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The `amazon-chroot` Packer builder is able to create Amazon AMIs backed by an EBS volume as the root device. For more information on the difference between instance storage and EBS-backed instances, storage for the root device section in the EC2 documentation.	docs	Amazon AMI Builder (chroot)

AMI Builder (chroot)

Type: amazon-chroot

The amazon-chroot Packer builder is able to create Amazon AMIs backed by an EBS volume as the root device. For more information on the difference between instance storage and EBS-backed instances, see the "storage for the root device" section in the EC2 documentation.

The difference between this builder and the amazon-ebs builder is that this builder is able to build an EBS-backed AMI without launching a new EC2 instance. This can dramatically speed up AMI builds for organizations who need the extra fast build.

~> This is an advanced builder If you're just getting started with Packer, we recommend starting with the amazon-ebs builder, which is much easier to use.

The builder does not manage AMIs. Once it creates an AMI and stores it in your account, it is up to you to use, delete, etc. the AMI.

How Does it Work?

This builder works by creating a new EBS volume from an existing source AMI and attaching it into an already-running EC2 instance. Once attached, a chroot is used to provision the system within that volume. After provisioning, the volume is detached, snapshotted, and an AMI is made.

Using this process, minutes can be shaved off the AMI creation process because a new EC2 instance doesn't need to be launched.

There are some restrictions, however. The host EC2 instance where the volume is attached to must be a similar system (generally the same OS version, kernel versions, etc.) as the AMI being built. Additionally, this process is much more expensive because the EC2 instance must be kept running persistently in order to build AMIs, whereas the other AMI builders start instances on-demand to build AMIs as needed.

Configuration Reference

There are many configuration options available for the builder. They are segmented below into two categories: required and optional parameters. Within each category, the available configuration keys are alphabetized.

Required:

access_key (string) - The access key used to communicate with AWS. Learn how to set this.
ami_name (string) - The name of the resulting AMI that will appear when managing AMIs in the AWS console or via APIs. This must be unique. To help make this unique, use a function like timestamp (see configuration templates for more info)
secret_key (string) - The secret key used to communicate with AWS. Learn how to set this.
source_ami (string) - The source AMI whose root volume will be copied and provisioned on the currently running instance. This must be an EBS-backed AMI with a root volume snapshot that you have access to. Note: this is not used when from_scratch is set to true.

Optional:

ami_description (string) - The description to set for the resulting AMI(s). By default this description is empty.
ami_groups (array of strings) - A list of groups that have access to launch the resulting AMI(s). By default no groups have permission to launch the AMI. all will make the AMI publicly accessible.
ami_product_codes (array of strings) - A list of product codes to associate with the AMI. By default no product codes are associated with the AMI.
ami_regions (array of strings) - A list of regions to copy the AMI to. Tags and attributes are copied along with the AMI. AMI copying takes time depending on the size of the AMI, but will generally take many minutes.
ami_users (array of strings) - A list of account IDs that have access to launch the resulting AMI(s). By default no additional users other than the user creating the AMI has permissions to launch it.
ami_virtualization_type (string) - The type of virtualization for the AMI you are building. This option is required to register HVM images. Can be "paravirtual" (default) or "hvm".
chroot_mounts (array of array of strings) - This is a list of devices to mount into the chroot environment. This configuration parameter requires some additional documentation which is in the "Chroot Mounts" section below. Please read that section for more information on how to use this.
command_wrapper (string) - How to run shell commands. This defaults to {{.Command}}. This may be useful to set if you want to set environmental variables or perhaps run it with sudo or so on. This is a configuration template where the .Command variable is replaced with the command to be run. Defaults to "{{.Command}}".
copy_files (array of strings) - Paths to files on the running EC2 instance that will be copied into the chroot environment prior to provisioning. Defaults to /etc/resolv.conf so that DNS lookups work.
device_path (string) - The path to the device where the root volume of the source AMI will be attached. This defaults to "" (empty string), which forces Packer to find an open device automatically.
enhanced_networking (boolean) - Enable enhanced networking (SriovNetSupport) on HVM-compatible AMIs. If true, add ec2:ModifyInstanceAttribute to your AWS IAM policy.
force_deregister (boolean) - Force Packer to first deregister an existing AMI if one with the same name already exists. Default false.
from_scratch (boolean) - Build a new volume instead of starting from an existing AMI root volume snapshot. Default false. If true, source_ami is no longer used and the following options become required: ami_virtualization_type, pre_mount_commands and root_volume_size. The below options are also required in this mode only:
- ami_block_device_mappings (array of block device mappings) An entry matching root_device_name should be set. See the amazon-ebs documentation for more details on this parameter.
- root_device_name (string) - The root device name. For example, xvda.
mount_path (string) - The path where the volume will be mounted. This is where the chroot environment will be. This defaults to /mnt/packer-amazon-chroot-volumes/{{.Device}}. This is a configuration template where the .Device variable is replaced with the name of the device where the volume is attached.
mount_partition (integer) - The partition number containing the / partition. By default this is the first partition of the volume.
mount_options (array of strings) - Options to supply the mount command when mounting devices. Each option will be prefixed with -o and supplied to the mount command ran by Packer. Because this command is ran in a shell, user discrestion is advised. See this manual page for the mount command for valid file system specific options
pre_mount_commands (array of strings) - A series of commands to execute after attaching the root volume and before mounting the chroot. This is not required unless using from_scratch. If so, this should include any partitioning and filesystem creation commands. The path to the device is provided by {{.Device}}.
post_mount_commands (array of strings) - As pre_mount_commands, but the commands are executed after mounting the root device and before the extra mount and copy steps. The device and mount path are provided by {{.Device}} and {{.MountPath}}.
root_volume_size (integer) - The size of the root volume for the chroot environment, and the resulting AMI
skip_region_validation (boolean) - Set to true if you want to skip validation of the ami_regions configuration option. Defaults to false.
tags (object of key/value strings) - Tags applied to the AMI.

Basic Example

Here is a basic example. It is completely valid except for the access keys:

{
  "type": "amazon-chroot",
  "access_key": "YOUR KEY HERE",
  "secret_key": "YOUR SECRET KEY HERE",
  "source_ami": "ami-e81d5881",
  "ami_name": "packer-amazon-chroot {{timestamp}}"
}

Chroot Mounts

The chroot_mounts configuration can be used to mount specific devices within the chroot. By default, the following additional mounts are added into the chroot by Packer:

/proc (proc)
/sys (sysfs)
/dev (bind to real /dev)
/dev/pts (devpts)
/proc/sys/fs/binfmt_misc (binfmt_misc)

These default mounts are usually good enough for anyone and are sane defaults. However, if you want to change or add the mount points, you may using the chroot_mounts configuration. Here is an example configuration which only mounts /prod and /dev:

{
  "chroot_mounts": [
    ["proc", "proc", "/proc"],
    ["bind", "/dev", "/dev"]
  ]
}

chroot_mounts is a list of a 3-tuples of strings. The three components of the 3-tuple, in order, are:

The filesystem type. If this is "bind", then Packer will properly bind the filesystem to another mount point.
The source device.
The mount directory.

Parallelism

A quick note on parallelism: it is perfectly safe to run multiple separate Packer processes with the amazon-chroot builder on the same EC2 instance. In fact, this is recommended as a way to push the most performance out of your AMI builds.

Packer properly obtains a process lock for the parallelism-sensitive parts of its internals such as finding an available device.

Gotchas

One of the difficulties with using the chroot builder is that your provisioning scripts must not leave any processes running or packer will be unable to unmount the filesystem.

For debian based distributions you can setup a policy-rc.d file which will prevent packages installed by your provisioners from starting services:

{
  "type": "shell",
  "inline": [
    "echo '#!/bin/sh' > /usr/sbin/policy-rc.d",
    "echo 'exit 101' >> /usr/sbin/policy-rc.d",
    "chmod a+x /usr/sbin/policy-rc.d"
  ]
},

// ...

{
  "type": "shell",
  "inline": [
    "rm -f /usr/sbin/policy-rc.d"
  ]
}

Building From Scratch

This example demonstrates the essentials of building an image from scratch. A 15G gp2 (SSD) device is created (overriding the default of standard/magnetic). The device setup commands partition the device with one partition for use as an HVM image and format it ext4. This builder block should be followed by provisioning commands to install the os and bootloader.

{
  "type": "amazon-chroot",
  "ami_name": "packer-from-scratch {{timestamp}}"
  "from_scratch": true,
  "ami_virtualization_type": "hvm",
  "device_setup_commands": [
    "parted {{.Device}} mklabel msdos mkpart primary 1M 100% set 1 boot on print",
    "mkfs.ext4 {{.Device}}1"
  ],
  "root_volume_size": 15,
  "root_device_name": "xvda",
  "ami_block_device_mappings": [
    {
      "device_name": "xvda",
      "delete_on_termination": true,
      "volume_type": "gp2"
    }
  ]
}

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