Unattended installation of VM images with Packer

Installing different OSes has usually been a cumbersome select-enter-point-click process. This post shows you how to auto-install VM images from an Ubuntu 14.04 Server ISO and Windows 2012 R2 ditto for running with KVM. To do so it uses the VM building tool Packer, which is also introduced.

Most people, these days, use pre-baked OSes—known as base images—as building blocks for their virtual server setup. This results in a significant productivity boost, avoiding repeated OS installs.

Now base images must come from somewhere, the easiest option at hand being some 3. party. But do you trust them? And what if the OS is not available? So I have often ended up installing nearly identical OSes in the usual timewasting repeated manner. The reason being a mix of complexity, lack of shareability of installation configurations, and non-generality of building tools.

The VM building tool Packer solves these issues, and this post shows how it works.

An example for the impatient

Install packer on Linux (for others look at Packer - Downloads):

$ wget https://dl.bintray.com/mitchellh/packer/packer_0.7.5_linux_amd64.zip
$ mkdir /usr/local/packer
$ unzip packer_0.7.5_linux_amd64.zip -d /usr/local/packer
$ ln -s /usr/local/packer/packer /usr/local/bin

Clone configuration templates:

$ git clone https://github.com/jakobadam/packer-qemu-templates

Install QEMU:

$ apt-get install qemu-kvm

Build Ubuntu 14.04 Server for running with QEMU:

$ cd packer-qemu-templates/ubuntu
$ packer build ubuntu-14.04-server-amd64.json

Build Windows 2012 R2 Server (virtio) for running with QEMU:

$ cd packer-qemu-templates/ubuntu
$ packer build windows-2012-R2-standard-amd64.json

And that's it. You should now have two qcow2 images for running with QEMU.

Short intro to Packer

Packer is VM image building tool that aims to streamline the OS installation process doing the heavy lifting of creating and provisioning virtual images. A central component of packer—from the users point of view—is the template.

Templates completely define the installation of an OS. Packer works by taking these, in JSON format, and outputs runnable provisioned virtual machine images without additional user input.

Packer Input Output

Packer has several virtual image output options runnable by Virtualbox, QEMU etc. Amazon is also supported but building machines here are a very different story, since they rely on both the presence of base images and building within Amazon.

Unfortunately, templates are specific for the output format, and including common template code is not (yet?) possible. The focus in this post is on the QEMU output format which we use for our cloud at https://origo.io/

Let's jump into it, and see how it works. I'll explain some of the essential elements in the templates for two different OSes: Ubuntu 14.04 Server and Windows 2012 R2.

Ubuntu 14.04 Server Installation

This section describes and explains the template for pre-baking a production-ready Ubuntu 14.04 server for running with QEMU. While reading this, it would be beneficial to have a look at the whole template at Github - Packer Ubuntu Template.

A template typically consists of three sections: variables, builders and provisioners.


The variables section contains user suppliable values substituted in the template. The Ubuntu template has three variables: username, password and disk size.

"variables": {
    "user": "adminubuntu",
    "password": "adminubuntu",
    "disk_size": 100000

Ubuntu 14.04 Template: Variables section

To replace the user default value when starting the packer build:

packer build -var 'user=u' ubuntu-14.04-server-amd64.json  


The builders section contains a list of building specifications, one for each output target, in our case only QEMU.

            "type": "qemu",
            "name": "ubuntu",
            "format": "qcow2",
            "accelerator": "kvm",
            "disk_size": "{{ user `disk_size`}}",

            "iso_url": "http://releases.ubuntu.com/14.04/ubuntu-14.04-server-amd64.iso",
            "iso_checksum": "01545fa976c8367b4f0d59169ac4866c",
            "iso_checksum_type": "md5",

            "http_directory": "http",

            "ssh_username": "{{user `user`}}",
            "ssh_password": "{{user `password`}}",
            "shutdown_command": "echo '{{user `password`}}'|sudo -S shutdown -P now",

            "boot_wait": "2s",
            "boot_command": [
                "/install/vmlinuz url=http://{{ .HTTPIP }}:{{ .HTTPPort }}/preseed.cfg ",
                "debian-installer=en_US auto locale=en_US kbd-chooser/method=us ",
                "hostname={{ .Name }} ",

                "keyboard-configuration/modelcode=SKIP ",
                "keyboard-configuration/layout=USA ",
                "keyboard-configuration/variant=USA ",

                "passwd/user-fullname={{user `user`}} ",
                "passwd/user-password-again={{user `password`}} ",
                "passwd/user-password={{user `password`}} ",
                "passwd/username={{user `user`}} ",

                "initrd=/install/initrd.gz -- <enter>"

Ubuntu 14.04 Template: Builders section

In order to slipstream the Ubuntu install process, a preseed.cgf file, with answers to install questions, must be supplied to the installer. This file is most easy to create by taking an existing, like example-preseed, massaging it and checking it by running:

debconf-set-selections -c preseed.cfg  

For more on preseeding, have a look at the debian handbook

The template includes an URL reference to preseed.cfg in the boot_command property. When Packer is started it fires up a web server serving the contents of the directory supplied in the http_directory property. The wired looking magic variables {{.HTTPIP}} and {{.HTTPPort}} are therefore part of the address to preseed.cfg on this web server.

Note: Variables are substituted in the template—not in files referred to from it, e.g., files served via HTTP or included on a floppy.

The boot command is an array of commands which are typed in via VNC. Since values for user and password are suppliable when starting a build, the adhering debconf values can't be put in preseed.cfg. Instead these settings are specified in the boot_command option in the template, where variables are substituted.

"passwd/user-fullname={{user `user`}} ",
"passwd/user-password={{user `password`}} ",
"passwd/user-password-again={{user `password`}} ",
"passwd/username={{user `user`}} ",


During install Packer continuously polls for SSH access. Upon successful login packer either shuts down the OS or continues to the provisioners step if supplied.

"provisioners": [
            "type": "shell",
            "scripts": [
            "execute_command": "echo '{{user `password`}}' | {{.Vars}} sudo -E -S bash '{{.Path}}'"

Ubuntu 14.04 Template: Provisioning step

In our template there is a single provisioner which uploads and executes three scripts on the machine: update.sh, packages.sh, and network.sh. The scripts update the machine, install some useful packages, and removes all traces from the network—MAC to network interface mapping—used during installation.

The somewhat arcane execute_command stems from the fact that the SSH user is not the root user, and therefore sudo is used to execute the scripts as superuser.

Windows 2012 R2 (virtio) installation

The template is available at Packer - Windows 2012 R2 template


The installation of the windows image is automated by utilizing Autounattend.xml—an incomprehensible XML answer file that the windows install process looks for on the floppy drive during startup. The easiest way to create this, is to take an existing file, and massage that one.

Microsoft provides a GUI tool which aids in creating the answer file, named Windows System Image Manager. The creation process, when running windows in a virtual machine, is as follows: Mount install DVD → copy D:/sources/install.wmi somewhere → Open up 'Windows System Image Manager' → Select 'Windows Image' → Select install.wim → Create new answer file.

Although, possible to create arbitrary configurations to windows in Autounattend.xml, I recommend sticking with the bare essentials, leaving additional configurations to the provisioning phase of packer. Just look at all the extra XML clutter to execute the single script, configure.bat:

<SynchronousCommand wcm:action="add">  
  <CommandLine>cmd.exe /c a:\configure.bat</CommandLine>
  <Description>Run configure script</Description>

Autounattend.xml: Execute script

Our template goes with the first option, as seen below, and attaches Autounattend.xml and relevant scripts referred from it in the floppy_files template variable. Note: All files are put on the floppy in one flat hierarchy, so don't duplicate file names.

"builders": [
            "type": "qemu",
            "name": "windows-2012R2",
            "accelerator": "kvm",
            "disk_size": "{{ user `disk_size`}}",

            "iso_url": "http://care.dlservice.microsoft.com/dl/download/6/2/A/62A76ABB-9990-4EFC-A4FE-C7D698DAEB96/9600.17050.WINBLUE_REFRESH.140317-1640_X64FRE_SERVER_EVAL_EN-US-IR3_SSS_X64FREE_EN-US_DV9.ISO",
            "iso_checksum_type": "md5",
            "iso_checksum": "5b5e08c490ad16b59b1d9fab0def883a",

            "boot_wait": "2m",
            "ssh_username": "Administrator",
            "ssh_password": "Administrator",
            "shutdown_command": "shutdown /s /t 10 /f /d p:4:1 /c \"Packer Shutdown\"",

            "floppy_files": [
            "qemuargs": [
                ["-m", "4096m"],
                ["-smp", 2]

Windows Template: Builders section

The configure script does two things. First, it disables the firewall, and then it installs SSH. It's essential to setup SSH in the building step to let packer do its thing. This, of course, is a hack and support for the windows alternative (WinRM) to SSH, for uploading and executing files, is on the way https://github.com/mitchellh/packer/issues/451. For now, we're left with Cygwin which is setup, during the building phase as the last step, before going to the provisioners section, which uses SSH.


The template property floppy_files instructs packer to attach the relevant virtio drivers on a floppy and make them available during the installation. The windows installation process is instructed by Autounattend.xml to look for these drivers on the floppy drive. Hence, the installation is performed with virtio, boosting performance.


"provisioners": [
            "type": "shell",
            "inline": ["net user Administrator {{ user `password`}}"]
            "type": "shell",
            "remote_path": "/cygdrive/c/Windows/Temp/script.bat",
            "execute_command": "cmd /c C:/Windows/Temp/script.bat",
            "scripts": [

Windows Template: Provisioners section

The provisioning step provides some saner defaults for most environments. It enables RDP, allows execution of user powershell scripts, and disables prompts for changing password. It also updates the password, hereby letting the user supply one as a parameter to packer build.


This post showed you how to efficiently create two different VM images—for running on KVM—with Packer. If you encounter missing OS templates, I hope that you will now contribute your own.

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