losetup man page on Ubuntu

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LOSETUP(8)		     MAINTENANCE COMMANDS		    LOSETUP(8)

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 limit]
		    [-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 corre‐
       sponding 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
       -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 limit
	      the  data	 end  is set to no more than sizelimit 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 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

	      # 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-ng package and is avail‐
       able from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.

Linux				  2003-07-01			    LOSETUP(8)
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