Tech Matchups: NACLs vs Security Groups
Overview
Network ACLs (NACLs) are stateless, subnet-level firewall rules that control inbound and outbound traffic for entire subnets in a VPC.
Security Groups are stateful, instance-level firewall rules that manage traffic to and from individual EC2 instances or resources.
Both secure VPCs: NACLs for broad subnet control, Security Groups for granular instance protection.
Section 1 - Rule Processing and Scope
NACLs operate at the subnet level, processing rules in numerical order—e.g., rule #100 (allow HTTP) is checked before #200 (deny all). Both inbound and outbound rules are required due to statelessness. Example:
Security Groups apply to instances, evaluating all rules simultaneously—e.g., allow SSH from 10.0.0.0/16. Stateful design auto-permits return traffic. Example:
NACLs affect all instances in a subnet (e.g., 100 EC2s); Security Groups target specific instances (e.g., 5 web servers). NACLs are broad; Security Groups are precise.
Scenario: NACLs block all subnet traffic on port 23; Security Groups allow HTTP only for web servers.
Section 2 - Stateful vs Stateless
NACLs are stateless—explicit rules are needed for both directions. Example: Allowing HTTP inbound (port 80) requires a separate outbound rule for responses (ephemeral ports 1024-65535).
Security Groups are stateful—allowing inbound HTTP auto-permits outbound responses. Example: One rule for TCP 80 covers both request and response.
Scenario: A web server with NACLs needs 2 rules (inbound 80, outbound 1024-65535); Security Groups need 1 (inbound 80). NACLs are rigid; Security Groups are flexible.
Section 3 - Use Case Scenarios
NACLs suit coarse-grained control—e.g., blocking all non-HTTP traffic to a public subnet. Ideal for compliance or baseline security (e.g., deny all except ports 80, 443).
Security Groups excel in fine-grained control—e.g., allowing only specific IPs to SSH into a database instance. Perfect for dynamic scaling (e.g., auto-scaling groups).
Scenario: NACLs lock down a subnet to HTTPS only; Security Groups permit SSH only from a bastion host. NACLs set boundaries; Security Groups refine access.
Section 4 - Performance and Limits
NACLs process rules sequentially, with a limit of 20 rules per NACL (extendable to 40). High rule counts may impact performance for large subnets.
Security Groups support up to 60 inbound and 60 outbound rules per group, with 5 groups per instance. Rules are evaluated in parallel, ensuring low latency.
Scenario: A subnet with 50 rules in NACLs slows traffic inspection; Security Groups with 10 rules per instance scale efficiently. NACLs are restrictive; Security Groups are agile.
Section 5 - Comparison Table
| Aspect | NACLs | Security Groups |
|---|---|---|
| Scope | Subnet-level | Instance-level |
| State | Stateless | Stateful |
| Rule Processing | Numerical Order | Simultaneous |
| Best For | Broad Control | Granular Access |
| Rule Limits | 20-40 per NACL | 60 per Group |
NACLs enforce subnet-wide policies; Security Groups protect specific resources. Use both for layered security.
Conclusion
NACLs and Security Groups are complementary VPC security tools. NACLs provide subnet-level, stateless control for broad policies, ideal for compliance or baseline restrictions. Security Groups offer instance-level, stateful precision, perfect for dynamic or granular access.
Weigh scope (subnet vs. instance), state (stateless vs. stateful), and complexity (broad vs. fine-grained). Use NACLs for subnet guardrails, Security Groups for instance-specific rules—or combine: NACLs for subnet lockdown, Security Groups for tailored access.