Question

Can you associate multiple Network ACLs with a subnet?

Answer 1

In AWS, you cannot associate multiple Network ACLs directly with a subnet. Each subnet in a VPC can have only one Network ACL associated with it at any given time. This design ensures that network traffic entering or leaving a subnet is governed by a single set of rules defined within that Network ACL.

How Network ACLs Work with Subnets

1. Default Network ACL:

   • Every VPC comes with a default Network ACL, which is initially associated with all subnets within the VPC.
   • The default Network ACL allows all inbound and outbound traffic by default until you modify its rules.

2. Custom Network ACLs:

   • You can create custom Network ACLs to define specific rules for inbound and outbound traffic.
   • Each custom Network ACL can have its own set of rules tailored to meet different security and access requirements.

3. Association with Subnets:

   • When you create a custom Network ACL, you have the option to associate it with specific subnets within the VPC.
   • You can associate one custom Network ACL with each subnet.
   • Once associated, the rules defined in the Network ACL apply to all traffic entering or leaving the instances within that subnet.

Use Case Considerations

• Layered Security: While you can't associate multiple Network ACLs with a single subnet, you can achieve layered security by strategically placing subnets with different Network ACLs in your VPC. This allows you to enforce different security policies for different parts of your network architecture.

• Security Best Practices: It’s generally recommended to use Security Groups for instance-level security (allowing or denying specific types of traffic to individual instances) and Network ACLs for subnet-level control (enforcing traffic rules for all instances within a subnet).

Management and Configuration

• Creating and Associating Network ACLs:

  • You can create custom Network ACLs using the AWS Management Console, AWS CLI, or SDKs.
  • Associate a custom Network ACL with a subnet using the AWS Management Console or AWS CLI (associate-network-acl command).

• Rule Evaluation:

  • Network ACL rules are evaluated in order, starting with the lowest numbered rule.
  • An explicit allow or deny rule can override subsequent rules in the list.

While AWS does not support associating multiple Network ACLs directly with a subnet, you can achieve flexible and granular network security by carefully planning your VPC architecture and using Security Groups and Network ACLs effectively. By strategically placing subnets with appropriate Network ACLs, you can enforce comprehensive security policies and control traffic flow within your AWS environment.