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How To Build Virtual Machine Images With Virt-builder In Linux

Quickly Build Virtual Machine Images With Virt-builder

By sk
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Virt-builder is a command line tool for building variety of virtual machine images for local or cloud use, easily as well as quickly. It also has many options to customize the images. You can install new application(s) on the VM image, set host name, set root password, run a command or script when the guest VM boots for the first time, add or edit files in the disk image and more. All of these tasks can be done from command line and doesn't require root permissions.

Virt-builder downloads the cleanly prepared, digitally signed OS templates, so you don't have to manually install the OS. All you have to do is just use the Virt-manager GUI or Virt-install command line tool to instantly fire up the VMs with the predefined templates. Virt-builder provides minimal OS templates for popular Linux and Unix variants. You can of course create your own template as well.

Install Virt-builder on Linux

Virt-builder is part of the Libguestfs library, so make sure you have installed it as described in the following guide.

Build Virtual Machine Images With Virt-builder

Building virtual machines' images with Virt-builder is quite easy and straight forward.

List available virtual machine templates

First, list the available OS templates. To do so, run:

$ virt-builder --list

As of writing this guide, the following templates were available:

centos-6                 x86_64     CentOS 6.6
centos-7.0               x86_64     CentOS 7.0
centos-7.1               x86_64     CentOS 7.1
centos-7.2               aarch64    CentOS 7.2 (aarch64)
centos-7.2               x86_64     CentOS 7.2
centos-7.3               x86_64     CentOS 7.3
centos-7.4               x86_64     CentOS 7.4
centos-7.5               x86_64     CentOS 7.5
centos-7.6               x86_64     CentOS 7.6
centos-7.7               x86_64     CentOS 7.7
centos-7.8               x86_64     CentOS 7.8
centos-8.0               x86_64     CentOS 8.0
centos-8.2               x86_64     CentOS 8.2
cirros-0.3.1             x86_64     CirrOS 0.3.1
cirros-0.3.5             x86_64     CirrOS 0.3.5
debian-10                x86_64     Debian 10 (buster)
debian-6                 x86_64     Debian 6 (Squeeze)
debian-7                 sparc64    Debian 7 (Wheezy) (sparc64)
debian-7                 x86_64     Debian 7 (wheezy)
debian-8                 x86_64     Debian 8 (jessie)
debian-9                 x86_64     Debian 9 (stretch)
fedora-26                aarch64    Fedora® 26 Server (aarch64)
fedora-26                armv7l     Fedora® 26 Server (armv7l)
fedora-26                i686       Fedora® 26 Server (i686)
fedora-26                ppc64      Fedora® 26 Server (ppc64)
fedora-26                ppc64le    Fedora® 26 Server (ppc64le)
fedora-26                x86_64     Fedora® 26 Server
fedora-27                aarch64    Fedora® 27 Server (aarch64)
fedora-27                armv7l     Fedora® 27 Server (armv7l)
fedora-27                i686       Fedora® 27 Server (i686)
fedora-27                ppc64      Fedora® 27 Server (ppc64)
fedora-27                ppc64le    Fedora® 27 Server (ppc64le)
fedora-27                x86_64     Fedora® 27 Server
fedora-28                i686       Fedora® 28 Server (i686)
fedora-28                x86_64     Fedora® 28 Server
fedora-29                aarch64    Fedora® 29 Server (aarch64)
fedora-29                i686       Fedora® 29 Server (i686)
fedora-29                ppc64le    Fedora® 29 Server (ppc64le)
fedora-29                x86_64     Fedora® 29 Server
fedora-30                aarch64    Fedora® 30 Server (aarch64)
fedora-30                i686       Fedora® 30 Server (i686)
fedora-30                x86_64     Fedora® 30 Server
fedora-31                x86_64     Fedora® 31 Server
fedora-32                x86_64     Fedora® 32 Server
freebsd-11.1             x86_64     FreeBSD 11.1
scientificlinux-6        x86_64     Scientific Linux 6.5
ubuntu-10.04             x86_64     Ubuntu 10.04 (Lucid)
ubuntu-12.04             x86_64     Ubuntu 12.04 (Precise)
ubuntu-14.04             x86_64     Ubuntu 14.04 (Trusty)
ubuntu-16.04             x86_64     Ubuntu 16.04 (Xenial)
ubuntu-18.04             x86_64     Ubuntu 18.04 (bionic)
fedora-18                x86_64     Fedora® 18
fedora-19                x86_64     Fedora® 19
fedora-20                x86_64     Fedora® 20
fedora-21                aarch64    Fedora® 21 Server (aarch64)
fedora-21                armv7l     Fedora® 21 Server (armv7l)
fedora-21                ppc64      Fedora® 21 Server (ppc64)
fedora-21                ppc64le    Fedora® 21 Server (ppc64le)
fedora-21                x86_64     Fedora® 21 Server
fedora-22                aarch64    Fedora® 22 Server (aarch64)
fedora-22                armv7l     Fedora® 22 Server (armv7l)
fedora-22                i686       Fedora® 22 Server (i686)
fedora-22                x86_64     Fedora® 22 Server
fedora-23                aarch64    Fedora® 23 Server (aarch64)
fedora-23                armv7l     Fedora® 23 Server (armv7l)
fedora-23                i686       Fedora® 23 Server (i686)
fedora-23                ppc64      Fedora® 23 Server (ppc64)
fedora-23                ppc64le    Fedora® 23 Server (ppc64le)
fedora-23                x86_64     Fedora® 23 Server
fedora-24                aarch64    Fedora® 24 Server (aarch64)
fedora-24                armv7l     Fedora® 24 Server (armv7l)
fedora-24                i686       Fedora® 24 Server (i686)
fedora-24                x86_64     Fedora® 24 Server
fedora-25                aarch64    Fedora® 25 Server (aarch64)
fedora-25                armv7l     Fedora® 25 Server (armv7l)
fedora-25                i686       Fedora® 25 Server (i686)
fedora-25                ppc64      Fedora® 25 Server (ppc64)
fedora-25                ppc64le    Fedora® 25 Server (ppc64le)
fedora-25                x86_64     Fedora® 25 Server
opensuse-13.1            x86_64     openSUSE 13.1
opensuse-13.2            x86_64     openSUSE 13.2
opensuse-42.1            x86_64     openSUSE Leap 42.1
opensuse-tumbleweed      x86_64     openSUSE Tumbleweed
List available virtual machines using Virt-builder

List available virtual machines using Virt-builder

As you can see, there are multiple OS templates available.

Before building a virtual machine image, you might want to look into the installation notes of the guest OS to know what is in there.

For example, to view the install notes of Debian 10, run:

$ virt-builder --notes debian-10

Sample output:

Debian 10 (buster)

This is a minimal Debian install.

This image is so very minimal that it only includes an ssh server
This image does not contain SSH host keys.  To regenerate them use:

    --firstboot-command "dpkg-reconfigure openssh-server"

This template was generated by a script in the libguestfs source tree:
    builder/templates/make-template.ml
Associated files used to prepare this template can be found in the
same directory.

Build a virtual machine image

I wanted to download the OS templates in a specific directory, so I created this directory:

$ mkdir virtbuilder
$ cd virtbuilder/

Let us build the Debian 10 virtual machine using command:

$ virt-builder debian-10

Sample output:

[   4.8] Downloading: http://builder.libguestfs.org/debian-10.xz
##################################################################################### 100.0%
[  83.2] Planning how to build this image
[  83.2] Uncompressing
[ 101.2] Opening the new disk
[ 119.8] Setting a random seed
virt-builder: warning: random seed could not be set for this type of guest
[ 119.9] Setting passwords
virt-builder: Setting random password of root to 66xW1CaIqfM8km2v
[ 121.5] Finishing off
                   Output file: debian-10.img
                   Output size: 6.0G
                 Output format: raw
            Total usable space: 5.8G
                    Free space: 4.9G (84%)
Build virtual machine images with virt-builder in Linux

Build virtual machine images with virt-builder in Linux

As you can see, this command has built the minimal Debian 10 image. It will not have any user accounts. It will only have random root password and the bare minimum installed software.

The output name of the image should be same as the template name. You can change it as per your liking using the -o option:

$ virt-builder debian-10 -o ostechnix.img

By default, the image format is img. You can convert it to different format, for example Qcow2, like below:

$ virt-builder debian-10 --format qcow2

By default, Virt-builder will build the image same as the host OS architecture. For example, if your host OS is 64-bit, it will then build 64-bit VM. You can change this to 32-bit (if available) using --arch option.

$ virt-builder debian-10 --arch i686

Want to build a custom-size image? It is also possible. The following command will build a VM with size 50 GB:

$ virt-builder debian-10 --size 50G

The guest OS is resized automatically using virt-resize command as it is copied to the output.

Set root password

Like I already mentioned, a random password will be assigned to the root user account while building the image. If you want to set a specific password for the root user, use --root-password option like below:

$ virt-builder centos-8.2 --format qcow2 --root-password password:centos

Sample output:

[   5.1] Downloading: http://builder.libguestfs.org/centos-8.2.xz
##################################################################################### 100.0%
[ 249.2] Planning how to build this image
[ 249.2] Uncompressing
[ 271.3] Converting raw to qcow2
[ 281.1] Opening the new disk
[ 319.9] Setting a random seed
[ 320.4] Setting passwords
[ 323.0] Finishing off
                   Output file: centos-8.2.qcow2
                   Output size: 6.0G
                 Output format: qcow2
            Total usable space: 5.3G
                    Free space: 4.0G (74%)

The above command will build CentOS 8.2 image and assign password for the root user as "centos".

You can also set password from a text file:

$ virt-builder centos-8.2 --root-password file:~/ostechnix.txt

To disable the root password, run:

$ virt-builder centos-8.2 --root-password disabled

Lock root account:

$ virt-builder centos-8.2 --root-password locked

Lock root account and disable root password:

$ virt-builder centos-8.2 --root-password locked:disabled

To assign root password, but lock the root account, use the following options:

--root-password locked:file:FILENAME
--root-password locked:password:PASSWORD

We can use the root password after unlocking the root user with "passwd -u" command.

Create users

To create user accounts while building a virtual machine image, run:

$ virt-builder centos-8.2 --firstboot-command 'useradd -m -p "" sk ; chage -d 0 sk'

The above command will create a user called "sk" with no password and force him to set password at his first login.

Set hostname

To set the hostname to the VM:

$ virt-builder centos-8.2 --hostname virt.ostechnix.local

Sample output:

[   4.7] Downloading: http://builder.libguestfs.org/centos-8.2.xz
[   7.2] Planning how to build this image
[   7.2] Uncompressing
[  31.0] Opening the new disk
[  41.9] Setting a random seed
[  42.0] Setting the hostname: virt.ostechnix.local
[  42.1] Setting passwords
virt-builder: Setting random password of root to MRn7fj1GSaeCAHQx
[  44.4] Finishing off
                   Output file: centos-8.2.img
                   Output size: 6.0G
                 Output format: raw
            Total usable space: 5.3G
                    Free space: 4.0G (74%)

Install software on VM image

To install packages on a VM, run:

$ virt-builder debian-10 --install vim

Sample output:

[   5.8] Downloading: http://builder.libguestfs.org/debian-10.xz
[   7.4] Planning how to build this image
[   7.4] Uncompressing
[  25.3] Opening the new disk
[  29.7] Setting a random seed
virt-builder: warning: random seed could not be set for this type of guest
[  29.8] Installing packages: vim
[  93.2] Setting passwords
virt-builder: Setting random password of root to 45Hj5yxh8vRqLDcu
[  94.9] Finishing off
                   Output file: debian-10.img
                   Output size: 6.0G
                 Output format: raw
            Total usable space: 5.8G
                    Free space: 4.8G (82%)

To install multiple packages, mention them within quotes, with comma-separated like below:

$ virt-builder debian-10 --install "apache2,htop"

Update all packages in the VM:

$ virt-builder centos-8.2 --update

If your VM uses SELinux, you need to do SELinux relabeling after installing or updating packages:

$ virt-builder centos-8.2 --update --selinux-relabel

Customize VM images

Virt-builder has many options to customize an image. For instance, you can run a specific command/script when the first time VM boots using command:

$ virt-builder debian-10 --firstboot-command 'apt -y update'

To add a line in the VM, run:

$ virt-builder centos-8.2 --append-line '/etc/hosts:192.168.225.1 server.ostechnix.local'

Caching templates

By default, all templates will be downloaded from the network for the first time. Since the size of the templates is large, the downloaded templates will be cached in user's home directory.

You can print the details of the cache directory and which templates are currently cached using the following command:

$ virt-builder --print-cache

Sample output:

cache directory: /home/sk/.cache/virt-builder
[...]
centos-7.8               x86_64     no
centos-8.0               x86_64     no
centos-8.2               x86_64     cached
cirros-0.3.1             x86_64     no
cirros-0.3.5             x86_64     no
debian-10                x86_64     cached
debian-6                 x86_64     no
debian-7                 sparc64    no
[...]

You can also verify it by manually looking in the cache folder:

$ ls $HOME/.cache/virt-builder
centos-8.2.x86_64.1 debian-10.x86_64.1

To download all available templates to your local cache folder, run:

$ virt-builder --cache-all-templates

If you don't want to cache the template while building the image, use --no-cache option.

To delete all cached templates, run:

$ virt-builder --delete-cache
[   0.0] Deleting: /home/sk/.cache/virt-builder

Importing disk images into hypervisor

Well, you have downloaded your desired OS and customized it as per your liking. Now what? Just import the image and run a VM using the newly created disk image with a hypervisor. We already have written a step by step guide to create a KVM virtual machine using Qcow2 image. This guide is written specifically for Qcow2, however it is same for importing .img format disk images too.

Virt-builder has hundreds of commands and options. I covered only the basic commands here. For more details, refer the manual age.

$ man virt-builder

Featured Photo by Igor Starkov from Pexels.

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