Loopback Device

A Loopback Device is a mechanism used to interpret files as real devices. The main advantage of this method is that all tools used on real disks can be used with a loopback device.

Linux Loopback Device

The linux loopback device can be used by root only, and needs to be enabled in the kernel before use.

Floppy Disk Images

First, lets create an empty image.

dd if=/dev/zero of=floppy.img bs=512 count=2880

Now, lets set it up for mounting.

losetup /dev/loop0 floppy.img

Now lets make it MSDOS formatted.

mkdosfs /dev/loop0

Mount!

mount -t msdos /dev/loop0 /mnt/myfloppy

Hard Disk Images

A hard disk image contains an MBR, then a number of partitions, but the 'mount' instruction in Linux works with disk partitions, not full disks. To mount a partition contained in our disk image, we need to make sure the 'mount' command only sees our partition, not the whole disk.

Creating an image

First create the empty file that we will use for our disk image. We will assume a disk geometry of #cylinders, 16 heads, 63 sectors/track, 512 bytes/sector, which means that each cylinder contains 516096 bytes (16*63*512). Decide how large you want your disk image to be, and choose an appropriate number of cylinders (I’ll be using #cylinders throughout).

Example: If I want a 500Mb disk, I would choose 1000 cylinders (approximation of (500*1000*1024)/516096).

Write the disk image (I'll assume the filename c.img throughout):

dd if=/dev/zero of=/path/to/c.img bs=516096c count=#cylinders

Explanation:

dd	Linux command for copy and convert a file
if=/dev/zero	Source file is /dev/zero which is...*drumroll*...an infinite source of zeros
of=/path/to/c.img	Destination file is our disk image (dd will create the file if it doesn't exist)
bs=516096c	Means read and write 516096 bytes at a time (This is just here to keep things simple)
count=#cylinders	Copy this number of blocks. Since we have set bs to 516096 bytes each block is one cylinder long

That leaves us with a nice sized file full of zeros that we'll use for our disk image.

Mounting

Now we attach the file to a loopback device. This lets us treat the file as though it were a physical disk.

Note: Under a normal Linux setup you will need to be root to use the losetup command (The same applies to most commands we'll be using).

losetup /dev/loop0 /path/to/c.img

Explanation:

losetup	Linux command to setup and control loop devices.
/dev/loop0	Loopback device 0. This will become our raw 'disk' device.
/path/to/c.img	File to attach to the the loopback device is our disk image.

If you run ps ax, you should now see a [loop0] process.

Partitioning

Now to create the MBR and partition table on the disk image (Usually you need to be root).

fdisk -u -C#cylinders -S63 -H16 /dev/loop0

Explanation:

fdisk	Linux DOS partition maintenance program.
-u	Display units in sectors not cylinders (We will need this).
-C#cylinders	Set the cylinders of disk to our value.
-S63	Set the sectors/track to 63.
-H16	Set the heads/track to 16.
/dev/loop0	Thanks to losetup this device represents our raw 'disk'.

Within fdisk use the following commands:

o - Create a new empty DOS partition table.
n - Create a new partition (For simplicity just make 1 primary partition covering the whole disk).
a - Toggle the bootable flag (Optional).
p - Print the partition table.

You should end up with a screen that looks something like this:

Disk /dev/loop0: 516 MB, 516096000 bytes
16 heads, 63 sectors/track, 1000 cylinders, total 1008000 sectors
Units = sectors of 1 * 512 = 512 bytes

      Device Boot      Start         End      Blocks   Id  System
/dev/loop0p1   *          63     1007999      503968+  83  Linux

Obviously the cylinder count, partition end and blocks will be different depending on the size of your image.

Make a note of the start sector (63 here) and the block count (503968 here).

Note: If you are intending to format the partition to something other than ext2fs then change the partition id here using the t command. I should also point out that disk manufacturers and programmers don't agree on how many bytes are in a megabyte.

w - Write partition table to our 'disk' and exit.

Ignore any errors about rereading the partition table. Since it's not a physical device we really don't care.

We now have a partition table on our disk image.

Unfortunately this also means that from here on out we have to account for the fact that our partition does not start at byte 0 of the image.

Detach our disk image from the loopback device.

Command:

losetup -d /dev/loop0

Explanation:

-d	Detach whatever is on the loopback device
/dev/loop0	The virtual drive

Ok, now we attach the file to the loopback device again, but in such a way that we skip everything before the start of our partition.

losetup -o32256 /dev/loop0 /path/to/c.img

Explanation

-o32256

Move the start of data 32256 bytes into the file

The reason we move 32256 bytes into the file is this is where the partition starts. Remember I said to note the start sector of the partition (63 is usual)? Well, since each sector is 512 bytes long we therefore know the starting byte of the partition is 32256 (63*512) bytes into the file. The reason behind this gap is that most (There is no real standard) fdisk programs don't use the first track for anything but the MBR. That space isn't always wasted though, some bootloaders (Eg GRUB) use it to store parts of their program.

Note: If you aren't using the suggested geometry then you'll have to calculate this for yourself.

We now have a device (/dev/loop0) which we can use in a similar fashion to a normal one for a partition (eg /dev/hda1).

Formatting the partition

For ext2fs, use:

mke2fs -b1024 /dev/loop0 #blocks

Explanation:

mke2fs	Create an ext2 filesytem
-b1024	Use block size of 1024
/dev/loop0	Device to make the filesystem on (Here /dev/loop0 is our 'partition')
#blocks	Remember I said to note the number of blocks from the fdisk section? This is why.

This gives us a clean ext2 formatted partition.

Note: mke2fs is smart enough to figure out block size and #blocks for itself, but if you ever want to use multiple partitions you'll need to know how to use those values.

For FAT32, use:

mkdosfs -F32 /dev/loop0 #blocks

Explanation:

mkdosfs	Create a DOS filesystem (This may be absent on some Linux systems, search for the dosfstools package if it is)
-F32	FAT 32 allocation tables (It should be obvious how to use FAT12/FAT16)
/dev/loop0	Same as for the ext2fs version
#blocks	Same as for the ext2fs version

This gives us a clean FAT32 formatted partition (Ignore the floppy warning).

Note: The reason for #blocks is the same as for ext2fs, ie possible multiple partitions.

Mount Partition

You should now be able to mount the partition (Because it is still setup on the loopback device).

Command:

mount -text2 /dev/loop0 /mnt/wherever

or

mount -tvfat /dev/loop0 /mnt/wherever

Explanation: mount Linux command to mount a filesystem -text2/-tvfat Filesystem being used, Linux can usually figure this out on its own. /dev/loop0 The device representing our PARTITION /mnt/wherever A directory to mount the partition on.

This should leave you with a nicely mounted partition. If you run df -Th you should end up with a line similar to:

Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/loop0    vfat    492M  4.0K  492M   1% /mnt/wherever

...or for ext2fs...

Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/loop0    ext2    477M   13K  452M   1% /mnt/wherever

(Yup, these are for the same disk image. By default ext2fs reserves/uses quite a bit of space even empty.)

Unmount, Detach

Ok, unmount the partition and detach the loopback device.

Command:

umount /dev/loop0
losetup -d /dev/loop0

Explanation: umount Linux command to unmount a filesystem. /dev/loop0 The device that was mounted

Making it Easier

One final thing to do, which is to simplify mounting and unmounting that partition.

Command: Mounting:

mount -text2 -oloop=/dev/loop0,offset=32256 /path/to/c.img /mnt/wherever

Unmounting:

umount /path/to/c.img

Explanation:

This is essentially a combination of the losetup and mount commands we used previously when formatting the partition. If used it also means we lose access to the raw 'disk' or 'partition' through /dev/loop0.

See also http://www.pixelbeat.org/scripts/lomount.sh

The End

That's it, you now know how to handle hard disk images under Linux. Whilst mounted you can use it in exactly the same way you use a normal disk partition. Multiple partitions are an extension of this, just change the offset of the losetup command according to the partition you want to use (And format using the correct number of blocks).

Things to remember: losetup type command will give you the equivalent of a raw disk device (Eg /dev/hda) losetup -o type command will give you the equivalent of a raw partition device (Eg /dev/hda1)

Loopback Device under FreeBSD

FreeBSD 4.x uses vnconfig FreeBSD 5.x uses mdconfig

First, use DD to create an empty floppy image (1.44mb in size)

FreeBSD 4.x

dd if=/dev/zero of=floppy.img bs=512 count=2880
vnconfig vn0 floppy.img
newfs_msdos -f 1440 /dev/vn0
mount -t msdosfs /dev/vn0 /mnt/myfloppy

To shut and image down, unmount and unconfigure it.

umount /tmp/myfloppy
vnconfig -c /dev/vn0

FreeBSD 5.x

Memdisks are allocated dynamically, and the name is displayed after the mdconfig command. This assumes that "md0" is printed.)

To mount:

dd if=/dev/zero of=floppy.img bs=512 count=2880
mdconfig -a -t vnode -f floppy.img
newfs_msdos -f 1440 /dev/md0
mount -t msdosfs /dev/md0 /mnt/myfloppy

To unmount:

umount /tmp/myfloppy
mdconfig -d -u md0