Comprehensive Guide to AWS Network ACL (NACL)

In Amazon Web Services (AWS), securing your resources is of utmost importance. One of the essential security mechanisms in AWS is the Network Access Control List (NACL). This guide focuses on AWS Network ACLs, explaining their functionality, use cases, and best practices for implementation.

What is a Network ACL (NACL)?

A Network ACL is an optional layer of security that operates at the subnet level within a Virtual Private Cloud (VPC). It acts as a stateless firewall to control inbound and outbound traffic to and from subnets based on rules.

Unlike Security Groups, which are stateful and operate at the instance level, NACLs require explicit rules for both inbound and outbound traffic. This stateless nature makes NACLs an effective tool for subnet-wide traffic filtering.

Key Features of Network ACLs

1. Stateless: NACLs do not remember past traffic sessions, meaning rules must explicitly allow both request and response traffic.

2. Rule Evaluation: NACL rules are evaluated sequentially, starting from the lowest-numbered rule. Once a rule matches, further rules are ignored.

3. Default vs. Custom NACL:

• The default NACL allows all inbound and outbound traffic.
• A custom NACL denies all traffic by default until rules are added.

4. Applicable at Subnet Level: Every subnet in a VPC must be associated with an NACL.

Structure of NACL Rules

Each NACL rule consists of:

• Rule Number: Determines the order of rule evaluation (lowest to highest).
• Protocol: Defines the traffic protocol (e.g., TCP, UDP, or ICMP).
• Port Range: Specifies the port(s) for traffic.
• Source/Destination: Defines the traffic's origin or destination IP address.
• Action: Specifies whether to ALLOW or DENY traffic.
• Traffic Direction: Applies to inbound or outbound traffic.

Creating and Managing NACLs

Step 1: Create a Custom Network ACL

1. Navigate to the VPC Console in AWS.
2. Select Network ACLs from the navigation pane.
3. Click on Create Network ACL, name it, and associate it with a VPC.

Step 2: Add Rules

• Select the NACL you created.
• Go to the Inbound Rules or Outbound Rules tab.

Add rules by specifying:

Rule number

Protocol

Port range

Source/Destination

Action (ALLOW/DENY)

4. Save changes.

Step 3: Associate Subnets

1. Select your NACL.
2. Click Subnet Associations.
3. Choose the subnets to associate with the NACL.

Example: Typical NACL Rule Table

• Inbound Rules: Allow HTTP (port 80), HTTPS (port 443), and SSH (port 22) traffic from specific sources.
• Outbound Rules: Typically mirror inbound rules or allow all traffic unless specified otherwise.

Use Cases for Network ACLs

1. Restricting Traffic to Subnets: Use NACLs to block unwanted IP ranges or restrict traffic for specific subnets.
2. Compliance Requirements: Enforce subnet-wide security rules for compliance with organizational policies.
3. Mitigating Threats: Quickly block malicious IP addresses at the subnet level.
4. Providing an Additional Security Layer: Enhance security by combining NACLs with Security Groups.

Best Practices for Using Network ACLs

1. Start with Default Deny: Begin with no rules, then explicitly define ALLOW rules as needed.
2. Be Specific with Rules: Avoid broad ALLOW rules; specify protocols, ports, and IP ranges clearly.
3. Document Rule Numbers and Purpose: Maintain a clear, documented order for rule evaluation.
4. Regularly Audit NACL Rules: Remove outdated or redundant rules to avoid unnecessary complexity.
5. Use Security Groups for Instance-Level Control: Combine NACLs with Security Groups for a multi-layered approach.
6. Test Before Deployment: Validate rules in a staging environment before applying them in production.

NACLs vs. Security Groups

Limitations of Network ACLs

1. No Application Layer Filtering: NACLs only operate at the network layer.
2. Stateless Nature: Rules must explicitly allow both directions of traffic.
3. Complexity with Multiple Subnets: Managing large numbers of rules across multiple subnets can become challenging.

Conclusion

AWS Network ACLs are a powerful tool for subnet-level traffic control, providing a foundational layer of security for your VPC. By carefully designing and managing NACL rules, you can enhance your security posture and meet organizational or compliance requirements. Always complement NACLs with Security Groups and other AWS security features for a robust, multi-layered defense.

-Ariful Islam Shawon

B.Sc. in Software Engineering

Software Engineer, DevOps Engineer

Cloud Engineer and Solution Architect

2x AWS Certified, AWS Certified DevOps Engineer?—?Professional

Amazon Web Services (AWS)