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losetup - Unix, Linux Command
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
Tag | Description |
-a
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Show status of all loop devices.
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-d
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Detach the file or device associated with the specified loop device.
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-E encryption_type
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Enable data encryption with specified number.
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-e encryption_name
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Enable data encryption with specified name.
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-f
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Find the first unused loop device. If a
file argument is present, use this device. Otherwise, print its name.
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-o offset
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The data start is moved offset bytes into the specified file or
device.
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-p num
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Read the passphrase from file descriptor with number
num instead of from the terminal.
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-r
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Setup read-only loop device.
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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 configured and 2 if an error occurred which prevented
losetup from determining the status of the device.
FILES
/dev/loop0, /dev/loop1, ... loop devices (major=7)
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EXAMPLE
If you are using the loadable module you must have the module loaded
first with the command
Tag | Description |
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# insmod loop.o
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Maybe also encryption modules are needed.
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# insmod des.o
# insmod cryptoloop.o
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The following commands can be used as an example of using the loop device.
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# 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
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If you are using the loadable module you may remove the module with
the command
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# rmmod loop
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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).
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