When designing secure architectures on AWS, understanding Security Groups and Network Access Control Lists (NACLs) is crucial. Both are essential components of the AWS network security model, but they serve different purposes and operate at different levels.
In this blog post, we’ll break down the differences between Security Groups and NACLs, explain when to use each, and help you build more secure and scalable cloud environments.
🔐 What Are Security Groups in AWS?
Security Groups act as virtual firewalls for your EC2 instances. They control inbound and outbound traffic at the instance level, meaning they define who can talk to your instances and who your instances can talk to.
✅ Key Features of Security Groups:
- Stateful: If you allow incoming traffic on a port, the response is automatically allowed.
- Instance-level: Applied directly to EC2 instances or ENIs.
- Default Deny: All traffic is denied unless explicitly allowed.
- Only allow rules: You cannot create “deny” rules.
- Evaluated as a whole: All rules are evaluated collectively.
🌐 What Are NACLs in AWS?
NACLs are used to control traffic at the subnet level. They serve as a firewall for VPC subnets, allowing or denying traffic into and out of an entire subnet.
✅ Key Features of NACLs:
- Stateless: Rules are evaluated separately for inbound and outbound traffic.
- Subnet-level: Applied to all resources in the subnet.
- Support both allow and deny rules.
- Rules are evaluated in order by rule number.
- More granular control for security policies.
🔄 Security Groups vs NACL – Detailed Comparison Table
| Feature | Security Groups | NACLs |
|---|---|---|
| Level | Instance level | Subnet level |
| Type | Stateful | Stateless |
| Rule Type | Allow only | Allow and Deny |
| Default Behavior | Deny all | Allow all (for new subnets) |
| Rule Evaluation | All rules evaluated together | Rules evaluated by priority number |
| Direction | Inbound and outbound | Inbound and outbound |
| Common Use Cases | Secure EC2 instances and load balancers | Secure entire subnet, fine-grained control |
| Rule Limits | 60 rules per group (soft limit) | 20 inbound + 20 outbound (default limit) |
💡 When to Use Security Groups
Use Security Groups when you want to:
- Protect individual EC2 instances
- Control application-level access
- Apply tight access control to services like RDS, ELB, etc.
- Keep rules easy to manage across instances
💡 When to Use NACLs
Use NACLs when you need to:
- Apply blanket policies at the subnet level
- Block specific IPs using deny rules
- Build multi-tier architectures with layered security
- Enable or restrict access between subnets
🔒 Best Practices
- Use Security Groups as your primary defense.
- Layer NACLs for broader control and specific deny rules.
- Keep rules organized and documented.
- Regularly audit both SG and NACL configurations.
- Use AWS Firewall Manager or CloudFormation for consistent deployment.
🧠 Final Thoughts
Both Security Groups and NACLs are critical tools for securing your AWS infrastructure. While they overlap in functionality, they are designed for different levels of control and work best when used together.
Understanding these differences can prevent misconfigurations that might leave your cloud environment vulnerable.