NACL

Network Access Control List

A security feature in Amazon Web Services (AWS) that functions as a virtual firewall for controlling inbound and outbound traffic at the subnet level within a Virtual Private Cloud (VPC). NACLs allow you to define rules that dictate what traffic is allowed to enter and exit subnets within your VPC.

Here are the key points to understand about NACLs:

1. Subnet-Level Security: NACLs operate at the subnet level. Each subnet in a VPC can be associated with a NACL, and the rules within the NACL apply to all resources (such as EC2 instances) within that subnet.
2. Stateless: Unlike security groups, which are stateful, NACLs are stateless. This means that if you allow inbound traffic from a specific IP address, you need to allow outbound traffic to that IP address as well explicitly.
3. Rule Order: NACLs process rules in order, from lowest rule number to highest. When a network packet matches a rule, it is allowed or denied, and the processing stops. Therefore, rule order is important.
4. Allow and Deny Rules: NACLs consist of numbered rules defining traffic allowed or denied based on criteria such as IP addresses, port ranges, and protocols.
5. Numbered Rules: NACL rules are numbered, each with an associated number, starting from 100 to 32766. Lower rule numbers are processed before higher numbers.
6. Rule Priority: NACL rules are prioritized based on rule numbers. If a packet matches multiple rules, the rule with the lowest number is applied.
7. Default Behavior: By default, NACLs deny all inbound and outbound traffic. This means that if you don’t explicitly allow traffic, it’s denied.
8. Logging: NACLs can be configured to log network traffic that the rules deny. This can be useful for monitoring and troubleshooting.
9. Limitations: NACLs have some limitations, such as not being able to reference security groups directly and not supporting rule-based modification for existing instances.
10. Use Cases: NACLs are used to add an additional layer of security to your VPC by controlling traffic entering and exiting subnets. They can be used to enforce specific network access policies or to filter traffic based on various criteria.

An NACL in AWS is a security mechanism that allows you to control and manage network traffic at the subnet level within your Virtual Private Cloud (VPC). It’s a key tool for enhancing the security posture of your cloud infrastructure.

• Abbreviation: NACL