Mounting QCOW2 images

Isn’t it fun that even before you start a VM out of an image, you can add files to that image, see and edit the directory and file structure of that VM?

I wanted to boot a VM out of a disk-image, but how will I know out of the 256 available IPs for that VM, which one actually got assigned? I tried vnc console, but the connection was terribly flaky. Even so, it was felt quite ugly to use an interface when I was trying to move to a keyboard-only (command line) world. So I just inserted a static IP into the /etc/network/interfaces file of that image! (I wasn’t aware of arp-scan before I discovered the trick described in this post)

We’ll mount the image, tweak the filesystem of that image, and then boot the image.

Install qemu-utils and enable ndb module

sudo apt-get install qemu-utils
sudo modprobe nbd

Use any qcow2 image, and if you don’t have any, download a small CirrOS cloud image (around 13MB).

wget http://download.cirros-cloud.net/0.3.2/cirros-0.3.2-x86_64-disk.img

Connect the image to the first nbd device

sudo qemu-nbd -c /dev/nbd0 cirros-0.3.2-x86_64-disk.img

Mount the image. For nbd0, see all the devices available (/dev/nbd0<some-number-or-string>) and try attaching to starting from the first one

sudo mount /dev/nbd0p1 /mnt

Now at /mnt, you can see the complete filesystem of that image, and make necessary changes. You can do all sorts of things – change sources.list, /etc/network/interfaces, put additional files inside the VM for particular users, etc.

After you’re done, unmount it.

sudo umount /mnt

And disconnect the loopback device too

sudo qemu-nbd -d /dev/nbd0

Done!

PS: I actually created two functions for mounting and unmounting, so that I don’t remember all these commands. Find them here.

Credits: Vigneshvar introduced me to qemu-nbd tool.

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