18.17. Encrypting Swap Space

Written by Christian Brüffer.

Swap encryption in FreeBSD is easy to configure and has been available since FreeBSD 5.3-RELEASE. Depending on which version of FreeBSD is being used, different options are available and configuration can vary slightly. From FreeBSD 6.0-RELEASE onwards, the gbde(8) or geli(8) encryption systems can be used for swap encryption. With earlier versions, only gbde(8) is available. Both systems use the encswap rc.d script.

The previous section, Encrypting Disk Partitions, includes a short discussion on the different encryption systems.

18.17.1. Why should Swap be Encrypted?

Like the encryption of disk partitions, encryption of swap space is done to protect sensitive information. Imagine an application that e.g. deals with passwords. As long as these passwords stay in physical memory, all is well. However, if the operating system starts swapping out memory pages to free space for other applications, the passwords may be written to the disk platters unencrypted and easy to retrieve for an adversary. Encrypting swap space can be a solution for this scenario.

18.17.2. Preparation

注意:

For the remainder of this section, ad0s1b will be the swap partition.

Up to this point the swap has been unencrypted. It is possible that there are already passwords or other sensitive data on the disk platters in cleartext. To rectify this, the data on the swap partition should be overwritten with random garbage:

# dd if=/dev/random of=/dev/ad0s1b bs=1m

18.17.3. Swap Encryption with gbde(8)

If FreeBSD 6.0-RELEASE or newer is being used, the .bde suffix should be added to the device in the respective /etc/fstab swap line:

# Device                Mountpoint      FStype  Options         Dump    Pass#
/dev/ad0s1b.bde         none            swap    sw              0       0
      

For systems prior to FreeBSD 6.0-RELEASE, the following line in /etc/rc.conf is also needed:

gbde_swap_enable="YES"

18.17.4. Swap Encryption with geli(8)

Alternatively, the procedure for using geli(8) for swap encryption is similar to that of using gbde(8). The .eli suffix should be added to the device in the respective /etc/fstab swap line:

# Device                Mountpoint      FStype  Options         Dump    Pass#
/dev/ad0s1b.eli         none            swap    sw              0       0
      

geli(8) uses the AES algorithm with a key length of 256 bit by default.

Optionally, these defaults can be altered using the geli_swap_flags option in /etc/rc.conf. The following line tells the encswap rc.d script to create geli(8) swap partitions using the Blowfish algorithm with a key length of 128 bit, a sectorsize of 4 kilobytes and the detach on last close option set:

geli_swap_flags="-a blowfish -l 128 -s 4096 -d"

Please refer to the description of the onetime command in the geli(8) manual page for a list of possible options.

18.17.5. Verifying that it Works

Once the system has been rebooted, proper operation of the encrypted swap can be verified using the swapinfo command.

If gbde(8) is being used:

% swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/ad0s1b.bde    542720        0   542720     0%
      

If geli(8) is being used:

% swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/ad0s1b.eli    542720        0   542720     0%
      

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