25.5 Encrypted Swap Partitions

Similar to encrypting disk partitions, encrypted swap space is used to protect sensitive information. For example, applications that handle passwords will not write them to disk as long as the passwords remain in physical memory, and they will be cleared after a reboot. However, if FreeBSD starts swapping out memory pages to free up space, passwords may be written to disk in unencrypted form. Encrypted swap space addresses this issue.

This section demonstrates how to configure an encrypted swap partition using geli. Use the swapinfo command to view the current swap partition:

$ swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/nda0p2       8388608        0  8388608     0%

The output indicates that the current swap partition is /dev/nda0p2.

Configuring Swap Partition Encryption

By default, the swap partition is not encrypted. Before enabling encryption, any sensitive data that may exist in the swap partition should be cleared first. Execute the following command to overwrite the current swap partition with random garbage:

# dd if=/dev/urandom of=/dev/nda0p2 bs=1m

Warning

This command carries risks; please carefully confirm the disk partition before proceeding!

To use geli(8) to encrypt the swap partition, edit the /etc/fstab file and modify the original swap partition entry by adding the .eli suffix. The following example configures an encrypted swap partition using the AES-XTS algorithm with a 256-bit key length and a 4 kilobyte sector size:

/dev/nda0p2.eli   none        swap    sw,ealgo=AES-XTS,keylen=256,sectorsize=4096    0       0

By default, geli(8) uses the AES-XTS encryption algorithm with a key length of 128 bits. For higher security, keylen=256 can be manually specified. The default settings are sufficient for most needs. If desired, these defaults can be changed in the options field in /etc/fstab. Available flags are as follows:

Flag	Description
aalgo	Data integrity verification algorithm used to ensure that encrypted data has not been tampered with. Supports HMAC/SHA1, HMAC/RIPEMD160, HMAC/SHA256, HMAC/SHA384, HMAC/SHA512; HMAC/SHA256 is recommended
ealgo	Encryption algorithm used to protect data. Supports AES-XTS, AES-CBC, Camellia-CBC (CBC mode is not recommended for new deployments); the default and recommended algorithm is AES-XTS
keylen	Key length for the encryption algorithm (in bits)
sectorsize	Block size into which data is divided before encryption. A larger sector size improves performance but increases storage overhead. A recommended size is 4096 bytes

Verifying the Encrypted Swap Partition

After rebooting the system, you can use swapinfo to verify that the encrypted swap partition is functioning correctly.

$ swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/nda0p2.eli   8388608        0  8388608     0%