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Sohrab Behdani
Sohrab Behdani

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How to install a minimal Debian system on Qemu

#debian #linux #qemu #emulation

I was working on a project that required a minimal debian installation. i didn't had the iso at the time, so i came up with an idea.

Debootstrap

Debian has a tool called debootstrap, which installs Debian base systems in a subdirectory of another, already installed system.

Debian wiki:

debootstrap doesn't require an installation CD, just access to a Debian repository. It can also be installed and run from another operating system - for example, you can use debootstrap to install Debian onto an unused partition from a running Gentoo system. It can also be used to create a rootfs for a machine of a different architecture, which is known as "cross-debootstrapping".

I'm Running Parch Linux which is a distribution based on Arch.
For start, i installed the qemu, debootstrap and other tools using pacman:

sudo pacman -S qemu-system-x86 qemu-img qemu-ui-gtk arch-install-scripts debootstrap
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Disk image

now we need to create the disk image that we want to install the debian on, we use the qemu-img command to create a new 60GB virtual disk for installation.

qemu-img create -f qcow2 debian.qcow2 60G
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after this, we would load the nbd module into the kernel for mounting the disk via qemu-nbd command.

qemu-nbd: QEMU Disk Network Block Device Server which allows us to expose disk image files such as .qcow2 , .vmdk , .vdi and etc as a Network Block Device (nbd) on linux.

for short: It enables a virtual disk image to be treated like a real block device.

Using nbd

for start, we need to load the module. for that we should run this command:

sudo modprobe nbd
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then after that, we would use the qemu-nbd command to mount the disk as a network block.

sudo qemu-nbd --connect=/dev/nbd0 debian.qcow2
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this would mount the virtual disk as /dev/nbd0

now we should partition the disk, you can use your preferd tool, i would use the parted itself.

sudo parted /dev/nbd0 -- mklabel msdos
sudo parted /dev/nbd0 -- mkpart primary ext4 1MiB 100%
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now we need to format the disk:

sudo mkfs.ext4 /dev/nbd0p1
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Installation

Now after partitioning we need to start installing the debian itself.

we would first mount the disk at /mnt:

sudo mount /dev/nbd0p1 /mnt
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Now we run this command to install the base system at /mnt which is our mounted disk:

sudo debootstrap --arch=amd64 trixie /mnt http://deb.debian.org/debian
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notes: I'm using Trixie, because it is now stable enough to be used.

You can change the mirror and arch to your preferred one.

the installation would start after running that command.

Post installation

now we need to chroot in our newly bootstrapped debian and configure it.

we have two options

For Arch users

if you are running Arch Linux or it forks, there is a handy tool called arch-chroot.

we need to simply run this command to chroot into it, then you can jump right to the next section.

sudo arch-chroot /mnt
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For other distros: 

sudo mount --bind /dev /mnt/dev

sudo mount --bind /sys /mnt/sys

sudo mount --bind /proc /mnt/proc

sudo chroot /mnt
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configuration:

We need to set our hostname and hosts file, for that you can run this command:

echo debian > /etc/hostname

echo "127.0.0.1 localhost" > /etc/hosts
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You can change the debian with whatever you like.

Now we need to configure the local time:

ln -sf /usr/share/zoneinfo/UTC /etc/localtime

/usr/sbin/dpkg-reconfigure tzdata
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Now we set the root user password:

passwd
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Now we need to install other system components to make it work properly:

apt update

apt install linux-image-amd64 grub-pc systemd-sysv sudo nano net-tools network-manager
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now we install the bootloader:

grub-install --target=i386-pc --recheck /dev/nbd0

/usr/sbin/grub-mkconfig -o /boot/grub/grub.cfg
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now we need to exit from chroot with exit command and generate the fstab file.

sudo su

genfstab -U /mnt >> /mnt/etc/fstab
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note: genfstab is a command from arch install scripts.

Booting into it

after all of this, we need to unmount our disk.

sudo umount /mnt

sudo qemu-nbd --disconnect /dev/nbd0
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after that, you need to run this command to boot it.

qemu-system-x86_64 -m 1024 -hda debian.qcow2 -enable-kvm
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Note: You can change the memory if you want or add more cpu cores, for that read the qemu docs.

Thanks for reading this, i might create a video from this later :)

Top comments (1)

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kamalmost profile image
KamalMostafa •

Thanks Sohrab. wasn't aware of this tool. keep it up.