Question

What is an AWS Network ACL (NACL)?

Answer 1

An AWS Network Access Control List (NACL), often referred to simply as a Network ACL, is a virtual firewall that operates at the subnet level within your Amazon Virtual Private Cloud (VPC). It acts as an additional layer of defense by controlling inbound and outbound traffic at the subnet boundary. Here are the key characteristics and features of AWS Network ACLs:

Key Features and Characteristics:

1. Subnet-Level Control:

   • Network ACLs are associated with subnets within your VPC, not with individual instances.
   • They help control traffic entering and leaving the subnet itself, affecting all instances within that subnet equally.

2. Stateless Filtering:

   • Network ACLs are stateless, meaning you must explicitly define rules for both inbound and outbound traffic separately.
   • Each rule applies to traffic in one direction (either inbound or outbound) and must be separately defined for the opposite direction.

3. Rule Evaluation and Order:

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

4. Rule Configuration:

   • Rules in Network ACLs are defined based on IP protocols (TCP, UDP, ICMP) and port number ranges.
   • You can create both allow and deny rules to control traffic flow to and from subnets.

5. Default Behavior:

   • By default, a Network ACL denies all inbound and outbound traffic.
   • You must explicitly define rules to allow specific types of traffic; otherwise, traffic is implicitly denied.

6. Logging and Monitoring:

   • You can enable logging on a Network ACL to capture detailed information about traffic flow, which helps in troubleshooting and monitoring network activities.

Use Cases:

• Network Segmentation: Implementing different security policies for different subnets based on organizational requirements.
• Compliance Requirements: Enforcing compliance standards by restricting traffic flow to and from subnets based on specific policies.
• Traffic Control: Controlling traffic between different subnets within the same VPC or to/from external networks.

Management and Configuration:

• Creation and Association:

  • Network ACLs are created using the AWS Management Console, AWS CLI (create-network-acl), or AWS SDKs.
  • You associate a Network ACL with a specific subnet within your VPC to apply its rules to all traffic entering and leaving that subnet.

• Rule Management:

  • Rules within a Network ACL can be managed through the AWS Management Console or AWS CLI (create-network-acl-entry for creating rules, replace-network-acl-entry for updating rules).

Summary:

AWS Network ACLs provide essential network security controls at the subnet level within your VPC. They help enforce traffic rules to ensure secure and compliant network communication between subnets and external networks. Understanding how to effectively configure and manage Network ACLs is crucial for designing robust and secure network architectures in AWS.