On Linux operating systems, the losetup command sets up and controls loop devices.

Syntax

losetup loopdev

losetup -a

losetup -j file [-o offset]

losetup -d loopdev…

losetup -f

losetup [{-e|-E} encryption] [-o offset] [–sizelimit size] [-p pfd] [-r] {-f[–show]|loopdev} file

losetup -c loopdev

losetup is used to associate loop devices with regular files or block devices, to detach loop devices and to query the status of a loop device.

  • Syntax
  • Options
  • Encryption
  • Exit status
  • Limitations
  • Examples
  • Related commands
  • Linux commands help

A loop device, also known as vnd (vnode disk) or lofi (loopback file interface), is a pseudo-device that makes a file accessible as a block device.

If only the loopdev argument is given, the status of the corresponding loop device is shown.

Options

The size and offset arguments may be followed by binary (2^N) suffixes KiB, MiB, GiB, TiB, PiB, and EiB (the “iB” is optional, e.g., “K” has the same meaning as “KiB”), or decimal (10^N) suffixes KB, MB, GB, PB, and EB.

Encryption

It is possible to specify transfer functions (for encryption/decryption or other purposes) using one of the -E and -e options. There are two mechanisms to specify the desired encryption: by number and by name. If an encryption is specified by number, then one has to make sure the Linux kernel knows about the encryption with that number, probably by patching the kernel. Standard numbers that are always present are 0 (no encryption) and 1 (XOR encryption). When the cryptoloop module is loaded (or compiled in), it uses number 18. This cryptoloop module takes the name of an arbitrary encryption type and finds the module that knows how to perform that encryption.

Exit status

losetup returns an exit status of 0 on success, and nonzero on failure. When losetup displays the status of a loop device, it returns 1 if the device is not configured, and 2 if an error occurred that prevented losetup from determining the status of the device.

Limitations

DES encryption is painfully slow. On the other hand, the simple XOR method is terribly weak. Both are considered insecure compared to newer algorithms. Additionally, some ciphers may require a license for you to be allowed to use them.

Cryptoloop is deprecated in favor of dm-crypt. For more details see cryptsetup.

Examples

If you are using the loadable loop device module, you must have the module loaded first with the command:

modprobe loop

Encryption modules may also be needed.

modprobe des

modprobe cryptoloop

The following commands are an example of using the loop device:

dd if=/dev/zero of=/file bs=1k count=100

losetup -e des /dev/loop0 /file

Password: Init (up to 16 hex digits):

mkfs -t ext2 /dev/loop0 100

mount -t ext2 /dev/loop0 /mnt

…

umount /dev/loop0

losetup -d /dev/loop0

If you are using the loadable module, you may remove the module with the command:

rmmod loop

dd — Copy and convert the encoding of files.mkfs — Build a Linux file system, usually a hard disk partition.mount — Mount a file system so that its data may be accessed.