2019-09-12

Packer by Hashicorp

About Packer

Packer is an open source tool for creating identical machine images for multiple platforms from a single source configuration. One of the worst things I have to do when creating a new VM is to go through the installation process as I were doing it in a physical machine. Imagine if you had to do it for different platforms (Vagrant, Xen, KVM)

About how to install Packer, you can go directly to the Packer installation page. TLDR if your are a Linux user you can download the binary directly and make it available in your shell PATH.

Packer builders

A builder in Packer describes how the image is going to be built for a specific platform (AWS EC2, KVM, Docker…​). In a Packer configuration file you can add as many builders as type of images you want to build. For instance, in this post I’m only using the Qemu builder because my target environment is KVM.

Configuration file

The Packer configuration file is just a JSON file with some group of properties:

  • variables: variable declaration. You can declare variables and default values and reference those variables along the rest of the configuration.

  • builders: As I mentioned in the beggining a builder represents the way in which the image machine is going to be built in a given environment (qemu in here)

  • provisioners: Provisioners can be used to configure or install new software in the image after booting (e.g Ansible as provisioner).

  • post-processors: Post processors run after the image is built and provisioners have finished (e.g uploading the image to a remote registry).

Example

My hello world example is to build a Debian 10 VM for Qemu environment. The structure of the project is: example is:

structure
+--http
|   +
|   |
|   +---debian-10.cfg
|
+--debian-10.json

  • debian-10.json: Packer configuration file

  • debian-10.cfg: Debian 10 preseed file

The Packer entry file is debian-10.json, is where the builders, provisioners, post-processors and variables are set:

.debian-10.json
{
    "builders":
    [
        {
            "type": "qemu",
            "vm_name": "debian-10-{{build_type}}",
            "headless": true,

            "iso_url": "https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-10.1.0-amd64-netinst.iso",
            "iso_checksum": "7915fdb77a0c2623b4481fc5f0a8052330defe1cde1e0834ff233818dc6f301e",
            "iso_checksum_type": "sha256",

            "memory": "2048",
            "disk_size": "5000",
            "cpus": 2,

            "ssh_username": "admindebian",
            "ssh_password": "Pa55w0rd",
            "shutdown_command": "echo 'Pa55w0rd'|sudo -S shutdown -h now",
            "ssh_timeout": "10m",

            "http_directory": "http",
            "boot_command": [
                "<esc><wait><wait>",
                "install ",
                "auto=true ",
                "priority=critical ",
                "interface=auto ",
                "url=http://{{ .HTTPIP }}:{{ .HTTPPort }}/debian-10.cfg ",
                "<enter>"
            ]
        }
    ],
    "provisioners": [
        {
            "type": "shell",
            "scripts": [
                "scripts/update.sh"
            ]
        }
    ]
}

This Packer file configures just one builder, which builds a Qemu compatible Debian 10 qcow2 image with the following properties:

builder basic properties
"type": "qemu",
"vm_name": "debian-10",
"headless": true,

The builder is of type qemu, the resulting virtual machine will be named debian-10 and the installation is automated so I don’t need to see it working (headless).

base image retrieval
"iso_url": "https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-10.1.0-amd64-netinst.iso",
"iso_checksum": "7915fdb77a0c2623b4481fc5f0a8052330defe1cde1e0834ff233818dc6f301e",
"iso_checksum_type": "sha256",

I’m taking an official image from the Debian site (iso_url), and to make sure the image hasn’t been tampered I’m going to check its signature (iso_checksum) with the right algorithm (iso_checksum_type).

phisical features
"memory": "2048",
"disk_size": "5000",
"cpus": 2,

When building the image, it will have 2GB of memory a disk_size of 5G and 2 cpus.

ssh information
"ssh_username": "admindebian",
"ssh_password": "Pa55w0rd",
"shutdown_command": "echo 'Pa55w0rd'|sudo -S shutdown -h now",
"ssh_timeout": "12m",

The ssh information is required to help Packer to communicate with the image.

It’s always a good idea to add a ssh_timeout, a connection loss or a timeout due to rebooting can happen at any time.

Preseed files

Because we want to automate the whole installation step-by-step process, we would like to pass like a guide to the installer in order to tell it what to do. That’s what the preseed files are for. In order to make the installer aware of the preseed file we need:

  • To tell Packer the directory where the preseed file is by setting the http property

  • To pass the url property to the boot command with the whole url where the preseed file is going to be available

debian-10.json
{
    ...
    "http_directory": "http",
    "boot_command": [
        ...
        "url=http://{{ .HTTPIP }}:{{ .HTTPPort }}/debian-10.cfg ",
        "<enter>"
    ]
}

In a preseed file you would find instructions for every step: networking, root account, setting the system locale, timezone…​ For example, here’s a sample of the preseed file for my Debian 10 image:

debian-10.cfg
### Networking
d-i netcfg/choose_interface select auto
d-i netcfg/wireless_wep string
d-i netcfg/get_hostname string unassigned-hostname
d-i netcfg/get_domain string unassigned-domain
d-i mirror/protocol string http
d-i mirror/country string manual
d-i mirror/http/hostname string httpredir.debian.org
d-i mirror/http/directory string /debian
d-i mirror/http/proxy string

### Users
d-i passwd/root-login boolean false
d-i passwd/user-fullname string Debian Admin
d-i passwd/username string admindebian
d-i passwd/user-password password Pa55w0rd
d-i passwd/user-password-again password Pa55w0rd
Every OS may need a different type of preseed file. Checkout some examples in this Github repository

Packer provisioners

Ok, once we’ve built the image, we may want to install some packages, or configure some services…​etc, that is to say to provision the image. You can reference several ways and tools to provision the image: Ansible, Puppet, shell script…​ For this example I’m using a simple shell script.

provisioners example
"provisioners": [
    {
        "type": "shell",
        "scripts": [
            "scripts/update.sh"
        ]
    }
]

As you may notice, the provisioners property is a list with possible provisioners. I’m just using the type shell pointing to a list with just one shell script. This script will be loaded to the built image and executed there (by default is copied to /tmp). The update.sh script is just updating apt indexes:

#!/usr/bin/env bash

echo "==> Provisioning image"
echo "==> Updating indexes [started]"

sudo apt-get -y update

echo "==> Updating indexes [finished]"

References