TIP: Working with Disk Images Under Linux

Introduction

At times I need to work with disk images under linux.  Now I’m not talking about an image file that is a single partition. I’m talking about a disk image that represents a hard drive with an MBR and multiple partition in it.

Example 1:  Mounting A Disk Image

We need to start by mounting the image file to a loop back device.
> losetup /dev/loop0 yourimage.imgReplace yourimage.img with the path to your image file. Now lets use fdisk to

see the partions on the disk image.
> fdisk -ul /dev/loop0

Fdisk showed that my image file had 2 partitions. One was a linux partition and the other was a linux lvm partition. Let’s mount the linux partition.
> mkdir /mnt/diskimg_p1
> lomount -diskimage /dev/loop0 -partition 1 /mnt/diskimg_p1

This partition turned out to be /boot. You can unmount it just as easy.
> umount /mnt/diskimg_p1

Now unmount the disk image
> losetup -d /dev/loop0

Example 2:  Mount A Disk Image Partition Using Offsets

Here’s another way to mount the partitions. We will use losetup with offsets. So start with the following:
> losetup /dev/loop0 yourimage.img
> fdisk -ul /dev/loop0

Which gives:

Disk /dev/loop0: 4294 MB, 4294967296 bytes
 255 heads, 63 sectors/track, 522 cylinders, total 8388608 sectors
 Units = sectors of 1 * 512 = 512 bytes

Device Boot Start End Blocks Id System
 /dev/loop0p1 * 63 208844 104391 83 Linux
 /dev/loop0p2 208845 8385929 4088542+ 8e Linux

Now the important part is the start blocks. Your start blocks will look different. My starts are 63 and 208845. The sector size is 512. So do 63*512 and 208845*512. That will give you the offset numbers to use in loset. For my setup I do the following.
> losetup -o 32256 /dev/loop1 /dev/loop0
> losetup -o 106928640 /dev/loop2 /dev/loop0
> mkdir /mnt/p1
> mkdir /mnt/p2
> mount /dev/loop1 /mnt/p1
> mount /dev/loop2 /mnt/p2

Now you have the partitions mounted.

Now lets unmount the partitions and cleanup.
> umount /mnt/p1
> umount /mnt/p2
> rmdir /mnt/p1
> rmdir /mnt/p2
> losetup -d /dev/loop1
> losetup -d /dev/loop2
> losetup -d /dev/loop0

Conclusion

Hopefully this tip will help you master the disk image.

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