ResiLien
/
terraform
2
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

website: guide for using the S3 backend with multiple AWS accounts 

Users commonly ask how the S3 backend can be used in an organization that
splits its infrastructure across many AWS accounts.

We've traditionally shied away from making specific recommendations here
because we can't possibly anticipate the different standards and
regulations that constrain each user. This new section attempts to
describe one possible approach that works well with Terraform's workflow,
with the goal that users make adjustments to it taking into account their
unique needs.

Since we are intentionally not being prescriptive here -- instead
considering this just one of many approaches -- it deviates from our usual
active writing style in several places to avoid giving the impression that
these are instructions to be followed exactly, which in some cases
requires the use of passive voice even though that is contrary to our
documentation style guide. For similar reasons, this section is also
light on specific code examples, since we do not wish to encourage users
to just copy-paste the examples without thinking through the consequences.
This commit is contained in:
Martin Atkins 2017-12-08 13:19:45 -08:00
parent bed1ea1542
commit c729bdff43
1 changed files with 218 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

218
website/docs/backends/types/s3.html.md
View File

@ -147,3 +147,221 @@ The following configuration options or environment variables are supported:
* `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](/docs/state/workspaces.html) 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](http://docs.aws.amazon.com/IAM/latest/UserGuide/id_users.html)
for system administrators that will log in to maintain infrastructure in
the other accounts.
* Optionally, one or more [IAM groups](http://docs.aws.amazon.com/IAM/latest/UserGuide/id_groups.html)
to differentiate between different groups of users that have different
levels of access to the other AWS accounts.
* An [S3 bucket](http://docs.aws.amazon.com/AmazonS3/latest/dev/UsingBucket.html)
that will contain the Terraform state files for each workspace.
* A [DynamoDB table](http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/HowItWorks.CoreComponents.html#HowItWorks.CoreComponents.TablesItemsAttributes)
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 `lock_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](http://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles.html)
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](http://docs.aws.amazon.com/IAM/latest/UserGuide/tutorial_cross-account-with-roles.html)
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:
```hcl
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`](/docs/providers/terraform/d/remote_state.html)
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 worspace 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](/guides/running-terraform-in-automation.html)
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](http://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_use_switch-role-ec2_instance-profiles.html)
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:
```json
{
"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.