Docunext Technology

When working with virtual machines and embedded devices, you’ll probably end up working with a disk image at some point.

To work with disk images, you can attach the file to the a device. To do so, you use the losetup command (which requires root privileges).

losetup
usage:
  losetup loop_device                                      # give info
  losetup -d loop_device                                   # delete
  losetup -f                                               # find unused
  losetup [ options ] {-f|loop_device} file                # setup
    where options include
    --offset , -o 
        start at offset  into file.
    –pass-fd , -p 
        read passphrase from file descriptor 
        instead of the terminal.
    –encryption , -e 
        encrypt with .
        Check /proc/crypto or /proc/crypto/cipher for available ciphers.
    –nohashpass, -N
        Don’t hash the password given.  (previous versions hash, non-debian doesn’t.
    –keybits , -k 
        specify number of bits in the hashed key given
        to the cipher.  Some ciphers support several key
        sizes and might be more efficient with a smaller
        key size.  Key sizes < 128 are generally not
        recommended

The challenge is if the disk image has multiple partitions. Though the loop device will be aware of the different partitions (which you can view with fdisk, and you’ll need to in order to get the offset number required to mount the last partition), you can’t access them all at once.

So far I only know how to mount the last partition on the disk image. You find out the offset of that partition, and attach it to the loopback device:

losetup -o52428800 /dev/loop0 debian-sarge-256-hda.img
mount -text3 /dev/loop0 /mnt/debian-sarge-cf/

It works! How to find out the offset? Use fdisk on the loopback device when the entire image is attached to it:

losetup /dev/loop0 debian-sarge-256-hda.img
fdisk /dev/loop0
Command (m for help): p

Disk /dev/loop0: 232 MB, 232783872 bytes
16 heads, 32 sectors/track, 888 cylinders
Units = cylinders of 512 * 512 = 262144 bytes

      Device Boot      Start         End      Blocks   Id  System
/dev/loop0p1   *           1          80       20479+  83  Linux
/dev/loop0p2              81         200       30720   83  Linux
/dev/loop0p3             201         888      176128   83  Linux

Command (m for help): u
Changing display/entry units to sectors

Command (m for help): p   

Disk /dev/loop0: 232 MB, 232783872 bytes
16 heads, 32 sectors/track, 888 cylinders, total 454656 sectors
Units = sectors of 1 * 512 = 512 bytes

      Device Boot      Start         End      Blocks   Id  System
/dev/loop0p1   *           1       40959       20479+  83  Linux
/dev/loop0p2           40960      102399       30720   83  Linux
/dev/loop0p3          102400      454655      176128   83  Linux

Use the “u” command in fdisk to change the units, then multiply the start number times 512, 102400 * 512 = 52428800.

Actually - turns out this also works on partitions other than the last one. For example, I just mounted the second partition by using the 40960 offset, multiplied by 512 = 20971520:

# losetup -o20971520 /dev/loop0 debian-sarge-256-hda.img
# mount -text3 /dev/loop0 /mnt/debian-sarge-cf/