Create an encrypted LUKS sparse file

For an updated version of this article, check this post!

I’m going to create a 2.7TB of file (sparse file) on my 3TB disk.
This file is a ‘thin provisionig‘ file, which means that it will expand only when/if used. FYI – it won’t shrink after usage. So a 2.7TB sparse file, once created, will be seen as 2.7T but it’s actually using a very little space (almost not noticeable). It will grow using it.
I will also encrypt it using cryptosetup LUKS format, to protect the content.

Why a sparse file?
Well, it’s practical, and it’s very handy if you want to move your files from a disk which is quite full already. In fact, I have my backups on this USB drive, EXT4 formatted. A 3TB drive half full.
A sparse file can be actually bigger than the available space on the disk. Of course, if you fill it up, it will error out. But while you want to move files, it’s… your saviour! ūüôā

Using my example, I have 1.2TB used on this 3TB disk. I want to move all this data in an encrypted container that can potentially use the whole disk. So? Sparse file is the solution!
Theoretically I could also have had like… 2.2/2.5TB of data (on a 2.7TB available disk). As long as you free up space¬†moving the files, things should work ūüôā

First of all… if you haven’t done it yet, just install the package:

Create the sparsefile

Let’s¬†create a 2.7TB sparse file in /3TB.
[/3TB is the mount point of my /dev/sdb1 USB device]

Create an encrypted LUKS container

We mount the sparse file just created and set the encrypted password. NOTE: you can change/add/remove this password (key) [later on explained how ūüėČ ]

Open the Vault

Now, it’s time to ‘turn on’ this encrypted volume device and check that all went well

And of course, we need to format the device. I’ll use EXT4.

At this point, you can mount this device /dev/mapper/myarchive as usual:

You are now able to use your encrypted sparse file! All files can be now copied/moved into/cryptoarchive

Close the Vault

To unmount the vault, you need to follow these steps. This is important when you turn off your server!

Add key to the container

LUKS allows for up to 8 passwords to each partition/vault.
You can add other 7 basically and use all of these 8 passwords to access your vault.

To add keys, vault needs to be ‘open’

Change the key of the container

To change the key, the vault/container needs to be ‘close’

Setup auto mount

This will generate a new file as a key, added to the vault and set to be used to auto mount at boot:

Than we need to setup /etc/crypttab and /etc/fstab

After that, I have personally created a specific init script. I couldn’t find the right way to run losetup /dev/loop0 /3TB/file_container.img before activating the archive, so…
First of all, I’ve disables cryptdisks via /etc/default/cryptdisks

Than, I’ve created this init script, saved as¬†/etc/init.d/crypt-myvault

This script requires also a /etc/defaults/crypt-myvault configuration file:

To finish, we need to enable the script:

We need to be sure that this script runs AFTER the USB drive is mounted. So… I’ve added ‘netatalk’ as required-start in the header of the init script, as I’ve noticed that once netatalk starts, the USB disk is already mounted.
I’m sure there is a better/nicer way, but this seems to work well for me ūüôā


LUKS passphrases: Changing, adding, removing