How Firewall Manager initiates network ACL management for a subnet - AWS WAF, AWS Firewall Manager, and AWS Shield Advanced

How Firewall Manager initiates network ACL management for a subnet

This section describes how Firewall Manager initiates network ACL management for a subnet.

Firewall Manager begins management of the network ACL for a subnet when it associates the subnet with a network ACL that Firewall Manager has created and tagged with FMManaged set to true.

Compliance with a network ACL policy requires the subnet's network ACL to have the policy's first rules positioned first, in the order specified in the policy, the last rules positioned last, in order, and any other custom rules positioned in the middle. These requirements can be satisfied by an unmanaged network ACL that the subnet is already associated with or by a managed network ACL.

When Firewall Manager applies a network ACL policy to a subnet that's associated with an unmanaged network ACL, Firewall Manager checks the following in order, stopping when it identifies a viable option:

  1. The associated network ACL is already compliant – If the network ACL that's currently associated with the subnet is compliant, then Firewall Manager leaves that association in place and does not start network ACL management for the subnet.

    Firewall Manager doesn't alter or otherwise manage a network ACL that it doesn't own, but as long as it's compliant, Firewall Manager leaves it in place and just monitors it for policy compliance.

  2. A compliant managed network ACL is available – If Firewall Manager is already managing a network ACL that complies with the required configuration, then this is an option. If remediation is enabled, Firewall Manager associates the subnet to it. If remediation is disabled, Firewall Manager marks the subnet noncompliant and offers replacing the network ACL association as a remediation option.

  3. Create a new compliant managed network ACL – If remediation is enabled, Firewall Manager creates a new network ACL and associates it with the subnet. Otherwise, Firewall Manager marks the subnet noncompliant and offers the remediation options of creating the new network ACL and replacing the network ACL association.

If these steps fail, Firewall Manager reports noncompliance for the subnet.

Firewall Manager follows these steps when a subnet first comes into scope and when a subnet's unmanaged network ACL is out of compliance.