Section: File Formats (5)
Updated: Jul 2006
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/etc/crypttab - encrypted block device table



The /etc/crypptab file describes encrypted block devices that are set up during system boot.

Empty lines and lines starting with the # character are ignored. Each of the remaining lines describes one encrypted block device, fields on the line are delimited by white space. The first two fields are mandatory, the remaining two are optional.

The first field contains the name of the resulting encrypted block device; the device is set up at /dev/mapper/name.

The second field contains a path to the underlying block device. If the block device contains a LUKS signature, it is opened as a LUKS encrypted partition; otherwise it is assumed to be a raw dm-crypt partition.

The third field specifies the encryption password. If the field is not present or the password is set to none, the password has to be manually entered during system boot. Otherwise the field is interpreted as a path to a file containing the encryption password. For swap encryption /dev/urandom can be used as the password file; using /dev/random may prevent boot completion if the system does not have enough entropy to generate a truly random encryption key.

The fourth field, if present, is a comma-delimited list of options. The following options are recognized:

Specifies the cipher to use; see cryptsetup(8) for possible values and the default value of this option. A cipher with unpredictable IV values, such as aes-cbc-essiv:sha256, is recommended.

Specifies the key size in bits; see cryptsetup(8) for possible values and the default value of this option.

Specifies the hash to use for password hashing; see cryptsetup(8) for possible values and the default value of this option.

If the the encryption password is read from console, it has to be entered twice (to prevent typos).

The encrypted block device will be used as a swap partition, and will be formatted as a swap partition after setting up the encrypted block device. The underlying block device will be formatted again as an unencrypted swap partition after destroying the encrypted block device. (This allows sharing a single swap partition between operating system installations, with some of them encrypting the swap partitions and some of them not.)

WARNING: Using the swap option will destroy the contents of the named partition during every boot, so make sure the underlying block device is specified correctly.

The encrypted block device will be prepared for using it as tmp partition: it will be formatted using mke2fs and its root directory will be set to mode 01777. The warning about the swap option applies here as well.

No options can be specified for LUKS encrypted partitions.



The /etc/crypptab file format is based on the Debian cryptsetup package, and is intended to be compatible.