AWS Network ACLs (Network Access Control Lists) and Security Groups are both essential components for managing and controlling network traffic within Amazon Web Services (AWS), but they serve different purposes and operate at different levels within the AWS networking architecture. Here are the key differences between Network ACLs and Security Groups:
Scope and Association
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Security Groups:
- Scope: Operate at the instance level.
- Association: Associated with individual EC2 instances (and other AWS resources like RDS instances).
- Function: Control inbound and outbound traffic to and from instances based on defined rules.
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Network ACLs:
- Scope: Operate at the subnet level.
- Association: Associated with subnets within a VPC.
- Function: Control traffic entering and leaving the subnet itself, affecting all instances within that subnet equally.
Statefulness
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Security Groups:
- Stateful: Automatically allow return traffic that corresponds to allowed inbound traffic.
- Example: If inbound SSH (port 22) is allowed, return traffic from the SSH session is automatically allowed.
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Network ACLs:
- Stateless: Require separate rules for inbound and outbound traffic.
- Example: Rules must be explicitly defined for both inbound and outbound traffic directions.
Rule Evaluation and Behavior
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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.
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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
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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.
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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
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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.
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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 is crucial for designing 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.
