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 is supported as well, using
UUID=<luks_uuid>.
The third field, key file, describes the file to use as a key for
decrypting the data of the source device. 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 precheck,
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 or the cipher, hash and size options.
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.
OPTIONS
cipher=<cipher>
Encryption algorithm (ignored for LUKS devices). See cryptsetup -c.
size=<size>
Encryption key size (ignored for LUKS devices). See cryptsetup -s.
hash=<hash>
Hash algorithm (ignored for LUKS devices). See cryptsetup -h.
offset=<offset>
Start offset (ignored for LUKS devices). Uses cryptsetup -o.
skip=<skip>
Skip sectors at the beginning (ignored for LUKS devices). Uses
cryptsetup -p.
verify
Verify password. Uses cryptsetup -y.
readonly
The backing device is read-only (eg: a dvd).
discard
Allow using of discards (TRIM) requests for device.
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.
Kernel version 3.1 or more recent is required. For older versions
is the option ignored.
luks
Use device with LUKS extensions.
swap
Run mkswap on the created device.
tmp=<tmpfs>
Run mkfs with filesystem type <tmpfs> on the created device.
Default is ext4.
precheck=<precheck>
Check the content of the source device by a suitable program; if
the check fails, the device is not created. If a program is
provided as an argument, it is run, giving the source device as
argument. Cryptdisks/cryptroot searches for the given progam in
/lib/cryptsetup/checks/ first, but full path to program is
supported as well.
Prechecks aren't invoked for LUKS devices, as these are checked
with isLuks anyway. Default for plain dm-crypt devices is set in
/etc/default/cryptdisks, or un_blkid otherwise. Set to /bin/true in
order to disable precheck for plain dm-crypt device.
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).
checkargs=<arguments>
Give <arguments> as the second argument to the check script. See
the CHECKSCRIPTS section for more information.
tries=<num>
The input of the passphrase is tried <num> times in case of
failure. If you want to disable retries, pass “tries=1”. Default is
3. Setting “tries=0” will ask for the passphrase until a correct
one has been submitted (infinitive retries).
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.
noauto
Entirely ignore the device at the boot process. It's still possible
to map the device manually using cryptdisks_start.
loud
Be loud. Print warnings if a device does not exist. This option
overwrites the option quiet.
quiet
Be quiet. Don't print warnings if a device does not exist. This
option overwrites the option loud.
keyscript=<path>
The executable at the indicated path is executed with the key file
from the third field of the crypttab as its only argument and the
output is used as the key. This also works with encrypted root
filesystems via initramfs if the executable is self-contained (i.e.
an executable which does not rely on any external program which is
not present in the initramfs environment).
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.
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_TRIED
Number of previous tries since start of cryptdisks (counts
until <term>CRYPTTAB_TRIES</term> is reached).
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.
keyslot=<slot>
Key slot (ignored for non-LUKS devices). See cryptsetup -S.
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-cbc-essiv:sha256,hash=ripemd160,size=256,swap
# Encrypted LUKS disk with interactive password, identified by UUID
cdisk0 UUID=12345678-9abc-def012345-6789abcdef01 none luks
# Encrypted ext4 disk with interactive password
# - retry 5 times if the check fails
cdisk1 /dev/sda2 none cipher=aes-cbc-essiv:sha256,hash=ripemd160,size=256,checkargs=ext4,tries=5
# Encrypted disk with interactive password
# - use a nondefault check script
# - no retries
cdisk2 /dev/hdc1 none cipher=aes-cbc-essiv:sha256,hash=ripemd160,size=256,check=customscript,tries=1
# Encrypted disk with interactive password
# - twofish as the cipher
cdisk3 /dev/sda3 none cipher=twofish,hash=ripemd160,size=256
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.
CRYPTDISKS_PRECHECK
Specifies the default checkscript to be run against the source
dm-crypt device, before cryptdisks has been invoked. The source
device is given as the first and only argument to the checkscript.
Takes effect if the precheck option is given in crypttab with no
value. See documentation for precheck 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 ALSOcryptsetup(8), cryptdisks_start(8), cryptdisks_stop(8)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>,
Jonas Meurer <jonas@freesources.org> and David Härdeman
<david@hardeman.nu>.
cryptsetup 2:1.6.1-1ubunt 2013-11-01 CRYPTTAB(5)
