The key differences between AWS Security Groups and Network ACLs lie in their scope, behavior, and how they enforce security policies within your AWS environment:
Scope:
- Security Groups:
- Scope: Operate at the instance level.
- Association: Associated with individual EC2 instances (or other supported AWS resources like RDS instances).
- Control: Control inbound and outbound traffic to and from these instances based on defined rules.
- Network ACLs:
- Scope: Operate at the subnet level.
- Association: Associated with subnets within a VPC.
- Control: Control traffic entering and leaving the subnet itself, affecting all instances in that subnet equally.
Statefulness:
-
Security Groups:
- Stateful: Automatically allows return traffic corresponding to allowed inbound traffic.
- Example: If you allow inbound SSH (port 22) traffic, the return traffic from the SSH session is automatically allowed.
-
Network ACLs:
- Stateless: Requires separate rules for inbound and outbound traffic.
- Each rule applies to traffic in one direction (inbound or outbound) and must be explicitly defined for both directions.
Rules and Configuration:
-
Security Groups:
- Rules: Define allow rules only; there are no explicit deny rules (implicit deny if not explicitly allowed).
- Behavior: Rules are based on protocols (TCP, UDP, ICMP) and port numbers.
- Dynamic: Security Groups are dynamically updated, and changes are applied immediately.
-
Network ACLs:
- Rules: Define both allow and deny rules. Rules are evaluated in order, and an explicit allow or deny rule determines the decision.
- Behavior: Rules are based on IP protocols (TCP, UDP, ICMP) and port number ranges.
- Static: Network ACLs must be updated manually; changes are not applied immediately.
Use Cases:
-
Security Groups:
- Ideal for allowing or restricting specific types of traffic to individual instances based on their roles (e.g., web server, database).
- Provide instance-level security and access control.
-
Network ACLs:
- Used to control traffic flow at the subnet level, affecting all instances within the subnet.
- Provide an 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.
Management and Configuration:
-
Security Groups:
- Managed and configured through the AWS Management Console, AWS CLI (create-security-group, authorize-security-group-ingress, authorize-security-group-egress), or AWS SDKs.
- Associated with instances during instance launch or modification.
-
Network ACLs:
- Managed and configured through the AWS Management Console, AWS CLI (create-network-acl, associate-network-acl), or AWS SDKs.
- Associated with subnets within a VPC to control traffic flow.
Summary:
Understanding these differences helps in designing and implementing effective network security architectures within AWS. Security Groups provide stateful, instance-level firewall rules, while Network ACLs offer stateless, subnet-level traffic controls. Both components play complementary roles in securing your AWS infrastructure based on specific security and compliance requirements.
