Introduction

When it comes to securing resources within an AWS Virtual Private Cloud (VPC), both Security Groups and Network Access Control Lists (NACLs) play vital roles. However, they operate at different layers of the network stack and serve distinct purposes. Let's explore the differences between Security Groups and NACLs:

Security Groups

Operational Layer

Operates at the Instance Level: Security Groups are stateful firewalls that operate at the instance level. They control inbound and outbound traffic for individual EC2 instances, RDS instances, and other resources within the same VPC.

Rule Configuration

Allow Rules Only: Security Groups consist of rules that allow traffic based on specified criteria (e.g., IP addresses, protocols, ports). By default, all traffic is denied unless explicitly allowed by a rule.

Filtering

Stateful: Security Groups are stateful, meaning they automatically allow return traffic for permitted inbound connections. For example, if an inbound rule allows traffic on port 80, the corresponding outbound traffic is allowed without requiring an explicit rule.

Dynamic Updates

Dynamic: Changes to Security Group rules take effect immediately. This flexibility allows for quick adjustments to network access based on evolving requirements.

Application

Instance-level Security: Security Groups are ideal for enforcing security policies specific to individual instances or resource groups. They provide granular control over network traffic and are well-suited for application-level security.

Network Access Control Lists (NACLs)

Operational Layer

Operates at the Subnet Level: NACLs are stateless firewalls that operate at the subnet level. They control traffic entering and exiting subnets within the VPC.

Rule Configuration

Allow and Deny Rules: NACLs consist of numbered rules that allow or deny traffic based on source and destination IP addresses, protocols, and ports. Unlike Security Groups, NACLs support both allow and deny rules.

Filtering

Stateless: NACLs are stateless, meaning they do not automatically allow return traffic. For each packet, both inbound and outbound rules are evaluated independently.

Priority Order

Evaluated in Order: NACL rules are evaluated in ascending order based on their rule numbers. If a packet matches a rule, the action (allow or deny) specified by that rule is applied, and subsequent rules are not evaluated.

Application

Subnet-level Security: NACLs provide an additional layer of security by controlling traffic flow at the subnet boundary. They are useful for implementing broad network security policies across multiple instances or services within a subnet.

Conclusion

Both Security Groups and NACLs are essential components of network security in AWS, and they are often used together to create multi-layered defense strategies for protecting resources within a VPC.