Building AWS AMIs with Packer

Packer is a powerful tool that allows you to automate the creation of machine images across various platforms, including AWS. In this guide, we will focus on creating an Amazon Machine Image (AMI) while specifying user data that runs at instance startup.

Configuration Overview

To achieve this, we will define a Packer template that includes a user data script. This script will execute unique configurations each time an instance is launched, rather than being baked into the AMI itself.

Packer Template Example

Here’s a sample configuration for creating an AWS AMI using Packer:

{
  "variables": {
    "ami_name": ""
  },
  "builders": [
    {
      "type": "amazon-ebs",
      "region": "us-east-1",
      "source_ami": "ami-c8580bdf",
      "instance_type": "t2.micro",
      "ssh_username": "ubuntu",
      "ami_name": "{{ user `ami_name` }}-{{ isotime | clean_ami_name }}",
      "user_data_file": "user_data.sh",
      "tags": {
        "os_version": "ubuntu",
        "built_by": "packer",
        "build_on": "{{ isotime | clean_ami_name }}",
        "Name": "{{ user `ami_name` }}"
      }
    }
  ],
  "provisioners": [
    {
      "type": "ansible",
      "playbook_file": "playbook.yml",
      "user": "ubuntu"
    }
  ]
}

Explanation of Key Components

• Builders: This section defines how the AMI will be built. We specify the region, source AMI, instance type, and user data file.
• User Data File: The user_data_file parameter points to a shell script (user_data.sh) that contains commands to be executed upon instance startup.
• Provisioners: This section allows you to run additional scripts or configuration management tools (like Ansible) to set up the instance before it is converted into an AMI.

Troubleshooting User Data Execution

After running the Packer build, if you find that the user data script did not execute (e.g., no logs in /var/lib/cloud/instance/user-data.txt), consider the following:

• Ensure that the user data script is correctly formatted and executable.
• Check the instance's cloud-init logs for any errors that might indicate why the script did not run.
• Verify that the instance type supports the features you are trying to use.

By following this guide, you should be able to successfully create an AWS AMI with Packer while ensuring that your user data scripts run as intended during instance initialization.