I’ve been building a simple NAS for my home, and I wanted to store the data on a secondary disk, encrypted with dm-crypt/LUKS. There are plenty of articles on how to do that, but when it comes to automatically mounting the disk at boot, all of them recommend writing the encryption key in a keyfile and store it on the local filesystem.
This approach wasn’t acceptable to me: while the data would be encrypted at rest, the key to open the encrypted partition would also be sitting in the same place. If someone were to steal the physical server (imagine this were a small Raspberry Pi!), they would have access to the data without any issue.
How could I have the LUKS encryption key stored in a secure, remote place, while at the same time being able to have the encrypted disk automatically mounted without manual intervention (e.g. in case of a reboot after a power outage)? In other words, how to have your cake and eat it too.
Turns out, there’s a relatively simple solution, which requires just two systemd units.
Note: this approach can not be used with encrypted root volumes, but only with secondary disks.
Step 1: Generate and store the keyfile
The first thing we need to do is to generate a keyfile. This should be 256-bytes long, and can be generated with:
dd bs=256 count=1 if=/dev/random | base64 > keyfile
I’m piping the encryption key through base64 so we don’t have to deal with binary files, making things more manageable.
You will then need to store the keyfile somewhere safe. You can pick and choose any place you’d like; some ideas include:
- A key vault like Azure Key Vault
- HTTPS servers, including object storage services such as AWS S3 or Azure Blob Storage; make sure you’re using TLS to protect the data while in transit, rather than basic HTTP
For a simple (but effective enough) solution, you can store the keyfile in Azure Blob Storage. You can see an example of doing this in the Appendix below.
Step 2: Create a script returning the keyfile
You will need to create a script that can return the keyfile when invoked, stored as
The content of the script completely depends on how and where you stored your keyfile. Following up on the example in the Appendix, with a keyfile stored on Azure Blob Storage, the script would look like this:
#!/bin/sh # Request the file from Azure Blob Storage using the URL with the SAS token, then pipe it through `base64 -d` to decode it from base64 curl -s "https://ln5bxfzbl0tlf5z.blob.core.windows.net/keyfiles/keyfile?se=2022-01-19T23%3A02Z&sp=r&spr=https&sv=2018-11-09&sr=b&sig=gkaN2OSzN2zj1WSAPiLJMgtkcXLi2Y8EOVdBUmZQh88%3D" | base64 -d
Whatever the content of your script (which could be a shell script or written in any other language), it’s important then to make it executable and readable by the
root user only:
# Ensure the owner of this file is "root" chown root:root /etc/luks/key.sh # Allow only the owner (root) to read and execute the script chmod 0500 /etc/luks/key.sh
Step 3: Encrypt the disk using LUKS
We’re now ready to get to the fun part, and encrypt the disk or partition.
To start, check the name of the disk you want to use, using
$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 30G 0 disk ├─sda1 8:1 0 29.9G 0 part / ├─sda14 8:14 0 4M 0 part └─sda15 8:15 0 106M 0 part /boot/efi sdb 8:16 0 4G 0 disk └─sdb1 8:17 0 4G 0 part /mnt sdc 8:32 0 32G 0 disk
In this example, I’m going to use the
sdc disk. This is likely going to be different for you, so make sure you replace the disk name in all the commands below.
Watch out! The commands below will delete all files on the drive you select.
Before we start, install the
# Debian, Ubuntu, Raspbian… apt install -y cryptsetup # CentOS, Fedora, RedHat yum install -y cryptsetup-luks
First, if your disk doesn’t have a partition yet (like mine), create a GPT partition table and a partition (without formatting it):
# Replace sdc with the drive you want to use parted /dev/sdc mklabel gpt parted -a opt /dev/sdc mkpart datadisk ext4 0% 100%
sdc1 partition using LUKS, create an ext4 volume in that partition, and then close the encrypted volume. In all commands that require a keyfile, we’re invoking the
/etc/luks/key.sh script that we created before, and telling
cryptsetup to read the keyfile from stdin.
# Encrypt the disk # Replace sdc1 with the correct partition! /etc/luks/key.sh | cryptsetup -d - -v luksFormat /dev/sdc1 # Open the encrypted volume, with the name "data" # Replace sdc1 with the correct partition! /etc/luks/key.sh | cryptsetup -d - -v luksOpen /dev/sdc1 data # Create a filesystem on the encrypted volume mkfs.ext4 -F /dev/mapper/data # Close the encrypted volume cryptsetup -v luksClose data
Step 4: Enable auto-mounting the encrypted disk
We’re almost done: ready to enable auto-mounting of the encrypted disk.
We’ll do that with two systemd units: one unlocking the encrypted device, and the other one actually mounting the disk.
To start, get the UUID of the
/dev/sdc1 partition, using
lsblk --fs. For example:
$ lsblk --fs NAME FSTYPE LABEL UUID MOUNTPOINT [...] sdc └─sdc1 crypto_LUKS a17db19d-5037-4cbb-b50b-c85e3e074864
In my example, that is
a17db19d-5037-4cbb-b50b-c85e3e074864; it will be different for you.
Create a systemd unit for unlocking the encrypted device and save it as
/etc/systemd/system/unlock-data.service. Make sure you replace the UUID in the command below!
[Unit] Description=Open encrypted data volume After=network-online.target local-fs.target StopWhenUnneeded=true [Service] Type=oneshot ExecStart=/bin/sh -c '/etc/luks/key.sh | /sbin/cryptsetup -d - -v luksOpen /dev/disk/by-uuid/a17db19d-5037-4cbb-b50b-c85e3e074864 data' RemainAfterExit=true ExecStop=/sbin/cryptsetup -d - -v luksClose data
StopWhenUnneeded=trueline: this will make systemd stop the unit (including running the
luksCloseoperation) automatically when the disk is unmounted.
Create another systemd unit with the mountpoint for
/mnt/data, and save it as
/etc/systemd/system/mnt-data.mount. Note that the unit’s name must match the path of the mountpoint!
[Unit] Requires=unlock-data.service After=unlock-data.service [Mount] What=/dev/mapper/data Where=/mnt/data Type=ext4 Options=defaults,noatime [Install] WantedBy=multi-user.target
We’re now ready, let’s enable the
mnt-data.mount unit so it’s activated at boot, and then mount it right away:
systemctl enable mnt-data.mount systemctl start mnt-data.mount
You can now check with
lsblk to see the encrypted disk mounted:
$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT [...] sdc 8:32 0 32G 0 disk └─sdc1 8:33 0 32G 0 part └─data 253:0 0 32G 0 crypt /mnt/data
Try rebooting the system, and you’ll see the partition being mounted automatically.
We’re done! However, keep in mind a few things:
- To mount and un-mount the encrypted disk you must use
systemctlrather than the usual
Mount the disk with
systemctl start mnt-data.mount, and un-mount with
systemctl stop mnt-data.mount
- The systemd units are executed only after the network and the other “normal” filesystems are mounted. If you have another service depending on the data disk’s availability, you need to explicitly make its systemd unit depending on the
- As mentioned at the beginning, this solution can’t be used with the root filesystem, but only with secondary data disks.
Appendix: Keyfiles on Azure Blob Storage
In this example, I’m storing the keyfile in Azure Blob Storage. While this doesn’t offer the same protection as a key vault, it can be enough for most people, depending on your threat model. I’m guaranteeing some level of security by:
- Configuring the Storage Account to accept only secure connections that use HTTPS
- Allowing connections only from the IP of my home (I have a “quasi-static” IP, that changes less than once per year)
- Requiring clients to use a SAS token to download the file
My threat model involves people stealing the disk and/or the server. The attacker wouldn’t be able to download the keyfile if they’re not in my network. By having the keyfile on a remote server, I can delete it right away if I need to, e.g. if the physical disk is stolen. This protection is enough for me, but depending on your threat model, you might want to look into more complex solutions, for example involving key vaults.
If you’re starting from scratch, you can create an Azure Storage Account, configure it per the requirements above, and upload your keyfile with the Azure CLI. You will need an Azure subscription, which is free and with this kind of usage you’ll be comfortably within their free tier.
This example assumes that you’ve generated a base64-encoded keyfile available in the
keyfilefile, in the current working directory, just as per Step 1 above.
# Login to Azure if you need to az login # Location where to store the keyfile; choose an Azure region close to you export LOCATION="eastus2" # If you have a fixed (or almost) IP address, you can restrict access to the storage account from that IP. # You can also use an IP range in the CIDR format. # Otherwise, leave this variable as an empty string export ALLOW_IP="184.108.40.206" # Generate a random name for the Storage Account export STORAGE_ACCOUNT_NAME=$(cat /dev/random | base64 | tr -dc 'a-zA-Z0-9' | tr '[:upper:]' '[:lower:]' | fold -w 15 | head -n 1) # Create a Resource Group and a Storage Account export RG_NAME="Keyfiles" az group create --name $RG_NAME --location $LOCATION az storage account create \ --name $STORAGE_ACCOUNT_NAME \ --resource-group $RG_NAME \ --location $LOCATION \ --sku Standard_LRS \ --kind StorageV2 # Require using TLS/HTTPS only az storage account update \ --name $STORAGE_ACCOUNT_NAME \ --https-only true # Allow from a specific IP only, if the variable ALLOW_IP isn't empty if [ ! -z "$ALLOW_IP" ]; then # Disallow access from anywhere by default az storage account update \ --resource-group $RG_NAME \ --name $STORAGE_ACCOUNT_NAME \ --default-action Deny # Allow the IP or IP range az storage account network-rule add \ --resource-group $RG_NAME \ --account-name $STORAGE_ACCOUNT_NAME \ --ip-address "$ALLOW_IP" # Disallow "Trusted Microsoft Services" az storage account update \ --resource-group $RG_NAME \ --name $STORAGE_ACCOUNT_NAME \ --bypass None fi # Create a blob container az storage container create \ --name "keyfiles" \ --public-access off \ --account-name $STORAGE_ACCOUNT_NAME # Upload the key az storage blob upload \ --account-name $STORAGE_ACCOUNT_NAME \ --container-name "keyfiles" \ --file "./keyfile" \ --name "keyfile"
Now that the file has been uploaded, we can get a link to it. Since the file is in a “private” container, it requires a special authentication token (SAS token) to be retrieved. SAS tokens add some extra protection thanks to having an expiration date and being tied to an account key, which can be revoked at any time. You can also add additional requirements on the SAS tokens, such as restrictions on IPs; this is a (less ideal) alternative to setting IP restrictions on the Storage Account as shown above.
You can generate a URL with a SAS token for the blob with:
# Create an expiration date 2 years in the future # On Linux: SAS_EXPIRY=$(date -u -d "2 years" '+%Y-%m-%dT%H:%MZ') # On macOS: SAS_EXPIRY=$(date -v+2y -u '+%Y-%m-%dT%H:%MZ') # Generate the URL with the SAS token az storage blob generate-sas \ --account-name $STORAGE_ACCOUNT_NAME \ --container-name "keyfiles" \ --name "keyfile" \ --https-only \ --permissions r \ --expiry "$SAS_EXPIRY" \ --full-uri
Result will be similar to:
You can use the URL above in the
/etc/luks/key.sh script, as per Step 2.
Cover photo by adrian (Unsplash)