Building VM image with Packer

Packer logoQuite often building a VM from scratch is not very wise. Unless server configuration is trivial, its provisioning might take significant amount of time. For example, creating an instance of a build server for my current project takes about 40 minutes. This includes installing updates, various SDKs and other dependencies. How is it possible then that I can add new build server to a cluster in about two or three minutes?

The secret is that the most of the software is baked into a VM image, so I never start from scratch. New VM still needs some steps like final configuration and registering within the cluster, but that’s fast. The slowest part is allocating resources for VM, not provisioning.

Historically, we’ve been using own tools for that, but there’re also free and open source ones. Like Packer.

What’s Packer

Packer is virtual machine image builder. It can create images for most of big cloud providers, like AWS, GCE, Azure and Digital Ocean, or even local hypervisors: VMWare, VirtualBox and few others. It also doesn’t discriminate and works well with both Linux and Windows images.

Packer needs something to build an image from and that something is template file. Template is regular JSON file with three main sections:

  • builders,
  • provisioners and
  • post-processors

While “builders” are responsible for creating virtual machine for future image and therefore vary from one cloud provider to another, “provisioners” are atomic configuration steps which are usually reusable. “post-processors” on the other hand are the final touches, like compressing output image or turning it into Vagrant box. Latter two sections are optional, so having just a builder is enough to create an image.

The plan

As I tend to reuse examples from one article to the other, let’s use something from Consul cluster we created before and turn it into a Packer image. I think creating Ubuntu image with Consul binaries baked into it would make some sense as an example.

We’ll use VirtualBox as a destination for image. It’s not that exciting like building an image in AWS or GCE, but it’s radically cheaper.

Installation

Packer is downloadable as an archive with binaries and unzipping concludes the installation. Installing VirtualBox involves more clicking, but it’s still trivial. Both tools are cross-platform and I’m using their Mac-compatible incarnations.

Setting up the “builder”

The worst part of using VirtualBox as Packer builder is that it’s more complex than all other cloud providers combined. The reason is simple: whereas we usually start creating cloud virtual machines from more or less ready VM image, VirtualBox VM starts right from operating system ISO file. Fortunately, mighty Google has enough of links to Packer/VirtualBox examples and this particular guy has templates for virtually everything.

VirtualBox Ubuntu builder needs two things:

  • builder section itself, which will describe what provider it uses (VirtualBox), where to pick OS ISO from, where to click during GUI installation and few virtual machine settings;
  • preseed.cfg file, which contains Ubuntu installation options. I just picked the first one found on internet.

This is the initial version of Packer template file with builder only:

Even though it looks cumbersome, it actually makes a lot of sense. type: virtualbox-iso means we’re using VirtualBox provider. iso_* properties define where OS ISO comes from. vboxmanage define VirtualBox VM hardware settings and boot_command actually specifies what keys to press when OS installer starts. I’d never memorized this command, but with the help of mighty Google I don’t have to.

"http_directory": "http" and preseed/url=http://{{.HTTPIP}}{{.HTTPPort}}/preseed.cfg are particularly interesting ones. In order to provide Ubuntu installer with default options (preseed/url) Packer starts its own embedded web server. However, it doesn’t come with a content to serve, so http_directory has to specify the place to serve the files from. I have local http directory with preseed.cfg in it (thanks again, Google!), so Ubuntu installation will go in totally unattended mode. You can take a look at this file at github.

Configuring “provisioners”

Dealing with provisioners is much simpler. Packer works well with configuration managers like Ansible or Chef, but in our case simple “shell” provisioner will be enough:

I picked the code for install-consul.sh from my previous post about Vagrant and Consul and changed it just a little bit:

And that’s it!

Test run

Now let’s validate template file we’ve just created and then try to build it.

Along the way you actually can see how the installation goes. Remember that ridiculously long <bs><bs>... sequence in the beginning? It’s used right here, for backspace‘ing boot options:

Packer create ubuntu image

And the rest of the process:

ubuntu install progress

And the whole process is fast. Like, few minutes fast. In the end it produces VMDK and OVF files, which are VM image equivalent in VirtualBox. We can import OVM as a new VM:

virtualbox ovf import

Then start the VM (login and password – ubuntu and ubuntu) and check if consul is there:

ubuntu consul image

Yup, it’s definitely there.

Conclusion

If you’re creating one-of-a-kind VM, it’s probably OK to create it from scratch in one go (automatically, obviously). But if you have a cluster of VMs that come and go, it makes sense to prepare an image that has most of configuration and software installed, and then use it as a base for new VMs. Packer is a tool exactly for that – for creating preconfigured images.

The best selling feature for me is that I can create images in different cloud providers from single configuration. I like Google Compute Engine a lot, but it’s nice to know that at any moment I can make few changes in “builders” and move the whole cluster to AWS.

As usual, the source code for this article can be found on github.

2 thoughts on “Building VM image with Packer”

  1. Well, it stuck at “Waiting for SSH to become available” for me. VirtualBox shows that the VM is stopped. And I’ve got some strange errors when tried to start it from UI.

    1. I need more details than that 🙂 What OS are you using? I just tried the sample one more time on another machine, Ubuntu 14.04. All worked.

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