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 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 Härdeman
       <david@hardeman.nu>.