crypttab

CRYPTTAB(5) cryptsetup manual CRYPTTAB(5)

NAME

   crypttab - static information about encrypted filesystems

DESCRIPTION

   The file /etc/crypttab contains descriptive information about encrypted devices. 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. crypttab entries are treated
   sequentially, so their order matters (dependencies need to listed first).

   Each encrypted device is described on a separate line. Fields on each line are separated by tabs or spaces. Lines starting with '#' are comments, and blank
   lines are ignored. Octal sequences \0num within a field are decoded, which can be used for values containing spaces or special characters. A backslash which
   doesn't start an octal sequence yields undefined behavior.

   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. 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, is an optional comma-separated list of options and/or flags describing the device type (luks, tcrypt, bitlk, fvault2, or plain
   which is also the default) and cryptsetup options associated with the encryption process. The supported options are described below. For plain dm-crypt
   devices the cipher, hash and size options are required. Some options can be changed on active mappings using cryptsetup refresh [<options>] <name>.
   Furthermore some options can be permanently written into metadata of LUKS2 headers using cryptsetup's --persistent flag.

   Note that the first three fields are required 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.

   bitlk
       Force BITLK (Windows BitLocker-compatible) mode. WARNING: crypttab support is currently experimental.

   fvault2
       Force Apple's FileVault2 mode. Only the (legacy) FileVault2 format based on Core Storage and HFS+ filesystem (introduced in MacOS X 10.7 Lion) is
       currently supported; the new version of FileVault based on the APFS filesystem used in recent macOS versions is not supported.

   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).

   same-cpu-crypt
       Perform encryption using the same cpu that IO was submitted on.

   submit-from-crypt-cpus
       Disable offloading writes to a separate thread after encryption.

   no-read-workqueue, no-write-workqueue
       Bypass dm-crypt internal workqueue and process read or write requests synchronously.

   swap
       Run mkswap on the created device.

       This option is ignored for initramfs devices.

   tmp[=<tmpfs>]
       Run mkfs with filesystem type <tmpfs> (or ext4 if omitted) on the created device.

       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 closed immediately. The program is being run with
       decrypted volume (target device) as first positional argument and, if the checkargs option is used, its value as second argument. See the CHECKSCRIPTS
       section for more information.

       The program is either specified by full path or relative to /lib/cryptsetup/checks/. If omitted, then the value of $CRYPTDISKS_CHECK set in
       /etc/default/cryptdisks is used (blkid by default).

       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 overrides the option quiet.

       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 overrides 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's standard output is passed to
       cryptsetup as decyption key. Its exit status is currently ignored, but no assumption should be made in that regard. When used in initramfs, the
       executable either needs to be self-contained (i.e. doesn't 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. The program is either specified by full path or relative to /lib/cryptsetup/scripts/.

       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, _CRYPTTAB_NAME
           The target name (after resp. before octal sequence decoding).

       CRYPTTAB_SOURCE, _CRYPTTAB_SOURCE
           The source device (after resp. before octal sequence decoding and device resolution).

       CRYPTTAB_KEY, _CRYPTTAB_KEY
           The value of the third field (after resp. before octal sequence decoding).

       CRYPTTAB_OPTIONS, _CRYPTTAB_OPTIONS
           A list of exported crypttab options (after resp. before octal sequence decoding).

       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 plain,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 plain,cipher=aes-xts-plain64,size=256,hash=sha1,check,checkargs=ext4,tries=5,discard

       # Encrypted disk with interactive password, discard enabled
       # - use a nondefault check script
       # - no retries
       cdisk2 /dev/sdc1 none plain,cipher=aes-xts-plain64,size=256,hash=sha1,check=customscript,tries=1,discard

       # Encrypted disk with interactive password, discard enabled
       # - Twofish as the cipher, RIPEMD-160 as the hash
       cdisk3 /dev/sda3 none plain,cipher=twofish,size=256,hash=ripemd160,discard

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.

SEE ALSO

   cryptsetup(8), cryptdisks_start(8), cryptdisks_stop(8), /usr/share/doc/cryptsetup-initramfs/README.initramfs.gz

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>.

cryptsetup 2:2.6.1-4~deb1 2023-04-20 CRYPTTAB(5)