📅 Day 14 | AWS NACL — Subnet-Level Security in AWS 🔐

NACL (Network Access Control List) is a subnet-level firewall in AWS VPC.

It controls what traffic is allowed in and out of each subnet, acting like a security gate at the subnet boundary.

If you’re learning AWS networking, mastering NACL is a key step in building secure cloud architectures.

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🔥 Why NACL Is Important in AWS & DevOps

• 🛡️ Adds an extra layer of security at the subnet level
• 🔁 Controls inbound & outbound traffic (stateless)
• 🚫 Supports both ALLOW and DENY rules
• 📦 Protects private subnets (App Servers, Databases, EKS Nodes)
• ⚙️ Often used in secure DevOps infrastructure (EKS, EC2, CI/CD, Load Balancers)

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🟧 What is NACL in AWS?

NACL (Network Access Control List) is a network-level firewall that controls traffic going in and out of a subnet in a VPC.

Think of it as a security gate for each subnet.

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🔥 Why NACL is important in DevOps?

As a DevOps engineer, you work with:

• VPC creation
• Subnets (public/private)
• EC2, Load balancers, NAT
• Kubernetes clusters (EKS)
• Terraform or CloudFormation

All of these use networking, and NACL helps control what traffic is allowed.

NACL ensures:

• ✔️ Traffic is restricted
• ✔️ Only safe ports are open
• ✔️ Subnet-to-subnet traffic is filtered
• ✔️ Security best practices are followed

This is important in DevOps pipelines, deployments, and infra automation.

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🟦 Key Features of NACL (Easy to Remember)

1️⃣ Subnet-Level Firewall

A NACL is attached to a subnet, not an EC2 instance.

2️⃣ Stateless

➡️ Allow inbound traffic? Then you must allow outbound traffic also.

Example: Allow port 80 IN → must allow port 80 OUT.

3️⃣ Allows both:

• ALLOW rules
• DENY rules

4️⃣ Rule number order matters

Lowest number checked first (100 → 101 → 102…).

5️⃣ Default NACL = Allow everything

6️⃣ Custom NACL = Deny everything unless allowed

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🟪 NACL Use Case in DevOps

Example 1: Public subnet

You deploy:

• EC2 instance (web server)
• ALB (load balancer)

NACL allows:

• HTTP (80)
• HTTPS (443)
• SSH (22)

Example 2: Private subnet

You deploy:

• App server
• Database
• EKS worker nodes

NACL allows only internal traffic:

• App → DB (3306)
• Node-to-node communication
• No public internet access

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💡 Difference Between NACL and Security Group (Interview)

Security Group	NACL
Instance level	Subnet level
Stateful	Stateless
Return traffic auto allowed	Must allow separately
Only ALLOW rules	ALLOW + DENY rules
Easier to manage	Used for high-level subnet control

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🟩 Simple Example to Understand

Public Subnet NACL

Inbound Rules:

• Allow HTTP (80)
• Allow HTTPS (443)
• Allow SSH (22)

Outbound Rules:

• Allow ALL

Private Subnet NACL

Inbound:

• Allow 3306 from app subnet

Outbound:

• Allow 3306 back to app subnet
• Deny internet traffic

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✅ Example NACL Rule Table

Rule No	Type	Protocol	Port Range	Source / Destination	Action
100	Inbound	TCP	80	0.0.0.0/0	ALLOW
110	Inbound	TCP	443	0.0.0.0/0	ALLOW
120	Inbound	TCP	22	0.0.0.0/0	ALLOW
1000	Outbound	ALL	ALL	0.0.0.0/0	ALLOW
*	-	-	-	-	DENY (implicit)

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🛠️ Terraform Example (minimal)

Use this as a starting point — update vpc_id, CIDRs and rule numbers as needed.

resource "aws_network_acl" "public_nacl" {
vpc_id = aws_vpc.main.id
tags = {
Name = "public-nacl"
}
}

resource "aws_network_acl_rule" "allow_http_in" {
network_acl_id = aws_network_acl.public_nacl.id
rule_number    = 100
egress         = false
protocol       = "6"       # TCP
rule_action    = "allow"
cidr_block     = "0.0.0.0/0"
from_port      = 80
to_port        = 80
}

resource "aws_network_acl_rule" "allow_https_in" {
network_acl_id = aws_network_acl.public_nacl.id
rule_number    = 110
egress         = false
protocol       = "6"
rule_action    = "allow"
cidr_block     = "0.0.0.0/0"
from_port      = 443
to_port        = 443
}

resource "aws_network_acl_rule" "allow_ssh_in" {
network_acl_id = aws_network_acl.public_nacl.id
rule_number    = 120
egress         = false
protocol       = "6"
rule_action    = "allow"
cidr_block     = "0.0.0.0/0"
from_port      = 22
to_port        = 22
}

resource "aws_network_acl_rule" "allow_all_out" {
network_acl_id = aws_network_acl.public_nacl.id
rule_number    = 1000
egress         = true
protocol       = "-1"      # all protocols
rule_action    = "allow"
cidr_block     = "0.0.0.0/0"
from_port      = 0
to_port        = 0
}

Thank You

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Thank You

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