LUKS header on the device:
	   cipher=, hash=, size=.

       noauto
	   This device will not be automatically unlocked on boot.

       nofail
	   The system will not wait for the device to show up and be unlocked
	   at boot, and not fail the boot if it does not show up.

       plain
	   Force plain encryption mode.

       read-only, readonly
	   Set up the encrypted block device in read-only mode.

       size=
	   Specifies the key size in bits. See cryptsetup(8) for possible
	   values and the default value of this option.

       swap
	   The encrypted block device will be used as a swap device, and will
	   be formatted accordingly after setting up the encrypted block
	   device, with mkswap(8). This option implies plain.

	   WARNING: Using the swap option will destroy the contents of the
	   named partition during every boot, so make sure the underlying
	   block device is specified correctly.

       tcrypt
	   Use TrueCrypt encryption mode. When this mode is used, the
	   following options are ignored since they are provided by the
	   TrueCrypt header on the device or do not apply: cipher=, hash=,
	   keyfile-offset=, keyfile-size=, size=.

	   When this mode is used, the passphrase is read from the key file
	   given in the third field. Only the first line of this file is read,
	   excluding the new line character.

	   Note that the TrueCrypt format uses both passphrase and key files
	   to derive a password for the volume. Therefore, the passphrase and
	   all key files need to be provided. Use tcrypt-keyfile= to provide
	   the absolute path to all key files. When using an empty passphrase
	   in combination with one or more key files, use "/dev/null" as the
	   password file in the third field.

       tcrypt-hidden
	   Use the hidden TrueCrypt volume. This implies tcrypt.

	   This will map the hidden volume that is inside of the volume
	   provided in the second field. Please note that there is no
	   protection for the hidden volume if the outer volume is mounted
	   instead. See cryptsetup(8) for more information on this limitation.

       tcrypt-keyfile=
	   Specifies the absolute path to a key file to use for a TrueCrypt
	   volume. This implies tcrypt and can be used more than once to
	   provide several key files.

	   See the entry for tcrypt on the behavior of the passphrase and key
	   files when using TrueCrypt encryption mode.

       tcrypt-system
	   Use TrueCrypt in system encryption mode. This implies tcrypt.

	   Please note that when using this mode, the whole device needs to be
	   given in the second field instead of the partition. For example: if
	   "/dev/sda2" is the system encrypted TrueCrypt patition, "/dev/sda"
	   has to be given.

       timeout=
	   Specifies the timeout for querying for a password. If no unit is
	   specified, seconds is used. Supported units are s, ms, us, min, h,
	   d. A timeout of 0 waits indefinitely (which is the default).

       tmp
	   The encrypted block device will be prepared for using it as /tmp;
	   it will be formatted using mke2fs(8). This option implies plain.

	   WARNING: Using the tmp option will destroy the contents of the
	   named partition during every boot, so make sure the underlying
	   block device is specified correctly.

       tries=
	   Specifies the maximum number of times the user is queried for a
	   password. The default is 3. If set to 0, the user is queried for a
	   password indefinitely.

       verify
	   If the encryption password is read from console, it has to be
	   entered twice to prevent typos.

       At early boot and when the system manager configuration is reloaded,
       this file is translated into native systemd units by systemd-
       cryptsetup-generator(8).

EXAMPLE
       Example 1. /etc/crypttab example

       Set up four encrypted block devices. One using LUKS for normal storage,
       another one for usage as a swap device and two TrueCrypt volumes.

	   luks	      UUID=2505567a-9e27-4efe-a4d5-15ad146c258b
	   swap	      /dev/sda7	      /dev/urandom	       swap
	   truecrypt  /dev/sda2	      /etc/container_password  tcrypt
	   hidden     /mnt/tc_hidden  /null		       tcrypt-hidden,tcrypt-keyfile=/etc/keyfile

SEE ALSO
       systemd(1), systemd-cryptsetup@.service(8), systemd-cryptsetup-
       generator(8), cryptsetup(8), mkswap(8), mke2fs(8)

systemd 208							   CRYPTTAB(5)
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