In AWS, both Network Access Control Lists (NACLs) and Security Groups are used to control traffic to and from resources within a Virtual Private Cloud (VPC). However, they operate at different levels and have different characteristics and use cases. Below is a comparison between NACLs and Security Groups:
Key Differences Between NACLs and Security Groups
| Feature | Security Groups | NACLs (Network Access Control Lists) |
|---|---|---|
| Level of Operation | Instance level (attached to EC2 instances) | Subnet level (applies to all instances within the subnet) |
| Statefulness | Stateful: Security groups automatically allow return traffic for inbound and outbound connections. You only need to allow traffic in one direction. | Stateless: NACLs do not automatically allow return traffic. You must explicitly allow both inbound and outbound traffic. |
| Default Behavior | By default, security groups deny all inbound traffic and allow all outbound traffic. | The default NACL allows all inbound and outbound traffic. Custom NACLs deny all traffic by default until rules are added. |
| Rule Application | Rules are applied to individual instances or ENIs. Multiple security groups can be attached to a single instance. | Rules are applied to entire subnets. All instances within a subnet are subject to the NACL associated with that subnet. |
| Evaluation of Rules | Rules are evaluated based on priority within the security group. Security groups do not have an explicit deny rule; only the absence of an allow rule results in traffic being denied. | Rules are evaluated in numerical order, starting with the lowest rule number. NACLs can explicitly allow or deny traffic. If no rule matches, the default action is to deny traffic. |
| Inbound and Outbound Rules | Separate inbound and outbound rules exist, but they are stateful. For example, allowing inbound HTTP traffic automatically allows the corresponding outbound traffic. | Separate inbound and outbound rules exist, and both must be configured to allow traffic in both directions. |
| Support for Allow/Deny Rules | Security groups can only include allow rules; they cannot explicitly deny traffic. If a security group does not allow traffic, it is implicitly denied. | NACLs can include both allow and deny rules, providing more granular control over traffic. |
| Logging and Monitoring | Security groups do not log traffic directly. However, you can monitor traffic using VPC Flow Logs, CloudWatch, or third-party solutions. | NACLs work with VPC Flow Logs, allowing you to monitor allowed and denied traffic at the subnet level. |
| Use Case | Typically used to control access to individual instances or groups of instances. Best suited for defining the security posture of specific resources (e.g., web servers, databases). | Typically used to control access to entire subnets. Best suited for broad, subnet-level traffic filtering and enforcing security boundaries between subnets. |
| Number of Rules | You can define up to 60 inbound and 60 outbound rules per security group (can be increased upon request). | You can define up to 20 inbound and 20 outbound rules per NACL (can be increased upon request). |
When to Use Security Groups vs. NACLs
- Use Security Groups When:
- You need to control traffic at the instance level.
- You require stateful filtering, where return traffic is automatically allowed.
- You want to apply security controls based on the role of the instance (e.g., web server, database server).
- You need to allow traffic based on specific protocols and ports for individual instances.
- Use NACLs When:
- You need to control traffic at the subnet level, affecting all instances within that subnet.
- You require stateless filtering and need to explicitly allow or deny both inbound and outbound traffic.
- You want to enforce broader security boundaries between subnets.
- You need to implement both allow and deny rules to create a more granular security policy.
Summary
- Security Groups are ideal for controlling traffic at the instance level with stateful filtering. They are simpler to manage for individual resources and are often used for more granular control over traffic to specific instances.
- NACLs are better suited for subnet-level security, providing stateless, more granular control over both inbound and outbound traffic with the ability to explicitly allow or deny traffic. They are useful for implementing broad security policies across entire subnets.
In practice, Security Groups and NACLs are often used together to provide layered security within a VPC, ensuring that traffic is controlled both at the instance level and across subnet boundaries.
