Security Groups vs NACLs: A Simple Breakdown

#aws #security #networking

AWS networking includes multiple layers of traffic control, and two of the most important components are Security Groups (SGs) and Network ACLs (NACLs). They’re often compared because both filter traffic, but they operate at different layers and behave differently. Understanding how each one works helps you build clean and predictable VPC architectures. In this article, we’ll look at how they differ and how they work together inside a VPC.

What Security Groups Actually Do

Security Groups act as virtual firewalls for individual resources, such as EC2 instances, load balancers, and RDS databases. They operate at the instance level, meaning the rules apply directly to the resource they’re attached to.

A few key characteristics define how they behave:

Stateful rules: If inbound traffic is allowed, the response is automatically allowed outbound. You don’t need to create a matching outbound rule.
Attachment-based: You attach SGs to resources, not subnets.
Allow-only model: SGs do not support explicit deny rules.
Fine-grained filtering: Perfect for controlling ports and traffic sources per service.

This makes Security Groups ideal for defining application-level behavior. For example, allowing only port 80 and 443 from the internet for a web server, or allowing a private app server to reach a database on port 3306.

Because they are stateful and resource-specific, SGs are usually the first layer people think about when securing workloads.

What NACLs Do in a VPC

Network ACLs (Network Access Control Lists) operate at the subnet level, meaning every instance inside that subnet is affected by the NACL rules. Unlike Security Groups, NACLs behave more like traditional network firewalls.

Important traits:

Stateless rules: Inbound and outbound rules must be defined separately.
Allow and deny options: You can explicitly block traffic.
Subnet-wide enforcement: All resources in the subnet follow the same NACL behavior.
Rule evaluation with numbering: Lower-numbered rules are evaluated first.

Because NACLs act at a broader layer, they make sense when you need consistent filtering across many instances or when you want to explicitly block certain traffic patterns.

For example, blocking a malicious IP at the subnet level or enforcing that a subnet can only accept HTTP/HTTPS traffic regardless of the SGs attached to its resources.

How They Work Together

Even though both control traffic, they don’t replace each other. Instead, they stack, and each layer has a role:

Security Groups determine which traffic can reach a specific instance.
NACLs determine which traffic can reach the subnet where the instance lives.

Traffic must pass both to reach the instance.

In practice:

SGs handle application-level access (ports and services).
NACLs handle broader network boundaries (subnet-level policies).

Keeping the logic clear helps avoid misconfigurations like accidentally blocking return traffic or denying traffic twice in different layers.

Choosing When to Use Which

In most AWS architectures, Security Groups do the heavy lifting. They’re flexible, easy to update, and precise.

NACLs are typically used when:

You want an extra guardrail at the subnet level.
You need explicit deny rules.
You’re segmenting environments (e.g., dev, staging, prod) with strict separation.
You're mitigating a known bad IP or port pattern.

For everyday use, SGs are more intuitive. NACLs become more useful in regulatory, high-security, or tightly segmented networks.

Practical Examples

Here are some common patterns you’ll see:

Public web server

Security Group: Allow inbound 80/443 from anywhere.
NACL: Allow ephemeral ports for return traffic.

Private application server

Security Group: Allow inbound 8080 only from public web tier.
NACL: Block all inbound from the internet ranges at subnet level.

Database subnet

Security Group: Allow inbound 3306 only from app SG.
NACL: No internet-bound rules; keep the subnet isolated.

Blocking a suspicious IP

NACL: Deny inbound from 203.x.x.x directly at subnet level.
SG: No need to modify instance-level rules — the subnet is already protected.

Operational Considerations

Managing SGs and NACLs affects operations in different ways:

Stateful SGs simplify troubleshooting because return traffic doesn’t need explicit rules.
NACLs require careful planning since one missing outbound ephemeral rule can break everything.
Too many SGs can become messy without naming conventions.
NACLs with hundreds of rules become difficult to maintain and audit.

Keeping both clean and well-documented saves time when debugging connectivity issues or scaling your VPC.

Tips for Working with SGs and NACLs

A few simple habits make daily work much smoother:

Name SGs and NACLs clearly based on purpose.
Keep NACLs simple unless you have a strong reason to do otherwise.
Review inbound and outbound flows regularly.
Document port usage for each tier or service.
Avoid overlapping or conflicting NACL rules.

These practices reduce mistakes and make both layers predictable.

Conclusion

Security Groups and Network ACLs both influence how traffic moves through your VPC, but they solve different problems. SGs secure individual resources through flexible, stateful rules, while NACLs enforce subnet-level boundaries with stateless filtering. Once you understand how the two layers interact, building secure and well-structured VPC networks becomes much easier to manage.