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 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).
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 /dev/null tcrypt-hidden,tcrypt-keyfile=/etc/keyfile
external /dev/sda3 keyfile:LABEL=keydev keyfile-timeout=10s
SEE ALSO
systemd(1), systemd-cryptsetup@.service(8), systemd-cryptsetup-
generator(8), fstab(5), cryptsetup(8), mkswap(8), mke2fs(8)
systemd 239 CRYPTTAB(5)
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