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[-s]|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 will take the name of an arbitrary encryption type and finds the module that knows how to perform that encryption.

OPTIONS

-a, --all
    show status of all loop devices

-d, --detach
    detach the file or device associated with the specified loop device

-e, -E, --encryption encryption_type
    enable data encryption with specified name or number

-f, --find
    find the first unused loop device. If a file argument is present, use this device. Otherwise, print its name.

-h, --help
    print help

-k, --keybits num
    set the number of bits to use in key to num.

-N, --nohashpass
    Do not hash the password. By default, Debian systems run the password through a hash function, non-Debian systems tend not to.

-o, --offset offset
    The data start is moved offset bytes into the specified file or device.

-p, --pass-fd num
    Read the passphrase from file descriptor with number num instead of from the terminal

-r, --read-only
    setup read-only loop device

-s, --show
    print device name if the -f option and a file argument are present

-v, --verbose
    verbose mode

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)

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. Both are insecure nowadays. Some ciphers may require a licence for you to be allowed to use them. Cryptoloop is deprecated in favor of dm-crypt. For more details see cryptsetup(8).

AVAILABILITY
The losetup command is part of the util-linux-ng package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.