Ubuntu Manpage: crypttab - static information about encrypted filesystems

Provided by: cryptsetup_2.2.2-3ubuntu2.4_amd64

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>:

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

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

           •   "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>:

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

           •   "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.

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 Härdeman <david@hardeman.nu> and Jonas Meurer
       <jonas@freesources.org>.

NAME

DESCRIPTION

ON DIFFERENT CRYPTTAB FORMATS

OPTIONS

CHECKSCRIPTS

EXAMPLES

ENVIRONMENT

KNOWN UPGRADE ISSUES

SEE ALSO

AUTHOR