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.
Loopback Device under Linux
The linux loopback device can be used by root only, and needs to be enabled in the kernel before use.
Floppy Disk Images With GRUB and EXT2
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 EXT2 formatted.
mkfs -t ext2 /dev/loop0
mount -t ext2 /dev/loop0 /mnt/myfloppy
Create GRUB directory.
cd /mnt/myfloppy mkdir grub
Copy GRUB's second stage files. (GRUB stage could also be located in /usr/lib/grub/)
cp /lib/grub/i386-pc/stage /mnt/myfloppy/grub
Create a device mapping for the GRUB installation. You need quotations around the first part.
echo "(fd0) /dev/loop0" > /mnt/myfloppy/grub/device.map
Start GRUB console for installation into the boot record.
grub --device-map=/mnt/myfloppy/grub/device.map /dev/loop0
In the GRUB console:
root (fd0) setup (fd0)
NOTE: You must unmount /mnt/myfloppy before using a emulator to directly read /dev/loop0, such as:
qemu -fda /dev/loop0
NOTE: When deleting the loop device, the original floppy.img file will be saved with the modified contents.
Floppy Disk Images With FAT16
Create an empty image.
dd if=/dev/zero of=floppy.img bs=512 count=2880
Set it up for mounting.
losetup /dev/loop0 floppy.img
Make it MSDOS formatted.
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
|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.
Now to create the MBR and partition table on the disk image (Usually you need to be root).
fdisk -u -C#cylinders -S63 -H16 /path/to/c.img
|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.|
|/path/to/c.img||fdisk is capable of partitioning image files directly.|
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 /path/to/c.img: 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 /path/to/c.img1 * 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.
Ok, now we attach the file to the loopback device, in such a way that we skip everything before the start of our partition.
losetup -o32256 /dev/loop0 /path/to/c.img
|-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
|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
|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.
You should now be able to mount the partition (Because it is still setup on the loopback device).
mount -text2 /dev/loop0 /mnt/wherever
mount -tvfat /dev/loop0 /mnt/wherever
|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.)
Ok, unmount the partition and detach the loopback device.
umount /dev/loop0 losetup -d /dev/loop0
|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.
mount -text2 -oloop=/dev/loop0,offset=32256 /path/to/c.img /mnt/wherever
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.
Finally, if you have to mount and umount that image very frequently and you're too lazy to type the sudo password each time, just add to /etc/fstab:
/path/to/c.img /mnt/wherever ext2 user,loop 0 0
now you can just call:
mount /mnt/wherever umount /mnt/wherever
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)
Don't forget to flush the filesystem buffers when manipulating with files on mounted disk image. On a Unix-like system, this can be simply done by executing the sync program in your shell.
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)
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 /mnt/myfloppy vnconfig -c /dev/vn0
Memdisks are allocated dynamically, and the name is displayed after the mdconfig command. This assumes that "md0" is printed.)
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
umount /mnt/myfloppy mdconfig -d -u md0
Loopback Device under OpenBSD
OpenBSD has used vnconfig(8) since version 2.2 (perhaps earlier..).
As root or using su/sudo, Here is an example scenario for configuring a vnode pseudo disk device.
Creating the floppy.img file using dd:
dd if=/dev/zero of=/path/to/floppy.img bs=512 count=2880
Configuring the vnd0 device:
vnconfig vnd0 /path/to/floppy.img
Listing configured devices:
vnconfig -l Output: vnd0: covering floppy.img on wd0a, inode 270473 vnd1: not in use vnd2: not in use vnd3: not in use
Creating a FAT12 file system and then mounting the device:
newfs_msdos -F 12 -f 1440 /dev/rvnd0c mount -t msdos /dev/vnd0i /mnt/floppy
Removing the device mount and uninstalling the vnd0 device:
umount /mnt/floppy vnconfig -u vnd0