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crypttab

man page of crypttab

crypttab: static information about encrypted filesystems

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 (which will be automatically mounted as a loop device) that should hold the encrypted data. 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 passphase; the passphase 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. See cryptsetup -c. size=<size> Encryption key size. See cryptsetup -s. hash=<hash> Hash algorithm. See cryptsetup -h. offset=<offset> Start offset. Uses cryptsetup -o. skip=<skip> Skip sectors at the beginning. Uses cryptsetup -p. verify Verify password. Uses cryptsetup -y. readonly The backing device is read-only (eg: a dvd). 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 ext2. 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. For the root device, "tries=0" enables infinitive retries due to a special case in the initramfs scripts. noearly The cryptsetup init scripts are invoked twice during the boot process - once before lvm, evms, 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 invokation 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). 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.
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 "ext3" [or another filesystem type like xfs, swap, crypto_LUKS, ...] - succeeds if ext3 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 "ext3" [or another filesystem type like xfs, swap, crypto_LUKS, ...] - succeeds if no ext3 filesystem is found on the device.

EXAMPLES

# Encrypted swap device cswap /dev/sda6 /dev/urandom swap # Encrypted luks disk with interactive password cdisk0 /dev/hda1 none luks # Encrypted ext2 disk with interactive password # - retry 5 times if the check fails cdisk1 /dev/sda2 none checkargs=ext2,tries=5 # Encrypted disk with interactive password # - use a nondefault check script # - no retries cdisk2 /dev/hdc1 none check=customscript,tries=1 # Encrypted disk with interactive password # - twofish as the cipher cdisk3 /dev/sda3 none cipher=twofish

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.

SEE ALSO

cryptsetup(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 Hardeman david@hardeman.nu. CRYPTTAB(5)
 
 
 

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