LOSETUP(8) MAINTENANCE COMMANDS LOSETUP(8)NAME
losetup - set up and control loop devices
SYNOPSIS
Get info:
losetup loop_device
losetup -a
Delete loop:
losetup -d loop_device
Print name of first unused loop device:
losetup -f
Setup loop device:
losetup [{-e|-E} encryption] [-o offset] [-p pfd] [-r]
{-f|loop_device} file
DESCRIPTION
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. If only the loop_device argument is given, the status of the
corresponding loop device is shown.
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 that 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 wil
take the name of an arbitrary encryption type and finds the module that
knows how to perform that encryption. (Thus, either one uses a number
different from 18 with the -E option, or one uses a name with the -e
option.)
OPTIONS-a Show status of all loop devices.
-d Detach the file or device associated with the specified loop
device.
-E encryption_type
Enable data encryption with specified number.
-e encryption_name
Enable data encryption with specified name.
-f Find the first unused loop device. If a file argument is
present, use this device. Otherwise, print its name.
-o offset
The data start is moved offset bytes into the specified file or
device.
-p num Read the passphrase from file descriptor with number num instead
of from the terminal.
-r Setup read-only loop device.
RETURN VALUE
losetup returns 0 on success, nonzero on failure. When losetup displays
the status of a loop device, it returns 1 if the device is not config‐
ured and 2 if an error occurred which prevented losetup from determin‐
ing the status of the device.
FILES
/dev/loop0, /dev/loop1, ... loop devices (major=7)
EXAMPLE
If you are using the loadable module you must have the module loaded
first with the command
# insmod loop.o
Maybe also encryption modules are needed.
# insmod des.o # insmod cryptoloop.o
The following commands can be used as 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
RESTRICTION
DES encryption is painfully slow. On the other hand, XOR is terribly
weak.
Cryptoloop is deprecated and unmaintained in 2.6 kernels. Use dm-crypt.
For more details see cryptsetup(8).
Linux 2003-07-01 LOSETUP(8)
