» S3

Kind: Standard (with locking via DynamoDB)

Stores the state as a given key in a given bucket on Amazon S3. This backend also supports state locking and consistency checking via Dynamo DB, which can be enabled by setting the dynamodb_table field to an existing DynamoDB table name.

» Example Configuration

terraform {
  backend "s3" {
    bucket = "mybucket"
    key    = "path/to/my/key"
    region = "us-east-1"
  }
}

This assumes we have a bucket created called mybucket. The Terraform state is written to the key path/to/my/key.

Note that for the access credentials we recommend using a partial configuration.

» S3 Bucket Permissions

Terraform will need the following AWS IAM permissions on the target backend bucket:

This is seen in the following AWS IAM Statement:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "s3:ListBucket",
      "Resource": "arn:aws:s3:::mybucket"
    },
    {
      "Effect": "Allow",
      "Action": ["s3:GetObject", "s3:PutObject"],
      "Resource": "arn:aws:s3:::mybucket/path/to/my/key"
    }
  ]
}

» DynamoDB Table Permissions

If you are using state locking, Terraform will need the following AWS IAM permissions on the DynamoDB table (arn:aws:dynamodb:::table/mytable):

This is seen in the following AWS IAM Statement:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "dynamodb:GetItem",
        "dynamodb:PutItem",
        "dynamodb:DeleteItem"
      ],
      "Resource": "arn:aws:dynamodb:*:*:table/mytable"
    }
  ]
}

» Using the S3 remote state

To make use of the S3 remote state we can use the terraform_remote_state data source.

data "terraform_remote_state" "network" {
  backend = "s3"
  config {
    bucket = "terraform-state-prod"
    key    = "network/terraform.tfstate"
    region = "us-east-1"
  }
}

The terraform_remote_state data source will return all of the root module outputs defined in the referenced remote state (but not any outputs from nested modules unless they are explicitly output again in the root). An example output might look like:

data.terraform_remote_state.network:
  id = 2016-10-29 01:57:59.780010914 +0000 UTC
  addresses.# = 2
  addresses.0 = 52.207.220.222
  addresses.1 = 54.196.78.166
  backend = s3
  config.% = 3
  config.bucket = terraform-state-prod
  config.key = network/terraform.tfstate
  config.region = us-east-1
  elb_address = web-elb-790251200.us-east-1.elb.amazonaws.com
  public_subnet_id = subnet-1e05dd33

» Configuration variables

The following configuration options or environment variables are supported:

  • bucket - (Required) The name of the S3 bucket.
  • key - (Required) The path to the state file inside the bucket. When using a non-default workspace, the state path will be /workspace_key_prefix/workspace_name/key
  • region / AWS_DEFAULT_REGION - (Optional) The region of the S3 bucket.
  • endpoint / AWS_S3_ENDPOINT - (Optional) A custom endpoint for the S3 API.
  • encrypt - (Optional) Whether to enable server side encryption of the state file.
  • acl - Canned ACL to be applied to the state file.
  • access_key / AWS_ACCESS_KEY_ID - (Optional) AWS access key.
  • secret_key / AWS_SECRET_ACCESS_KEY - (Optional) AWS secret access key.
  • kms_key_id - (Optional) The ARN of a KMS Key to use for encrypting the state.
  • lock_table - (Optional, Deprecated) Use dynamodb_table instead.
  • dynamodb_table - (Optional) The name of a DynamoDB table to use for state locking and consistency. The table must have a primary key named LockID. If not present, locking will be disabled.
  • profile - (Optional) This is the AWS profile name as set in the shared credentials file.
  • shared_credentials_file - (Optional) This is the path to the shared credentials file. If this is not set and a profile is specified, ~/.aws/credentials will be used.
  • token - (Optional) Use this to set an MFA token. It can also be sourced from the AWS_SESSION_TOKEN environment variable.
  • role_arn - (Optional) The role to be assumed.
  • assume_role_policy - (Optional) The permissions applied when assuming a role.
  • external_id - (Optional) The external ID to use when assuming the role.
  • session_name - (Optional) The session name to use when assuming the role.
  • workspace_key_prefix - (Optional) The prefix applied to the state path inside the bucket. This is only relevant when using a non-default workspace. This defaults to "env:"
  • skip_credentials_validation - (Optional) Skip the credentials validation via the STS API.
  • skip_get_ec2_platforms - (Optional) Skip getting the supported EC2 platforms.
  • skip_region_validation - (Optional) Skip validation of provided region name.
  • skip_requesting_account_id - (Optional) Skip requesting the account ID.
  • skip_metadata_api_check - (Optional) Skip the AWS Metadata API check.

» Multi-account AWS Architecture

A common architectural pattern is for an organization to use a number of separate AWS accounts to isolate different teams and environments. For example, a "staging" system will often be deployed into a separate AWS account than its corresponding "production" system, to minimize the risk of the staging environment affecting production infrastructure, whether via rate limiting, misconfigured access controls, or other unintended interactions.

The S3 backend can be used in a number of different ways that make different tradeoffs between convenience, security, and isolation in such an organization. This section describes one such approach that aims to find a good compromise between these tradeoffs, allowing use of Terraform's workspaces feature to switch conveniently between multiple isolated deployments of the same configuration.

Use this section as a starting-point for your approach, but note that you will probably need to make adjustments for the unique standards and regulations that apply to your organization. You will also need to make some adjustments to this approach to account for existing practices within your organization, if for example other tools have previously been used to manage infrastructure.

Terraform is an administrative tool that manages your infrastructure, and so ideally the infrastructure that is used by Terraform should exist outside of the infrastructure that Terraform manages. This can be achieved by creating a separate administrative AWS account which contains the user accounts used by human operators and any infrastructure and tools used to manage the the other accounts. Isolating shared administrative tools from your main environments has a number of advantages, such as avoiding accidentally damaging the administrative infrastructure while changing the target infrastructure, and reducing the risk that an attacker might abuse production infrastructure to gain access to the (usually more privileged) administrative infrastructure.

» Administrative Account Setup

Your administrative AWS account will contain at least the following items:

  • One or more IAM user for system administrators that will log in to maintain infrastructure in the other accounts.
  • Optionally, one or more IAM groups to differentiate between different groups of users that have different levels of access to the other AWS accounts.
  • An S3 bucket that will contain the Terraform state files for each workspace.
  • A DynamoDB table that will be used for locking to prevent concurrent operations on a single workspace.

Provide the S3 bucket name and DynamoDB table name to Terraform within the S3 backend configuration using the bucket and dynamodb_table arguments respectively, and configure a suitable workspace_key_prefix to contain the states of the various workspaces that will subsequently be created for this configuration.

» Environment Account Setup

For the sake of this section, the term "environment account" refers to one of the accounts whose contents are managed by Terraform, separate from the administrative account described above.

Your environment accounts will eventually contain your own product-specific infrastructure. Along with this it must contain one or more IAM roles that grant sufficient access for Terraform to perform the desired management tasks.

» Delegating Access

Each Administrator will run Terraform using credentials for their IAM user in the administrative account. IAM Role Delegation is used to grant these users access to the roles created in each environment account.

Full details on role delegation are covered in the AWS documentation linked above. The most important details are:

  • Each role's Assume Role Policy must grant access to the administrative AWS account, which creates a trust relationship with the administrative AWS account so that its users may assume the role.
  • The users or groups within the administrative account must also have a policy that creates the converse relationship, allowing these users or groups to assume that role.

Since the purpose of the administrative account is only to host tools for managing other accounts, it is useful to give the administrative accounts restricted access only to the specific operations needed to assume the environment account role and access the Terraform state. By blocking all other access, you remove the risk that user error will lead to staging or production resources being created in the administrative account by mistake.

When configuring Terraform, use either environment variables or the standard credentials file ~/.aws/credentials to provide the administrator user's IAM credentials within the administrative account to both the S3 backend and to Terraform's AWS provider.

Use conditional configuration to pass a different assume_role value to the AWS provider depending on the selected workspace. For example:

variable "workspace_iam_roles" {
  default = {
    staging    = "arn:aws:iam::STAGING-ACCOUNT-ID:role/Terraform"
    production = "arn:aws:iam::PRODUCTION-ACCOUNT-ID:role/Terraform"
  }
}

provider "aws" {
  # No credentials explicitly set here because they come from either the
  # environment or the global credentials file.

  assume_role = "${var.workspace_iam_roles[terraform.workspace]}"
}

If workspace IAM roles are centrally managed and shared across many separate Terraform configurations, the role ARNs could also be obtained via a data source such as terraform_remote_state to avoid repeating these values.

» Creating and Selecting Workspaces

With the necessary objects created and the backend configured, run terraform init to initialize the backend and establish an initial workspace called "default". This workspace will not be used, but is created automatically by Terraform as a convenience for users who are not using the workspaces feature.

Create a workspace corresponding to each key given in the workspace_iam_roles variable value above:

$ terraform workspace new staging
Created and switched to workspace "staging"!

...

$ terraform workspace new production
Created and switched to workspace "production"!

...

Due to the assume_role setting in the AWS provider configuration, any management operations for AWS resources will be performed via the configured role in the appropriate environment AWS account. The backend operations, such as reading and writing the state from S3, will be performed directly as the administrator's own user within the administrative account.

$ terraform workspace select staging
$ terraform apply
...

» Running Terraform in Amazon EC2

Teams that make extensive use of Terraform for infrastructure management often run Terraform in automation to ensure a consistent operating environment and to limit access to the various secrets and other sensitive information that Terraform configurations tend to require.

When running Terraform in an automation tool running on an Amazon EC2 instance, consider running this instance in the administrative account and using an instance profile in place of the various administrator IAM users suggested above. An IAM instance profile can also be granted cross-account delegation access via an IAM policy, giving this instance the access it needs to run Terraform.

To isolate access to different environment accounts, use a separate EC2 instance for each target account so that its access can be limited only to the single account.

Similar approaches can be taken with equivalent features in other AWS compute services, such as ECS.

» Protecting Access to Workspace State

In a simple implementation of the pattern described in the prior sections, all users have access to read and write states for all workspaces. In many cases it is desirable to apply more precise access constraints to the Terraform state objects in S3, so that for example only trusted administrators are allowed to modify the production state, or to control reading of a state that contains sensitive information.

Amazon S3 supports fine-grained access control on a per-object-path basis using IAM policy. A full description of S3's access control mechanism is beyond the scope of this guide, but an example IAM policy granting access to only a single state object within an S3 bucket is shown below:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "s3:ListBucket",
      "Resource": "arn:aws:s3:::myorg-terraform-states"
    },
    {
      "Effect": "Allow",
      "Action": ["s3:GetObject", "s3:PutObject"],
      "Resource": "arn:aws:s3:::myorg-terraform-states/myapp/production/tfstate"
    }
  ]
}

It is not possible to apply such fine-grained access control to the DynamoDB table used for locking, so it is possible for any user with Terraform access to lock any workspace state, even if they do not have access to read or write that state. If a malicious user has such access they could block attempts to use Terraform against some or all of your workspaces as long as locking is enabled in the backend configuration.