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772ae5f54d
Add optional parameter field to dmcrypt table and support "allow_discards" option. Discard requests bypass crypt queue processing. Bio is simple remapped to underlying device. Note that discard will be never enabled by default because of security consequences. It is up to the administrator to enable it for encrypted devices. (Note that userspace cryptsetup does not understand new optional parameters yet. Support for this will come later. Until then, you should use 'dmsetup' to enable and disable this.) Signed-off-by: Milan Broz <mbroz@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
77 lines
2.5 KiB
Plaintext
77 lines
2.5 KiB
Plaintext
dm-crypt
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=========
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Device-Mapper's "crypt" target provides transparent encryption of block devices
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using the kernel crypto API.
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Parameters: <cipher> <key> <iv_offset> <device path> \
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<offset> [<#opt_params> <opt_params>]
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<cipher>
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Encryption cipher and an optional IV generation mode.
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(In format cipher[:keycount]-chainmode-ivopts:ivmode).
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Examples:
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des
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aes-cbc-essiv:sha256
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twofish-ecb
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/proc/crypto contains supported crypto modes
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<key>
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Key used for encryption. It is encoded as a hexadecimal number.
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You can only use key sizes that are valid for the selected cipher.
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<keycount>
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Multi-key compatibility mode. You can define <keycount> keys and
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then sectors are encrypted according to their offsets (sector 0 uses key0;
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sector 1 uses key1 etc.). <keycount> must be a power of two.
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<iv_offset>
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The IV offset is a sector count that is added to the sector number
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before creating the IV.
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<device path>
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This is the device that is going to be used as backend and contains the
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encrypted data. You can specify it as a path like /dev/xxx or a device
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number <major>:<minor>.
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<offset>
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Starting sector within the device where the encrypted data begins.
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<#opt_params>
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Number of optional parameters. If there are no optional parameters,
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the optional paramaters section can be skipped or #opt_params can be zero.
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Otherwise #opt_params is the number of following arguments.
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Example of optional parameters section:
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1 allow_discards
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allow_discards
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Block discard requests (a.k.a. TRIM) are passed through the crypt device.
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The default is to ignore discard requests.
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WARNING: Assess the specific security risks carefully before enabling this
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option. For example, allowing discards on encrypted devices may lead to
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the leak of information about the ciphertext device (filesystem type,
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used space etc.) if the discarded blocks can be located easily on the
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device later.
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Example scripts
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===============
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LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
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encryption with dm-crypt using the 'cryptsetup' utility, see
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http://code.google.com/p/cryptsetup/
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[[
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#!/bin/sh
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# Create a crypt device using dmsetup
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dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
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]]
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[[
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#!/bin/sh
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# Create a crypt device using cryptsetup and LUKS header with default cipher
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cryptsetup luksFormat $1
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cryptsetup luksOpen $1 crypt1
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]]
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