How AWS Control Tower Works

This section describes at a high level how AWS Control Tower works. Your landing zone is a well-architected multi-account environment for all of your AWS resources. You can use this environment to enforce compliance regulations on all of your AWS accounts.

Structure of an AWS Control Tower Landing Zone

The structure of a landing zone in AWS Control Tower is as follows:

• Root – The parent that contains all other OUs in your landing zone.

• Core OU – This OU contains the log archive and audit member accounts. These accounts often are referred to as shared accounts.

• Custom OU – The custom OU is created when you launch your landing zone. This and other member OUs contain the member accounts that your users work with to perform their AWS workloads.

• AWS SSO directory – This directory houses your AWS SSO users. It defines the scope of permissions for each AWS SSO user.

• AWS SSO users – These are the identities that your users can assume to perform their AWS workloads in your landing zone.

What Happens When You Set Up a Landing Zone

When you set up a landing zone, AWS Control Tower performs the following actions in your management account on your behalf:

• Creates three Organizations organizational units (OUs): Root, Core, and Custom.

• Creates two shared accounts: the log archive account and audit account.

• Creates a cloud-native directory in AWS SSO, with preconfigured groups and single sign-on access.

• Applies 20 preventive guardrails to enforce policies.

• Applies six detective guardrails to detect configuration violations.

• Preventive guardrails are not applied to the management account.

• Except for the management account, guardrails are applied to the organization as a whole.

Safely Managing Resources Within Your AWS Control Tower Landing Zone and Accounts

• When you create your landing zone, a number of AWS resources are created. To use AWS Control Tower, you must not modify or delete these AWS Control Tower managed resources outside of the supported methods described in this guide. Deleting or modifying these resources will cause your landing zone to enter an unknown state. For details, see Guidance for Creating and Modifying AWS Control Tower Resources

• When you enable guardrails with strongly recommended guidance, AWS Control Tower creates AWS resources that it manages in your accounts. Do not modify or delete resources created by AWS Control Tower. Doing so can result in the guardrails entering an unknown state. For more information, see Guardrail Reference.

What Are the Shared Accounts?

In AWS Control Tower, three shared accounts in your landing zone are not provisioned in Account Factory: the management account, the log archive account, and the audit account.

What is the management account?

This is the account that you created specifically for your landing zone. This account is used for billing for everything in your landing zone. It's also used for Account Factory provisioning of accounts, as well as to manage OUs and guardrails.

Note

It is not recommended to run any type of production workloads from an AWS Control Tower management account. Create a separate AWS Control Tower account to run your workloads.

When you set up your landing zone, the following AWS resources are created within your management account.

AWS service	Resource type	Resource name
AWS Organizations	Accounts	audit log archive
AWS Organizations	OUs	Core Custom
AWS Organizations	Service Control Policies	aws-guardrails-*
AWS CloudFormation	Stacks	AWSControlTowerBP-BASELINE-CLOUDTRAIL-MASTER
AWS CloudFormation	StackSets	AWSControlTowerBP-BASELINE-CLOUDTRAIL AWSControlTowerBP-BASELINE-CLOUDWATCH AWSControlTowerBP-BASELINE-CONFIG AWSControlTowerBP-BASELINE-ROLES AWSControlTowerBP-BASELINE-SERVICE-ROLES AWSControlTowerBP-SECURITY-TOPICS AWSControlTowerGuardrailAWS-GR-AUDIT-BUCKET-PUBLIC-READ-PROHIBITED AWSControlTowerGuardrailAWS-GR-AUDIT-BUCKET-PUBLIC-WRITE-PROHIBITED AWSControlTowerLoggingResources AWSControlTowerSecurityResources
AWS Service Catalog	Product	AWS Control Tower Account Factory
AWS CloudTrail	Trail	aws-controltower-BaselineCloudTrail
Amazon CloudWatch	CloudWatch Logs	aws-controltower/CloudTrailLogs
AWS Identity and Access Management	Roles	AWSControlTowerAdmin AWSControlTowerStackSetRole AWSControlTowerCloudTrailRolePolicy
AWS Identity and Access Management	Policies	AWSControlTowerServiceRolePolicy AWSControlTowerAdminPolicy AWSControlTowerCloudTrailRolePolicy AWSControlTowerStackSetRolePolicy
AWS Single Sign-On	Directory groups	AWSAccountFactory AWSAuditAccountAdmins AWSControlTowerAdmins AWSLogArchiveAdmins AWSLogArchiveViewers AWSSecurityAuditors AWSSecurityAuditPowerUsers AWSServiceCatalogAdmins
AWS Single Sign-On	Permission Sets	AWSAdministratorAccess AWSPowerUserAccess AWSServiceCatalogAdminFullAccess AWSServiceCatalogEndUserAccess AWSReadOnlyAccess AWSOrganizationsFullAccess

What is the log archive account?

This account works as a repository for logs of API activities and resource configurations from all accounts in the landing zone.

When you set up your landing zone, the following AWS resources are created within your log archive account.

AWS service	Resource type	Resource Name
AWS CloudFormation	Stacks	StackSet-AWSControlTowerGuardrailAWS-GR-AUDIT-BUCKET-PUBLIC-READ-PROHIBITED- StackSet-AWSControlTowerGuardrailAWS-GR-AUDIT-BUCKET-PUBLIC-WRITE-PROHIBITED StackSet-AWSControlTowerBP-BASELINE-CLOUDWATCH- StackSet-AWSControlTowerBP-BASELINE-CONFIG- StackSet-AWSControlTowerBP-BASELINE-CLOUDTRAIL- StackSet-AWSControlTowerBP-BASELINE-SERVICE-ROLES- StackSet-AWSControlTowerBP-BASELINE-ROLES- StackSet-AWSControlTowerLoggingResources-
AWS Config	AWS Config Rules	AWSControlTower_AWS-GR_AUDIT_BUCKET_PUBLIC_READ_PROHIBITED AWSControlTower_AWS-GR_AUDIT_BUCKET_PUBLIC_WRITE_PROHIBIT
AWS CloudTrail	Trails	aws-controltower-BaselineCloudTrail
Amazon CloudWatch	CloudWatch Event Rules	aws-controltower-ConfigComplianceChangeEventRule
Amazon CloudWatch	CloudWatch Logs	aws-controltower/CloudTrailLogs /aws/lambda/aws-controltower-NotificationForwarder
AWS Identity and Access Management	Roles	aws-controltower-AdministratorExecutionRole aws-controltower-CloudWatchLogsRole aws-controltower-ConfigRecorderRole aws-controltower-ForwardSnsNotificationRole aws-controltower-ReadOnlyExecutionRole AWSControlTowerExecution
AWS Identity and Access Management	Policies	AWSControlTowerServiceRolePolicy
Amazon Simple Notification Service	Topics	aws-controltower-SecurityNotifications
AWS Lambda	Applications	StackSet-AWSControlTowerBP-BASELINE-CLOUDWATCH-*
AWS Lambda	Functions	aws-controltower-NotificationForwarder
Amazon Simple Storage Service	Buckets	aws-controltower-logs-* aws-controltower-s3-access-logs-*

What is the audit account?

The audit account is a restricted account that's designed to give your security and compliance teams read and write access to all accounts in your landing zone. From the audit account, you have programmatic access to review accounts, by means of a role that is granted to Lambda functions only. The audit account does not allow you to log in to other accounts manually. For more information about Lambda functions and roles, see Configure a Lambda function to assume a role from another AWS account.

When you set up your landing zone, the following AWS resources are created within your audit account.

AWS service	Resource type	Resource name
AWS CloudFormation	Stacks	StackSet-AWSControlTowerGuardrailAWS-GR-AUDIT-BUCKET-PUBLIC-READ-PROHIBITED- StackSet-AWSControlTowerGuardrailAWS-GR-AUDIT-BUCKET-PUBLIC-WRITE-PROHIBITED- StackSet-AWSControlTowerBP-BASELINE-CLOUDWATCH- StackSet-AWSControlTowerBP-BASELINE-CONFIG- StackSet-AWSControlTowerBP-BASELINE-CLOUDTRAIL- StackSet-AWSControlTowerBP-BASELINE-SERVICE-ROLES- StackSet-AWSControlTowerBP-SECURITY-TOPICS- StackSet-AWSControlTowerBP-BASELINE-ROLES- StackSet-AWSControlTowerSecurityResources-*
AWS Config	Aggregator	aws-controltower-GuardrailsComplianceAggregator
AWS Config	AWS Config Rules	AWSControlTower_AWS-GR_AUDIT_BUCKET_PUBLIC_READ_PROHIBITED AWSControlTower_AWS-GR_AUDIT_BUCKET_PUBLIC_WRITE_PROHIBITED
AWS CloudTrail	Trail	aws-controltower-BaselineCloudTrail
Amazon CloudWatch	CloudWatch Event Rules	aws-controltower-ConfigComplianceChangeEventRule
Amazon CloudWatch	CloudWatch Logs	aws-controltower/CloudTrailLogs /aws/lambda/aws-controltower-NotificationForwarder
AWS Identity and Access Management	Roles	aws-controltower-AdministratorExecutionRole aws-controltower-CloudWatchLogsRole aws-controltower-ConfigRecorderRole aws-controltower-ForwardSnsNotificationRole aws-controltower-ReadOnlyExecutionRole aws-controltower-SecurityAdministratorRole aws-controltower-SecurityReadOnlyRole AWSControlTowerExecution
AWS Identity and Access Management	Policies	AWSControlTowerServiceRolePolicy
Amazon Simple Notification Service	Topics	aws-controltower-AggregateSecurityNotifications aws-controltower-AllConfigNotifications aws-controltower-SecurityNotifications
AWS Lambda	Functions	aws-controltower-NotificationForwarder

How Guardrails Work

A guardrail is a high-level rule that provides ongoing governance for your overall AWS environment. Each guardrail enforces a single rule, and it's expressed in plain language. Compliance needs evolve, and you can change the elective or strongly recommended guardrails that are in force, at any time, from the AWS Control Tower console. Mandatory guardrails are always applied, and they can't be changed.

Preventive guardrails prevent actions from occurring. For example, the Disallow policy changes to log archive guardrail prevents any IAM policy changes within the log archive shared account. Any attempt to perform a prevented action is denied and logged in CloudTrail. The resource is also logged in AWS Config.

Detective guardrails detect specific events when they occur and log the action in CloudTrail. For example, the Enable encryption for EBS volumes attached to EC2 instances detects if an unencrypted Amazon EBS volume is attached to an EC2 instance in your landing zone.

Related Topics

• Guardrails in AWS Control Tower

• Detect and resolve drift in AWS Control Tower

How AWS Regions Work With AWS Control Tower

Currently, AWS Control Tower is supported in the following AWS Regions:

• US East (N. Virginia)

• US East (Ohio)

• US West (Oregon)

• Canada (Central) Region

• Asia Pacific (Sydney)

• Asia Pacific (Singapore) Region

• Europe (Frankfurt) Region

• Europe (Ireland)

• Europe (London) Region

• Europe (Stockholm) Region

When you create a landing zone, the region that you're using for access to the AWS Management Console becomes your home AWS Region for AWS Control Tower. During the creation process, some resources are provisioned in the home AWS Region. Other resources, such as OUs and AWS accounts, are global.

Currently, all preventive guardrails work globally. Detective guardrails, however, only work in regions where AWS Control Tower is supported. For more information about the behavior of guardrails when you activate AWS Control Tower in a new region, see Deploying AWS Control Tower to a new AWS Region.

How AWS Control Tower Works With Roles to Create and Manage Accounts

AWS Control Tower creates a customer's account by calling the CreateAccount API of AWS Organizations. When AWS Organizations creates this account, it creates a role within that account, which AWS Control Tower names by passing in a parameter to the API. The name of the role is AWSControlTowerExecution.

AWS Control Tower takes over the AWSControlTowerExecution role for all accounts created by Account Factory. Using this role, AWS Control Tower baselines the account and applies mandatory (and any other enabled) guardrails, which results in creation of other roles. These roles in turn are used by other services, such as AWS Config.

Note

To baseline an account is to set up its blueprints and guardrails. The baselining process also sets up the centralized logging and security audit roles on the account, as part of deploying the blueprints. AWS Control Tower baselines are contained in the roles that you apply to every managed account.

The AWSControlTowerExecution role, explained

The AWSControlTowerExecution role allows AWS Control Tower to manage your individual accounts and report information about them to your audit and logging accounts.

• AWSControlTowerExecution allows auditing by the AWS Control Tower audit account.

• AWSControlTowerExecution helps you configure your organizations's logging, so that all the logs for every account are sent to the logging account.

After you’ve completed setting up accounts, AWSControlTowerExecution ensures that your selected AWS Control Tower guardrails apply automatically to every individual account in your organization, as well as to every new account you create in AWS Control Tower. Therefore, you can provide compliance and security reports with ease, based on the auditing and logging features embodied by AWS Control Tower guardrails. Your security and compliance teams can verify that all requirements are met, and that no organizational drift has occurred. For more information about drift, see the AWS Control Tower User Guide.

To summarize, the AWSControlTowerExecution role and its associated policy gives you flexible control of security and compliance across your entire organization. Therefore, breaches of security are less likely to occur.

How AWS Control Tower aggregates unmanaged OUs and accounts

The AWS Control Tower management account creates an organization-level aggregator, which assists in detecting external AWS Config rules, so that AWS Control Tower does not need to gain access to unmanaged accounts. The AWS Control Tower console shows you how many externally created AWS Config rules you have for a given account, and links you to the AWS Config console, where you can view details about those external rules.

To create the aggregator, AWS Control Tower adds a role with the permissions required to describe an organization and list the accounts under it. The AWSControlTowerConfigAggregatorRoleForOrganizations role requires the AWSConfigRoleForOrganizations managed policy and a trust relationship with config.amazonaws.com.

Here is the artifact for the role:

    
  {
    "Version": "2012-10-17",
      "Statement": [
       {
        "Effect": "Allow",
        "Action": [
          "organizations:ListAccounts",
          "organizations:DescribeOrganization",
          "organizations:ListAWSServiceAccessForOrganization"
         ],
       "Resource": "*"
      }
    ]
  }
  
    

Here is the AWSControlTowerConfigAggregatorRoleForOrganizations trust relationship:

 
  {
    "Version": "2012-10-17",
      "Statement": [
      {
        "Effect": "Allow",
        "Principal": {
        "Service": "config.amazonaws.com"
        },
        "Action": "sts:AssumeRole"
      }
    ]
  }
}

To deploy this functionality in the management account, the following permissions are added in the managed policy AWSControlTowerServiceRolePolicy, which is used by the AWSControlTowerAdmin role when it creates the AWS Config aggregator:

   
{
  "Version": "2012-10-17",
    "Statement": [
      {
        "Effect": "Allow",
        "Action": [
          "config:PutConfigurationAggregator",
          "config:DeleteConfigurationAggregator",
          "iam:PassRole"
          ],
        "Resource": [
          "arn:aws:iam:::role/service-role/AWSControlTowerConfigAggregatorRoleForOrganizations",
          "arn:aws:config:::config-aggregator/"
          ]
        },
      {
        "Effect": "Allow",
        "Action": "organizations:EnableAWSServiceAccess",
        "Resource": "*"
      }
    ]
}

New resources created: AWSControlTowerConfigAggregatorRoleForOrganizations and aws-controltower-ConfigAggregatorForOrganizations

When you are ready, you can enroll accounts individually, or enroll them as a group by registering an OU. When you've enrolled an account, if you create a rule in AWS Config, AWS Control Tower detects the new rule and updates the account's list of enabled guardrails in the AWS Control Tower console to show that you have an external rule. The aggregator provides the number of external rules and provides a link to the AWS Config console where you can view the details of each external rule for your account. Use the information in the AWS Config console and the AWS Control Tower console to determine whether you have the appropriate guardrails enabled for the account.

Note

To link directly from the AWS Control Tower console to your aggregated list of AWS Config rules, configure your AWS Config console with the Config Recorder and Delivery Channel in the home region of your management account.

How AWS Control Tower Works With StackSets

AWS Control Tower uses CloudFormation StackSets to set up resources in your accounts. Each stack set has StackInstances that correspond to multiple accounts, and to multiple AWS Regions per account. Control Tower applies updates to certain accounts and AWS Regions selectively, based on certain CloudFormation parameters. When updates are applied to some stack instances, other stack instances may be left in Outdated status. This behavior is expected and normal.

When a stack instance goes into Outdated status, it usually means that the stack corresponding to that stack instance is not aligned with the latest template in the stack set. The stack remains in the older template, so it might not include the latest resources or parameters. The stack is still completely usable.

Here's a quick summary of what behavior to expect, based on AWS CloudFormation parameters that are specified during an update:

If the stack set update includes changes to the template (that is, if the TemplateBody or TemplateURL properties are specified), or if the Parameters property is specified, AWS CloudFormation marks all stack instances with a status of Outdated prior to updating the stack instances in the specified accounts and AWS Regions. If the stack set update does not include changes to the template or parameters, AWS CloudFormation updates the stack instances in the specified accounts and Regions, while leaving all other stack instances with their existing stack instance status. To update all of the stack instances associated with a stack set, do not specify the Accounts or Regions properties.

For more information, see Update Your Stack Set in the AWS CloudFormation User Guide.