Understanding AWS Security Groups and NACLs: A Comprehensive Guide

When securing your AWS infrastructure, two fundamental components are Security Groups (SGs) and Network Access Control Lists (NACLs). Both act as virtual firewalls but operate at different layers and have distinct behaviors. In this blog, we’ll explore their differences, use cases, and best practices for implementation.

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1. What is an AWS Security Group?

Security Groups (SGs) are stateful firewalls that operate at the instance level (EC2, RDS, Lambda, etc.). They control inbound and outbound traffic based on rules you define.

Key Features:

✅ Stateful: If you allow inbound traffic, the response is automatically allowed, even if no outbound rule exists.
✅ Acts at the instance level: Applied to ENIs (Elastic Network Interfaces).
✅ Supports "Allow" rules only: You cannot create "deny" rules.
✅ Evaluates all rules: If multiple SGs are attached, all rules are evaluated.

Example Security Group Rule:

Inbound Rule:  
- Type: SSH  
- Protocol: TCP  
- Port Range: 22  
- Source: 203.0.113.1/32  

Outbound Rule:  
- Type: All traffic  
- Protocol: All  
- Port Range: All  
- Destination: 0.0.0.0/0

Best Practices for Security Groups:

🔹 Least Privilege Principle: Only allow necessary ports (e.g., 80 for HTTP, 443 for HTTPS).
🔹 Use Security Group References: Instead of IPs, reference other SGs for better manageability.
🔹 Regularly Audit Rules: Remove unused or overly permissive rules.

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2. What is a Network ACL (NACL)?

Network Access Control Lists (NACLs) are stateless firewalls that operate at the subnet level. They provide an additional layer of security by filtering traffic entering or leaving a subnet.

Key Features:

✅ Stateless: You must explicitly allow inbound and outbound responses.
✅ Works at the subnet level: Applies to all instances in the subnet.
✅ Supports "Allow" and "Deny" rules: More granular control than SGs.
✅ Rule Evaluation is Order-Based: Rules are processed in numbered order (lowest first).

Example NACL Rule:

Rule #100 (Inbound):  
- Type: HTTP  
- Protocol: TCP  
- Port Range: 80  
- Source: 0.0.0.0/0  
- Action: ALLOW  

Rule #200 (Inbound):  
- Type: SSH  
- Protocol: TCP  
- Port Range: 22  
- Source: 203.0.113.1/32  
- Action: ALLOW  

Rule #* (Default Deny):  
- Type: ALL  
- Protocol: ALL  
- Port Range: ALL  
- Source: 0.0.0.0/0  
- Action: DENY

Best Practices for NACLs:

🔹 Explicit Deny for Security: Always have a final rule (e.g., Rule #1000) to deny all unmatched traffic.
🔹 Order Rules Carefully: Lower-numbered rules are processed first.
🔹 Use for Subnet-Level Protection: Ideal for blocking malicious IPs or restricting inter-subnet traffic.

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3. Security Group vs. NACL: Key Differences

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4. When to Use Security Groups vs. NACLs?

• Use Security Groups for:

  • Instance-level traffic filtering.

  • Stateful rule management (e.g., allowing return traffic automatically).

  • Micro-segmentation within a VPC.

• Use NACLs for:

  • Subnet-wide traffic control (e.g., blocking a malicious IP range).

  • Creating explicit deny rules (e.g., blocking FTP traffic).

  • Compliance requirements requiring stateless filtering.

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5. Conclusion

Security Groups and NACLs serve different but complementary roles in AWS security:

• Security Groups are ideal for fine-grained, instance-level protection.

• NACLs provide an additional subnet-level firewall with deny capabilities.

For robust security, use both in combination:
✔ Security Groups for granular access control.
✔ NACLs for broad subnet-level restrictions.

By understanding their differences and best practices, you can design a secure and efficient AWS network architecture.

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#AWS #CloudSecurity #SecurityGroups #NACL #DevOps #CyberSecurity