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 are delimited by white space.
Each line is in the form
name encrypted-device password options
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 an 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:
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.
discard
Allow discard requests to be passed through the encrypted
block device. This improves performance on SSD storage but has security
implications.
hash=
Specifies the hash to use for password hashing. See
cryptsetup(8) for possible values and the default value of this
option.
header=
Use a detached (separated) metadata device or file where
the LUKS header is stored. This option is only relevant for LUKS devices. 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.
key-slot=
Specifies the key slot to compare the passphrase or key
against. If the key slot does not match the given passphrase or key, but
another would, the setup of the device will fail regardless. This option
implies
luks. See
cryptsetup(8) for possible values. The default
is to try all key slots in sequential order.
keyfile-timeout=
Specifies the timeout for the device on which the key
file resides and falls back to a password if it could not be mounted. See
systemd-cryptsetup-generator(8) for key files on external
devices.
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=.
_netdev
Marks this cryptsetup device as requiring network. It
will be started after the network is available, similarly to
systemd.mount(5) units marked with
_netdev. The service unit to
set up this device will be ordered between remote-fs-pre.target and
remote-cryptsetup.target, instead of cryptsetup-pre.target and
cryptsetup.target.
Hint: if this device is used for a mount point that is specified
in fstab(5), the _netdev option should also be used for the
mount point. Otherwise, a dependency loop might be created where the mount
point will be pulled in by local-fs.target, while the service to configure
the network is usually only started after the local file system has
been mounted.
noauto
This device will not be added to cryptsetup.target. This
means that it will not be automatically unlocked on boot, unless something
else pulls it in. In particular, if the device is used for a mount point,
it'll be unlocked automatically during boot, unless the mount point itself is
also disabled with noauto.
nofail
This device will not be a hard dependency of
cryptsetup.target. It'll be still pulled in and started, but the system will
not wait for the device to show up and be unlocked, and boot will not fail if
this is unsuccessful. Note that other units that depend on the unlocked device
may still fail. In particular, if the device is used for a mount point, the
mount point itself is also needs to have noauto option, or the boot
will fail if the device is not unlocked successfully.
offset=
Start offset in the backend device, in 512-byte sectors.
This option is only relevant for plain devices.
plain
Force plain encryption mode.
read-only, readonly
Set up the encrypted block device in read-only
mode.
skip=
How many 512-byte sectors of the encrypted data to skip
at the beginning. This is different from the
offset= option with
respect to the sector numbers used in initialization vector (IV) calculation.
Using
offset= will shift the IV calculation by the same negative
amount. Hence, if
offset=n is given, sector
n will
get a sector number of 0 for the IV calculation. Using
skip= causes
sector
n to also be the first sector of the mapped device, but with its
number for IV generation being
n.
This option is only relevant for plain devices.
size=
Specifies the key size in bits. See
cryptsetup(8)
for possible values and the default value of this option.
sector-size=
Specifies the sector size in bytes. 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 option 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 option
implies tcrypt.
tcrypt-veracrypt
Check for a VeraCrypt volume. VeraCrypt is a fork of
TrueCrypt that is mostly compatible, but uses different, stronger key
derivation algorithms that cannot be detected without this flag. Enabling this
option could substantially slow down unlocking, because VeraCrypt's key
derivation takes much longer than TrueCrypt's. This option implies
tcrypt.
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.
x-systemd.device-timeout=
Specifies how long systemd should wait for a device to
show up before giving up on the entry. The argument is a time in seconds or
explicitly specified units of "s", "min", "h",
"ms".
At early boot and when the system manager configuration is
reloaded, this file is translated into native systemd units by
systemd-cryptsetup-generator(8).