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Understanding Security Group and Network Access Control List (NACL) in AWS

AWS provides robust security measures to protect resources within its cloud infrastructure. Two fundamental components of AWS security are Security Groups and Network Access Control Lists (NACLs). While both serve similar purposes – controlling traffic to and from AWS resources – they operate at different layers of the network stack and offer distinct features.

Security Groups

Security Groups

Security Groups act as virtual firewalls for EC2 instances and other AWS resources. They regulate inbound and outbound traffic by defining rules that specify which type of traffic is allowed or denied. Here are some key points about Security Groups:

  1. Security Groups operate at the instance level and are stateful, meaning if you allow inbound traffic, the return traffic is automatically allowed regardless of outbound rules. This simplifies the management of security policies.

  2. You define inbound and outbound rules to permit specific types of traffic. For example, you can allow SSH (port 22) for administration or HTTP (port 80) for web traffic. If a rule doesn't explicitly allow traffic, it's implicitly denied.

  3. Security Groups are easy to configure through the AWS Management Console, CLI, or SDKs. You can modify rules dynamically without restarting instances.

  4. A single Security Group can be attached to multiple instances, enabling consistent security policies across resources.

Network Access Control Lists (NACLs)

Network Access Control Lists (NACLs)

NACLs are another layer of defense for controlling traffic at the subnet level. Unlike Security Groups, which operate at the instance level, NACLs function at the subnet level. Here's what you need to know about NACLs:

  1. Unlike Security Groups, NACLs are stateless. This means that if you allow inbound traffic, you must explicitly allow the corresponding outbound traffic and vice versa.

  2. NACLs have numbered rules that are evaluated in ascending order. Once a rule is matched, subsequent rules are not processed. This order matters when defining complex access control policies.

  3. NACLs support both allow and deny rules. However, the order of rules and the stateless nature of NACLs make it essential to carefully plan and configure rules.

  4. NACLs are associated with subnets, allowing you to control traffic entering and leaving the subnet. Each subnet in a VPC must be associated with a NACL, and by default, it allows all traffic.

Best Practices

To effectively secure your AWS infrastructure using Security Groups and NACLs, consider the following best practices:

  1. *Follow the principle of least privilege by only permitting necessary traffic. Restrict access to ports and protocols that are required for the application to function.

  2. Implement multiple layers of security using both Security Groups and NACLs. While Security Groups provide instance-level security, NACLs offer subnet-level control, adding an extra layer of defense.

  3. Regularly review and audit your security rules to ensure they align with your organization's security policies. Remove any unnecessary rules or overly permissive configurations.

  4. Utilize AWS APIs, CLI, or Infrastructure as Code (IaC) tools like AWS CloudFormation or Terraform to automate the configuration and management of Security Groups and NACLs, ensuring consistency and scalability.


In conclusion, Security Groups and Network Access Control Lists are essential components of AWS security, offering different levels of control over inbound and outbound traffic within your VPC. By understanding their differences and best practices, you can effectively secure your AWS resources and protect them from unauthorized access and malicious activity.

You can see a video tutorial about NACL and SG describing the strengths, weaknesses, usage example, and a hands-on demo showing how you can create a NACL and SG and even more - YouTube video

NACL and Security Group in AWS | Demo Tutorial 2024 SG & Network Access Control List - YouTube

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