---
description: >
  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.
layout: docs
page_title: Amazon chroot - Builders
sidebar_title: 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](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ComponentsAMIs.html#storage-for-the-root-device).

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](/docs/builders/amazon-ebs), 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](https://en.wikipedia.org/wiki/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.

## Chroot Specific Configuration Reference

There are many configuration options available for the builder. In addition to
the items listed here, you will want to look at the general configuration
references for [AMI](#ami-configuration),
[BlockDevices](#block-devices-configuration) and
[Access](#access-config-configuration) configuration references, which are
necessary for this build to succeed and can be found further down the page.

#### Required:

@include 'builder/amazon/chroot/Config-required.mdx'

#### Optional:

@include 'builder/amazon/chroot/Config-not-required.mdx'

## General Common Configuration Reference

Following will be a set of fields that are also settable for other aws
builders.

### AMI Configuration

#### Required:

@include 'builder/amazon/common/AMIConfig-required.mdx'

#### Optional:

@include 'builder/amazon/common/AMIConfig-not-required.mdx'

### Block Devices Configuration

Block devices can be nested in the
[ami_block_device_mappings](#ami_block_device_mappings) array.

@include 'builder/amazon/common/BlockDevice.mdx'

#### Optional:

@include 'builder/amazon/common/BlockDevice-not-required.mdx'

### Access Config Configuration

#### Required:

@include 'builder/amazon/common/AccessConfig-required.mdx'

#### Optional:

@include 'builder/amazon/common/AccessConfig-not-required.mdx'

## Basic Example

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

```json
{
  "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 `/proc` and `/dev`:

```json
{
  "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

### Unmounting the Filesystem

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](http://people.debian.org/~hmh/invokerc.d-policyrc.d-specification.txt)
file which will prevent packages installed by your provisioners from starting
services:

```json
({
  "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"]
})
```

### Ansible provisioner

Running ansible against `amazon-chroot` requires changing the Ansible
connection to chroot and running Ansible as root/sudo.

### Using Instances with NVMe block devices.

In C5, C5d, M5, and i3.metal instances, EBS volumes are exposed as NVMe block
devices
[reference](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/nvme-ebs-volumes.html).
In order to correctly mount these devices, you have to do some extra legwork,
involving the `nvme_device_path` option above. Read that for more information.

A working example for mounting an NVMe device is below:

```json
{
  "variables": {
    "region": "us-east-2"
  },
  "builders": [
    {
      "type": "amazon-chroot",
      "region": "{{user `region`}}",
      "source_ami_filter": {
        "filters": {
          "virtualization-type": "hvm",
          "name": "amzn-ami-hvm-*",
          "root-device-type": "ebs"
        },
        "owners": ["137112412989"],
        "most_recent": true
      },
      "ena_support": true,
      "ami_name": "amazon-chroot-test-{{timestamp}}",
      "nvme_device_path": "/dev/nvme1n1p",
      "device_path": "/dev/sdf"
    }
  ],

  "provisioners": [
    {
      "type": "shell",
      "inline": ["echo Test > /tmp/test.txt"]
    }
  ]
}
```

Note that in the `nvme_device_path` you must end with the `p`; if you try to
define the partition in this path (e.g. `nvme_device_path`: `/dev/nvme1n1p1`)
and haven't also set the `"mount_partition": 0`, a `1` will be appended to the
`nvme_device_path` and Packer will fail.

## 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.

```json
{
  "type": "amazon-chroot",
  "ami_name": "packer-from-scratch {{timestamp}}",
  "from_scratch": true,
  "ami_virtualization_type": "hvm",
  "pre_mount_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"
    }
  ]
}
```

## Build template data

In configuration directives marked as a template engine above, the following
variables are available:

- `BuildRegion` - The region (for example `eu-central-1`) where Packer is
  building the AMI.
- `SourceAMI` - The source AMI ID (for example `ami-a2412fcd`) used to build
  the AMI.
- `SourceAMICreationDate` - The source AMI creation date (for example `"2020-05-14T19:26:34.000Z"`).
- `SourceAMIName` - The source AMI Name (for example
  `ubuntu/images/ebs-ssd/ubuntu-xenial-16.04-amd64-server-20180306`) used to
  build the AMI.
- `SourceAMIOwner` - The source AMI owner ID.
- `SourceAMIOwnerName` - The source AMI owner alias/name (for example `amazon`).
- `SourceAMITags` - The source AMI Tags, as a `map[string]string` object.

## Build function template engine variables

For the build function of [template engine](/docs/templates/engine), the following
variables are available:

- `SourceAMIName` - The source AMI Name (for example
  `ubuntu/images/ebs-ssd/ubuntu-xenial-16.04-amd64-server-20180306`) used to
  build the AMI.
- `Device` - Root device path.
- `MountPath` - Device mounting path.
