crypttab

CRYPTTAB(5) cryptsetup manual CRYPTTAB(5)

NAME
crypttab - static information about encrypted filesystems

DESCRIPTION
The file /etc/crypttab contains descriptive information about encrypted
filesystems. crypttab is only read by programs (e.g. cryptdisks_start
and cryptdisks_stop), and not written; it is the duty of the system
administrator to properly create and maintain this file. Each
filesystem is described on a separate line; fields on each line are
separated by tabs or spaces. Lines starting with "#" are comments,
empty lines are ignored. The order of records in crypttab is important
because the init scripts sequentially iterate through crypttab doing
their thing.

The first field, target, describes the mapped device name. It must be a
plain filename without any directory components. A mapped device which
encrypts/decrypts data to/from the source device will be created at
/dev/mapper/target by cryptsetup.

The second field, source device, describes either the block special
device or file that contains the encrypted data. Instead of giving the
source device explicitly, the UUID (resp. LABEL, PARTUUID and
PARTLABEL) is supported as well, using "UUID=<uuid>" (resp.
"LABEL=<label>", "PARTUUID=<partuuid>" and "PARTLABEL=<partlabel>").

The third field, key file, describes the file to use as a key for
decrypting the data of the source device. In case of a keyscript, the
value of this field is given as argument to the keyscript. Values with
spaces and special characters need to be escaped using octal sequences,
like for fstab(5). Note that the entire key file will be used as the
passphrase; the passphrase must not be followed by a newline character.

It can also be a device name (e.g. /dev/urandom), note however that
LUKS requires a persistent key and therefore does not support random
data keys.

If the key file is the string none, a passphrase will be read
interactively from the console. In this case, the options check,
checkargs and tries may be useful.

The fourth field, options, describes the cryptsetup options associated
with the encryption process. At minimum, the field should contain
either the string luks respectively tcrypt or the cipher, hash and size
options. Some options can be changed on active devices using cryptsetup
refresh [<options>] <name>. Moreover some options can be permanently
written to the metada of LUKS2 headers using the --persistent option
flag.

Options are in the format: key=value [,key=value ...]. The supported
options are described below.

Note that all four fields are mandatory and that a missing field will
lead to unspecified behaviour.

ON DIFFERENT CRYPTTAB FORMATS
Please note that there are several independent cryptsetup wrappers with
their own crypttab format. This manpage covers Debian's implementation
for initramfs scripts and SysVinit init scripts. systemd brings its own
crypttab implementation. We try to cover the differences between the
systemd and our implementation in this manpage, but if in doubt, better
check the systemd crypttab(5) manpage, e.g. online at
https://www.freedesktop.org/software/systemd/man/crypttab.html.

OPTIONS
cipher=<cipher>
Encryption algorithm (ignored for LUKS and TCRYPT devices). See
cryptsetup -c.

size=<size>
Encryption key size (ignored for LUKS and TCRYPT devices). See
cryptsetup -s.

sector-size=<bytes>
Sector size. See cryptsetup(8) for possible values and the default
value of this option.

hash=<hash>
Hash algorithm (ignored for LUKS and TCRYPT devices). See
cryptsetup -h.

offset=<offset>
Start offset (ignored for LUKS and TCRYPT devices). Uses cryptsetup
-o.

skip=<skip>
Skip sectors at the beginning (ignored for LUKS and TCRYPT
devices). Uses cryptsetup -p.

keyfile-offset=<keyfile-offset>
Specifies the number of bytes to skip at the start of the key file.

keyfile-size=<keyfile-size>
Specifies the maximum number of bytes to read from the key file.
The default is to read the whole file up to the compiled-in
maximum, that can be queried with cryptsetup --help. This option is
ignored for plain dm-crypt devices, as the key file size is then
given by the encryption key size (option size).

keyslot=<slot>, key-slot=<slot>
Key slot (ignored for non-LUKS devices). See cryptsetup -S.

header=<path>
Detached header file (ignored for plain dm-crypt devices). See
cryptsetup --header.

verify
Verify password. Uses cryptsetup -y.

readonly, read-only
Set up a read-only mapping.

tries=<num>
Try to unlock the device <num> before failing. It's particularly
useful when using a passphrase or a keyscript that asks for
interactive input. If you want to disable retries, pass "tries=1".
Default is "3". Setting "tries=0" means infinitive retries.

discard
Allow using of discards (TRIM) requests for device.

Starting with Debian 10 (Buster), this option is added per default
to new dm-crypt devices by the Debian Installer. If you don't care
about leaking access patterns (filesystem type, used space) and
don't have hidden truecrypt volumes inside this volume, then it
should be safe to enable this option. See the following warning for
further information.

WARNING: Assess the specific security risks carefully before
enabling this option. For example, allowing discards on encrypted
devices may lead to the leak of information about the ciphertext
device (filesystem type, used space etc.) if the discarded blocks
can be located easily on the device later.

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=

plain
Force plain encryption mode.

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=

veracrypt, tcrypt-veracrypt
Use VeraCrypt extension to TrueCrypt device. Only useful in
conjunction with tcrypt option (ignored for non-TrueCrypt devices).

tcrypthidden, tcrypt-hidden
Use hidden TCRYPT header (ignored for non-TCRYPT devices).

swap
Run mkswap on the created device.

This option is ignored for initramfs devices.

tmp=<tmpfs>
Run mkfs with filesystem type <tmpfs> on the created device.
Default is ext4.

This option is ignored for initramfs devices.

check=<check>
Check the content of the target device by a suitable program; if
the check fails, the device is removed. If a program is provided as
an argument, it is run, giving the decrypted volume (target device)
as first argument, and the value of the checkargs option as second
argument. Cryptdisks/cryptroot searches for the given program in
/lib/cryptsetup/checks/ first, but full path to program is
supported as well.

Default is set in /etc/default/cryptdisks (blkid).

This option is specific to the Debian crypttab format. It's not
supported by systemd.

checkargs=<arguments>
Give <arguments> as the second argument to the check script. See
the CHECKSCRIPTS section for more information.

This option is specific to the Debian crypttab format. It's not
supported by systemd.

initramfs
The initramfs hook processes the root device, any resume devices
and any devices with the initramfs option set. These devices are
processed within the initramfs stage of boot. As an example, that
allows the use of remote unlocking using dropbear.

This option is specific to the Debian crypttab format. It's not
supported by systemd.

noearly
The cryptsetup init scripts are invoked twice during the boot
process - once before lvm, raid, etc. are started and once again
after that. Sometimes you need to start your encrypted disks in a
special order. With this option the device is ignored during the
first invocation of the cryptsetup init scripts.

This option is ignored for initramfs devices and specific to the
Debian crypttab format. It's not supported by systemd.

noauto
Entirely ignore the device at the boot process. It's still possible
to map the device manually using cryptdisks_start.

This option is ignored for initramfs devices and specific to the
Debian crypttab format. It's not supported by systemd.

loud
Be loud. Print warnings if a device does not exist. This option
overwrites the option loud.

This option is ignored for initramfs devices and specific to the
Debian crypttab format. It's not supported by systemd.

quiet
Be quiet. Don't print warnings if a device does not exist. This
option overwrites the option loud.

This option is ignored for initramfs devices and specific to the
Debian crypttab format. It's not supported by systemd.

keyscript=<path>
The executable at the indicated path is executed with the value of
the third field as only argument. The keyscript output is passed to
cryptsetup as decyption key. When used in initramfs, the executable
either needs to be self-contained (i.e. does'nt rely on any
external program which is not present in the initramfs environment)
or the dependencies have to added to the initramfs image by other
means.

LIMITATIONS: All binaries and files on which the keyscript depends
must be available at the time of execution. Special care needs to
be taken for encrypted filesystems like /usr or /var. As an
example, unlocking encrypted /usr must not depend on binaries from
/usr/(s)bin.

This option is specific to the Debian crypttab format. It's not
supported by systemd.

WARNING: With systemd as init system, this option might be ignored.
At the time this is written (December 2016), the systemd cryptsetup
helper doesn't support the keyscript option to /etc/crypttab. For
the time being, the only option to use keyscripts along with
systemd is to force processing of the corresponding crypto devices
in the initramfs. See the 'initramfs' option for further
information.

All fields of the appropriate crypttab entry are available to the
keyscript as exported environment variables:

CRYPTTAB_NAME
The target name

CRYPTTAB_SOURCE
The source device

CRYPTTAB_KEY
The key file

CRYPTTAB_OPTIONS
A list of exported crypttab options

CRYPTTAB_OPTION_<option>
The value of the appropriate crypttab option, with value set to
'yes' in case the option is merely a flag. For option aliases,
such as 'readonly' and 'read-only', the variable name refers to
the first alternative listed (thus 'CRYPTTAB_OPTION_readonly'
in that case). If the crypttab option name contains '-'
characters, then they are replaced with '_' in the exported
variable name. For instance, the value of the
'CRYPTTAB_OPTION_keyfile_offset' environment variable is set to
the value of the 'keyfile-offset' crypttab option.

CRYPTTAB_TRIED
Number of previous tries since start of cryptdisks (counts
until maximum number of tries is reached).

CHECKSCRIPTS
blkid
Checks for any known filesystem. Supports a filesystem type as
argument via <checkargs>:

o no checkargs - succeeds if any valid filesystem is found on the
device.

o "none" - succeeds if no valid filesystem is found on the
device.

o "ext4" [or another filesystem type like xfs, swap, crypto_LUKS,
...] - succeeds if ext4 filesystem is found on the device.

un_blkid
Checks for no known filesystem. Supports a filesystem type as
argument via <checkargs>:

o no checkargs - succeeds if no valid filesystem is found on the
device.

o "ext4" [or another filesystem type like xfs, swap, crypto_LUKS,
...] - succeeds if no ext4 filesystem is found on the device.

EXAMPLES
# Encrypted swap device
cswap /dev/sda6 /dev/urandom cipher=aes-xts-plain64,size=256,hash=sha1,swap

# Encrypted LUKS disk with interactive password, identified by its UUID, discard enabled
cdisk0 UUID=12345678-9abc-def012345-6789abcdef01 none luks,discard

# Encrypted TCRYPT disk with interactive password, discard enabled
tdisk0 /dev/sr0 none tcrypt,discard

# Encrypted ext4 disk with interactive password, discard enabled
# - retry 5 times if the check fails
cdisk1 /dev/sda2 none discard,cipher=aes-xts-plain64,size=256,hash=sha1,checkargs=ext4,tries=5

# Encrypted disk with interactive password, discard enabled
# - use a nondefault check script
# - no retries
cdisk2 /dev/sdc1 none discard,cipher=aes-xts-plain64,size=256,hash=sha1,check=customscript,tries=1

# Encrypted disk with interactive password, discard enabled
# - Twofish as the cipher, RIPEMD-160 as the hash
cdisk3 /dev/sda3 none dscard,cipher=twofish,size=256,hash=ripemd160

ENVIRONMENT
CRYPTDISKS_ENABLE
Set to yes to run cryptdisks initscripts at startup. Set to no to
disable cryptdisks initscripts. Default is yes.

CRYPTDISKS_MOUNT
Specifies the mountpoints that are mounted before cryptdisks is
invoked. Takes mountpoints configured in /etc/fstab as arguments.
Separate mountpoints by space. This is useful for keys on removable
devices, such as cdrom, usbstick, flashcard, etc. Default is unset.

CRYPTDISKS_CHECK
Specifies the default checkscript to be run against the target
device, after cryptdisks has been invoked. The target device is
passed as the first and only argument to the checkscript. Takes
effect if the check option is given in crypttab with no value. See
documentation for check option above for more information.

KNOWN UPGRADE ISSUES
The upstream defaults for encryption cipher, hash and keysize have
changed several times in the past, and they're expected to change again
in future, for example if security issues arise. On LUKS devices, the
used settings are stored in the LUKS header, and thus don't need to be
configured in /etc/crypttab. For plain dm-crypt devices, no information
about used cipher, hash and keysize are available at all. Therefore we
strongly suggest to configure the cipher, hash and keysize in
/etc/crypttab for plain dm-crypt devices, even if they match the
current default.

SEE ALSO
cryptsetup(8), cryptdisks_start(8), cryptdisks_stop(8),
/usr/share/doc/cryptsetup-initramfs/README.initramfs.gz

AUTHOR
This manual page was originally written by Bastian Kleineidam
<calvin@debian.org> for the Debian distribution of cryptsetup. It has
been further improved by Michael Gebetsroither <michael.geb@gmx.at>,
David Hardeman <david@hardeman.nu> and Jonas Meurer
<jonas@freesources.org>.

cryptsetup 2:2.2.2-3ubunt 2022-01-18 CRYPTTAB(5)

crypttab

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