trusty (8) losetup.8.gz

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NAME

       losetup - set up and control loop devices

SYNOPSIS

       Get info:

            losetup loopdev

            losetup -a

            losetup -j file [-o offset]

       Delete loop:

            losetup -d loopdev...

       Print name of first unused loop device:

            losetup -f

       Setup loop device:

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

       Resize loop device:

            losetup -c loopdev

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  loopdev  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
       find the module that knows how to perform that encryption.

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.

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

       -c, --set-capacity loopdev
              force loop driver to reread size of the file associated with the specified loop device

       -d, --detach loopdev...
              detach the file or device associated with the specified loop device(s)

       -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

       -j, --associated file
              show status of all loop devices associated with given file

       -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

       --sizelimit size
              the data end is set to no more than size bytes after the data start

       -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

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

              The  short  form  of  this  option (-s) is deprecated.  This short form could be in collision with
              Loop-AES implementation where the same option is used for --sizelimit.

       -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

              # modprobe loop

       Maybe also encryption modules are needed.

              # modprobe des # modprobe cryptoloop

       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   package   and   is   available   from
       ftp://ftp.kernel.org/pub/linux/utils/util-linux/.