This guide explains how to set up an encrypted swap backed by a swap-file. The steps are based on the Arch-wiki: dm-crypt/Swap encryption guide. I verified the method on Debian 11, 12, and Proxmox VE 8, but it should work on other Linux systems.
Sometimes virtualization host systems (currently my host environments include Proxmox VE and vanilla Debian), run nested containerization and virtualization environments (e.g., VMs that host Kubernetes and similar clusters in dev environments). Some of these envionments may require the host OS to run without swap enabled due to security considerations.
However, when such "host OS" itself is nested and if the parent host system uses a swap partition (without the knowledge of the nested OS), this could lead to potential security vunerabilities. Therefore, configuring the virtualization host system with encrypted swap is more secure than using an unencrypted swap partition or file.
In addition to the CPU overhead needed for encryption, there are additional caveats of using encrypted swap partitions on machines with the suspend-to-disk mode (a.k.a. "hybernation") support such as laptops and desktop PCs. In short, to ensure the desired security benefits of encrypted swap, the suspend-to-disk mode must be diabled. For more details please see the Arch Wiki page.
If other persistent-storage-backed swap is enabled on the system, make sure to disable those and remove them from the system configuration.
Create a backing swap file of the desired size with dd or truncate (truncate is quicker because it does not write the bytes to the disk).
File size must be divisible by the encryption block size in the following step. In this guide we use 4096-byte sector size. For example, if we want 2G swap, instead of 2'000'000'000 bytes, we should create a file of size 2 GiB or 2'147'483'648 (=2*1024^3) bytes.
With dd:
dd bs=1M count=2048 if=/dev/zero of=/swap0.encryptedWith truncate:
truncate -s 2147483648 /swap0.encryptedExample sizes:
| In GiB | In 1M blocks | In bytes |
|---|---|---|
| 2 | 2048 | 2147483648 |
| 4 | 4096 | 4294967296 |
| 8 | 8192 | 8589934592 |
| 16 | 16384 | 17179869184 |
| 24 | 24576 | 25769803776 |
| 32 | 32768 | 34359738368 |
| 48 | 49152 | 51539607552 |
| 64 | 65536 | 68719476736 |
| 72 | 73728 | 77309411328 |
| 96 | 98304 | 103079215104 |
| 128 | 131072 | 137438953472 |
Similar to how /etc/fstab is used to define filesystems, /etc/crypttab is used to define encrypted partitions to be opened at system boot.
Put the following configuration in /etc/crypttab (create a new file if it does not already exist)
# <name> <device> <password> <options>
swap0 /swap0.encrypted /dev/urandom swap,cipher=aes-xts-plain64,size=256,sector-size=4096
It instructs dm-crypt to create a mapping /dev/mapper/swap0 for /swap0.encrypted encrypted with a random password without storing the encryption headers in the file itself.
Warning: When using a partition instead of a file, DO NOT use the partition path in the form of /dev/sdb2! Device numbering can change between boots (and when additional devices, e.g. USB are added or removed) and dm-crypt could overwrite a different disk partition. Instead, we can use the partition identifier, which is (most likely) unique and it does not change when the partition is reformatted (i.e., when it is re-encrypted with a new password after every reboot):
# <name> <device> <password> <options>
swap0 PARTUUID=6889ce20-0eea-41a9-afed-65e25277cc3a /dev/urandom swap,cipher=aes-xts-plain64,size=256,sector-size=4096
Finally, we can configure the swap space in /etc/fstab by appending:
/dev/mapper/swap0 none swap defaults 0 0
Works on Debian 12 and Proxmox 8, may work on other systemd-managed systems, but I have not tested it.
/usr/lib/systemd/system-generators/systemd-cryptsetup-generator /run/systemd/generator
systemctl daemon-reload
systemctl restart cryptsetup.targetThis tells systemd to generate unit cryptsetup unit files based on the current /etc/crypttab, reload the configuration, and execute them.
The configuration changes will be applied after reboot.
The changes can be verified with lsblk and swapon.
The following example shows a Proxmox VE 8 node with 16 GiB encrypted swap configured:
# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
loop0 7:0 0 16G 0 loop
└─swap0 252:6 0 16G 0 crypt [SWAP]
...
# swapon
NAME TYPE SIZE USED PRIO
/dev/dm-0 partition 16G 0B -2
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