Ubuntu Manpages

crypttab

static information about encrypted filesystems

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.

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.

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.

# 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

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.

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.

cryptsetup(8), cryptdisks_start(8), cryptdisks_stop(8)

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>.