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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.
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.
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 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.
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.
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).
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".
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 /dev/null tcrypt-hidden,tcrypt-keyfile=/etc/keyfile
SEE ALSO
systemd(1), systemd-cryptsetup@.service(8), systemd-cryptsetup-generator(8), fstab(5),
cryptsetup(8), mkswap(8), mke2fs(8)
systemd 219 CRYPTTAB(5)
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On Apache/2.4.6 (CentOS)
Under GNU General Public License
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