path: root/mount/losetup.8

.TH LOSETUP 8 "2003-07-01" "Linux" "MAINTENANCE COMMANDS"
.SH NAME
losetup \- set up and control loop devices
.SH SYNOPSIS
.ad l
Get info:
.sp
.in +5
.B losetup
.I loopdev
.sp
.B losetup -a
.sp
.B losetup -j
.I file
.RB [ \-o
.IR offset ]
.sp
.in -5
Delete loop:
.sp
.in +5
.B "losetup \-d"
.IR loopdev ...
.sp
.in -5
Print name of first unused loop device:
.sp
.in +5
.B "losetup \-f"
.sp
.in -5
Setup loop device:
.sp
.in +5
.B losetup
.RB [{ \-e | \-E }
.IR encryption ]
.RB [ \-o
.IR offset ]
.RB [ \-\-sizelimit
.IR size ]
.in +8
.RB [ \-p
.IR pfd ]
.RB [ \-r ]
.RB { \-f [ \-\-show ]| \fIloopdev\fP }
.I file
.sp
.in -13
Resize loop device:
.sp
.in +5
.B "losetup \-c"
.I loopdev
.in -5
.ad b
.SH DESCRIPTION
.B 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
\fIloopdev\fP argument is given, the status of the corresponding loop
device is shown.

.SS "Encryption"
It is possible to specify transfer functions (for encryption/decryption
or other purposes) using one of the
.B \-E
and
.B \-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.
.SH OPTIONS
The \fIsize\fR and \fIoffset\fR 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.

.IP "\fB\-a, \-\-all\fP"
show status of all loop devices
.IP "\fB\-c, \-\-set-capacity\fP \fIloopdev\fP
force loop driver to reread size of the file associated with the specified loop device
.IP "\fB\-d, \-\-detach\fP \fIloopdev\fP..."
detach the file or device associated with the specified loop device(s)
.IP "\fB\-e, \-E, \-\-encryption \fIencryption_type\fP"
enable data encryption with specified name or number
.IP "\fB\-f, \-\-find\fP"
find the first unused loop device. If a
.I file
argument is present, use this device. Otherwise, print its name
.IP "\fB\-h, \-\-help\fP"
print help
.IP "\fB\-j, \-\-associated \fIfile\fP"
show status of all loop devices associated with given
.I file
.IP "\fB\-k, \-\-keybits \fInum\fP"
set the number of bits to use in key to \fInum\fP.
.IP "\fB\-N, \-\-nohashpass\fP"
Do not hash the password.  By default, Debian systems run the password through a
hash function, non-Debian systems tend not to.
.IP "\fB\-o, \-\-offset \fIoffset\fP"
the data start is moved \fIoffset\fP bytes into the specified file or
device
.IP "\fB\-\-sizelimit \fIsize\fP"
the data end is set to no more than \fIsize\fP bytes after the data start
.IP "\fB\-p, \-\-pass-fd \fInum\fP"
read the passphrase from file descriptor with number
.I num
instead of from the terminal
.IP "\fB\-r, \-\-read-only\fP"
setup read-only loop device
.IP "\fB\-\-show\fP"
print device name if the
.I -f
option and a
.I file
argument are present.

The short form of this option (\fB\-s\fP) is deprecated.  This short form could
be in collision with Loop-AES implementation where the same option is used for
\fB\-\-sizelimit\fP.
.IP "\fB\-v, \-\-verbose\fP"
verbose mode

.SH RETURN VALUE
.B losetup
returns 0 on success, nonzero on failure. When
.B 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
.B losetup
from determining the status of the device.

.SH FILES
.nf
/dev/loop0, /dev/loop1, ...   loop devices (major=7)
.fi
.SH EXAMPLE
If you are using the loadable module you must have the module loaded
first with the command
.IP
# modprobe loop
.LP
Maybe also encryption modules are needed.
.IP
# modprobe des
# modprobe cryptoloop
.LP
The following commands can be used as an example of using the loop device.
.nf
.IP
# 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
.fi
.LP
If you are using the loadable module you may remove the module with
the command
.IP
# rmmod loop
.LP
.fi
.SH 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
.BR cryptsetup (8).
.SH AVAILABILITY
The losetup command is part of the util-linux package and is available from
ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
.\" .SH AUTHORS
.\" .nf
.\" Original version: Theodore Ts'o <tytso@athena.mit.edu>
.\" Original DES by: Eric Young <eay@psych.psy.uq.oz.au>
.\" .fi