dm-crypt/System configuration

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Revision as of 21:11, 9 February 2014 by Indigo (Talk | contribs) (Boot loader: expanding cryptkey options for its two forms)

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Reason: Aggregate here all the generic information on system configuration from the other sub-articles of Dm-crypt. (Discuss in Talk:Dm-crypt/System configuration#)

Back to Dm-crypt.

mkinitcpio

When encrypting a system it is necessary to regenerate the initial ramdisk after properly configuring mkinitcpio. Depending on the particular scenarios, a subset of the following hooks will have to be enabled:

  • encrypt: always needed when encrypting the root partition, or a partition that needs to be mounted before root; it must come before the filesystems hook; it is not needed in all the other cases, as system initialization scripts like /etc/crypttab take care of unencrypting any other partitions.
  • shutdown: recommended before mkinitcpio 0.16 to ensure controlled unmounting during system shutdown. It is still functional, but not deemed necessary anymore.
  • keymap: provides support for foreign keymaps for typing encryption passwords; it must come before the encrypt hook.
  • keyboard: needed to make USB keyboards work in early userspace.
    • usbinput: deprecated, but can be given a try in case keyboard does not work.

Other hooks needed should be clear from other manual steps followed during the installation of the system.

Boot loader

In order to enable booting an encrypted root partition, a subset of the following kernel parameters need to be set. See kernel parameters for instructions specific to your boot loader.

cryptdevice

This parameter will make the system prompt for the passphrase to unlock the root device on a cold boot. It is parsed by the encrypt hook to identify which device contains the encrypted system:

cryptdevice=device:dmname
  • device is the path to the raw encrypted device. Usage of Persistent block device naming is advisable.
  • dmname is the device-mapper name given to the device after decryption, which will be available as /dev/mapper/dmname.

root

This parameter is needed when not using GRUB. The reason GRUB does not require this is because the auto-generated grub.cfg is meant to handle specifying the root for you.

root=device
  • device is the device file of the actual (decrypted) root file system. If the file system is formatted directly on the decrypted device file this will be /dev/mapper/dmname.

resume

resume=device
  • device is the device file of the decrypted (swap) filesystem used for suspend2disk. If swap is on a separate partition, it will be in the form of /dev/mapper/swap. See also Dm-crypt/Swap Encryption.

cryptkey

This parameter is required by the encrypt hook for reading a keyfile to unlock the cryptdevice. It can have two parameter sets, depending on whether the keyfile exists as a file or a bitstream starting on a specific location.

For a file the format is:

cryptkey=device:fstype:path
  • device is the raw block device where the key exists.
  • fstype is the filesystem type of device (or auto).
  • path is the absolute path of the keyfile within the device.

Example: cryptkey=//dev/usbstick:vfat:/secretkey

For a bitstream on a device the key's location is specified with the following:

cryptkey=device:offset:size 

Example: cryptkey=/dev/sdZ:0:512 reads a 512 bit keyfile starting at the beginning of the device.

See also Dm-crypt/Device Encryption#Cryptsetup and keyfiles.

crypttab

The /etc/crypttab (or, encrypted device table) file contains a list of encrypted devices that are to be unlocked when the system boots, similar to fstab. This file can be used for automatically mounting encrypted swap devices or secondary filesystems. It is read before fstab, so that dm-crypt containers can be unlocked before the filesystem inside is mounted. Note that crypttab is read after the system has booted, so it is not a replacement for mkinitcpio hooks or boot loader options in the case of an encrypted root scenario. See the crypttab man page for details.

/etc/crypttab
 #Example crypttab file. Fields are: name, underlying device, passphrase, cryptsetup options.
 #Mount /dev/lvm/swap as /dev/mapper/swap using plain dm-crypt with passphrase "SWAP"
 swap	/dev/lvm/swap	SWAP		-c aes-xts-plain -h whirlpool -s 512
 #Mount /dev/lvm/tmp as /dev/mapper/tmp using plain dm-crypt with a random passphrase, making its contents unrecoverable after it is dismounted.
 tmp	/dev/lvm/tmp	/dev/urandom	-c aes-xts-plain -s 512
 #Mount /dev/lvm/home as /dev/mapper/home using LUKS, and prompt for the passphrase at boot time.
 home   /dev/lvm/home
 #Mount /dev/sdb1 as /dev/mapper/backup using LUKS, with a passphrase stored in a file.
 backup /dev/sdb1       /home/alice/backup.key