The main differences between a Security Group and a Network ACL in AWS lie in their scope, statefulness, and functionality within the AWS networking architecture:
Scope
- Security Group:
- Operates at the instance level.
- Applies to individual EC2 instances (and other AWS resources like RDS instances).
- Controls inbound and outbound traffic to and from these instances based on defined rules.
- Network ACL:
- Operates at the subnet level.
- Applies to all instances (and other resources) within a subnet in a VPC.
- Controls traffic entering and leaving the subnet itself, affecting all instances in that subnet equally.
Statefulness
-
Security Group:
- Stateful: Automatically allows return traffic that corresponds to allowed inbound traffic.
- For example, if you allow inbound SSH (port 22) traffic, the return traffic from the SSH session is automatically allowed, regardless of outbound rules.
-
Network ACL:
- Stateless: Requires separate rules for inbound and outbound traffic.
- Each rule applies to traffic in one direction only (either inbound or outbound) and must be explicitly defined for both directions.
Rules and Functionality
-
Security Group:
- Rules are based on allowing traffic.
- You define rules that permit traffic based on protocols (TCP, UDP, ICMP) and port ranges.
- No explicit deny rules (implicit deny if not explicitly allowed).
- Dynamic and updated immediately when modified.
-
Network ACL:
- Rules can allow or deny traffic.
- Rules are evaluated in order (lowest numbered rule first).
- Supports protocol (TCP, UDP, ICMP) and port number ranges for defining rules.
- Requires explicit allow rules for traffic to be permitted; otherwise, traffic is denied by default.
- Static and must be updated manually; changes are not applied immediately.
Use Cases
-
Security Group:
- Ideal for allowing or restricting specific types of traffic to individual instances based on their roles (e.g., web server, database).
- Provides instance-level security and access control.
-
Network ACL:
- Used to control traffic flow at the subnet level, affecting all instances within the subnet.
- Provides additional layer of security and compliance enforcement within the VPC.
- Useful for enforcing network security policies and restricting traffic between subnets or to/from external networks.
Security Groups are stateful, instance-level firewalls that control traffic based on instance roles and defined rules, while Network ACLs are stateless, subnet-level filters that control traffic flow within the VPC subnet, affecting all instances equally within that subnet. Understanding these differences helps in effectively implementing and managing network security within AWS environments.
