In AWS, Security Groups (SGs) and Network ACLs (NACLs) are both used for controlling inbound and outbound traffic to and from resources within a Virtual Private Cloud (VPC). However, they work at different levels and have distinct functionalities.
Let’s break down the differences in detail:
🔹 What is a Security Group?
A Security Group (SG) is like a firewall at the instance level that controls which traffic is allowed in and out of an EC2 instance.
🔹 Acts as a virtual firewall for EC2 instances.
🔹 Applies at the instance level (not at the subnet level).
🔹 Only allows defined traffic; all other traffic is denied by default.
🔹 Stateful: If you allow inbound traffic, the corresponding outbound response is automatically allowed.
🔹 Can have multiple security groups per instance, each with different rules.
🔹 What is a Network ACL (NACL)?
A Network ACL (NACL) is like a firewall at the subnet level that controls traffic entering and leaving a subnet.
🔹 Works at the subnet level, not the instance level.
🔹 Allows and denies traffic based on rules set at the network level.
🔹 Stateless: Each inbound and outbound rule must be explicitly defined.
🔹 Evaluates rules in numerical order, from lowest to highest.
🔹 Applies to all instances in the subnet.
🛠 Key Differences Between Security Groups and NACLs
| Feature | Security Groups (SGs) | Network ACLs (NACLs) |
|---|---|---|
| Level of Control | Works at the instance level | Works at the subnet level |
| Stateful or Stateless? | Stateful (If you allow inbound traffic, the corresponding outbound response is automatically allowed) | Stateless (Each rule must be explicitly defined for inbound and outbound traffic) |
| Default Behavior | All inbound and outbound traffic is denied by default | All inbound and outbound traffic is allowed by default (except the default NACL) |
| Rule Processing | Evaluates all rules before deciding | Evaluates rules in order (lowest number first) |
| How Many Can Be Assigned? | Multiple Security Groups can be attached to a single instance | One NACL per subnet (affects all instances in the subnet) |
| Use Case | Used for controlling access to individual EC2 instances | Used for controlling access at the subnet level |
📌 How Security Groups and NACLs Work Together
🔸 Example Scenario: Securing a WordPress Blog on AWS
Imagine you have a VPC with a public and private subnet.
- The public subnet has an EC2 instance running WordPress.
- The private subnet has a database instance (RDS) that should not be publicly accessible.
Here’s how you would use Security Groups and NACLs to secure the environment:
🔹 Security Group for the Web Server (EC2)
✅ Inbound Rules:
- Allow HTTP (port 80) and HTTPS (port 443) from the internet.
- Allow SSH (port 22) only from your IP (for security).
✅ Outbound Rules:
- Allow all outgoing traffic.
🔹 Security Group for the Database Server (RDS)
✅ Inbound Rules:
- Allow MySQL (port 3306) only from the web server’s security group.
✅ Outbound Rules:
- Allow all outgoing traffic.
🔹 NACL Rules for the Public Subnet
✅ Inbound Rules:
- Allow HTTP, HTTPS from anywhere (port 80, 443).
- Allow SSH only from your IP.
✅ Outbound Rules:
- Allow all outgoing traffic.
🔹 NACL Rules for the Private Subnet
✅ Inbound Rules:
- Allow only MySQL (3306) from the web server’s security group.
✅ Outbound Rules:
- Allow all outgoing traffic.
🚀 When to Use Security Groups vs. NACLs?
✔️ Use Security Groups when you need to control access at the EC2 instance level.
✔️ Use NACLs when you need to control access at the subnet level (for broader rules).
✔️ For best security, use both together!
Would you like help setting up Security Groups and NACLs for your AWS environment? Let us know! 🚀
