"A detailed comparison between AWS Security Groups and NACLs, including their features, differences, and best practices."
original post url - https://jayeshdevops.hashnode.dev/aws-security-group-vs-nacl
When working with AWS infrastructure, controlling inbound and outbound traffic is essential for security. AWS provides two key components to manage this: Security Groups and Network Access Control Lists (NACLs). Although they serve similar purposes, their scope, roles, and behavior differ significantly. In this post, we’ll compare these two and understand when to use each.
What is a Security Group?
A Security Group (SG) acts as a virtual firewall for your EC2 instances or other resources. It allows you to control what traffic is allowed or denied at the instance level.
Key Features:
- Stateful: If an inbound rule allows traffic, the response is automatically allowed without needing an explicit outbound rule.
- Instance-level control: Security groups are applied to individual EC2 instances, and each instance can have multiple security groups.
- Supports inbound and outbound rules: You define both types of rules explicitly.
- Limited scope: Security Groups apply only at the instance level (e.g., EC2, RDS, etc.).
What is a Network Access Control List (NACL)?
A Network ACL (NACL) is an optional firewall-like layer for controlling traffic at the subnet level within a VPC. It acts as a gatekeeper for all inbound and outbound traffic to and from the subnet.
Key Features:
- Stateless: Both inbound and outbound rules must be defined explicitly (allowing inbound traffic doesn’t automatically allow outbound traffic).
- Subnet-level control: NACLs apply to all resources within the subnet.
- Numbered rules: Rules are evaluated in order, starting with the lowest-numbered rule.
- Default behavior: NACLs come with a default rule that allows all traffic, but this can be customized.
Key Differences Between Security Groups and NACLs
| Feature | Security Group | NACL |
|---|---|---|
| Level | Instance-level (e.g., EC2) | Subnet-level |
| State | Stateful | Stateless |
| Rules Evaluation | All rules are evaluated | Evaluated in order of priority |
| Inbound/Outbound Behavior | Implicit response rules | Explicit rules for both |
| Usage Scope | Specific instances only | Entire subnet |
| Default Rule | Denies all by default | Allows all by default |
When to Use Security Groups vs NACLs
Use Security Groups:
When you need resource-specific control, such as allowing SSH traffic to a specific EC2 instance.Use NACLs:
To enforce network-wide rules, like blocking a specific IP across an entire subnet.
Conclusion
Both Security Groups and NACLs are essential for managing network security in AWS. Security Groups are best for fine-grained, resource-level control, while NACLs provide broader control at the subnet level. Understanding their differences will help you apply them effectively to secure your cloud infrastructure.
Happy securing your cloud infrastructure! 🚀
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