Section: cryptsetup manual (5)
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 respectively tcrypt or the cipher, hash and size options.
Options are in the format: key=value [,key=value ...]. The supported options are described below.
: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.
: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 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).
: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:
# Encrypted swap device cswap /dev/sda6 /dev/urandom cipher=aes-xts-plain64,size=256,hash=sha1,swap # Encrypted LUKS disk with interactive password, identified by UUID cdisk0 UUID=12345678-9abc-def012345-6789abcdef01 none luks # Encrypted TCRYPT disk with interactive password tdisk0 /dev/sr0 none tcrypt # Encrypted ext4 disk with interactive password # - retry 5 times if the check fails cdisk1 /dev/sda2 none cipher=aes-xts-plain64,size=256,hash=sha1,checkargs=ext4,tries=5 # Encrypted disk with interactive password # - use a nondefault check script # - no retries cdisk2 /dev/sdc1 none cipher=aes-xts-plain64,size=256,hash=sha1,check=customscript,tries=1 # Encrypted disk with interactive password # - Twofish as the cipher, RIPEMD-160 as the hash cdisk3 /dev/sda3 none cipher=twofish,size=256,hash=ripemd160
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.
This manual page was originally written by Bastian Kleineidam <[email protected]> for the Debian distribution of cryptsetup. It has been further improved by Michael Gebetsroither <[email protected]>, Jonas Meurer <[email protected]> and David Härdeman <[email protected]>.
Tutoriais de Tecnologia Web