Question
Answer and Explanation
An AWS Network Access Control List (NACL) association refers to the binding of a NACL to one or more subnets within a Virtual Private Cloud (VPC). In simpler terms, it's how you apply the rules defined in a NACL to control network traffic in your AWS environment.
Here's a breakdown of the key concepts:
1. Network Access Control Lists (NACLs):
- NACLs are stateless firewalls that act as a security layer for your subnets within a VPC.
- They consist of a numbered list of rules that determine whether traffic is allowed (ALLOW) or denied (DENY), both inbound (ingress) and outbound (egress).
- Unlike Security Groups, which are instance-level stateful firewalls, NACLs operate at the subnet level and are stateless, meaning they do not remember previous connections.
2. Subnets:
- Subnets are subdivisions of a VPC's IP address range, providing a way to organize your resources. Each subnet resides within a specific Availability Zone.
3. The Association:
- When you create a NACL, it's initially not associated with any subnet. You must explicitly associate the NACL with one or more subnets to enforce its rules.
- A subnet can only be associated with one NACL at a time. However, multiple subnets can be associated with the same NACL.
How Association Works:
- Once a NACL is associated with a subnet, all network traffic entering or leaving that subnet is evaluated against the NACL's rules.
- NACL rules are processed in order of their rule number, with lower numbers having higher precedence. The first matching rule, whether it's an ALLOW or DENY, is applied. If no rule matches, the default DENY rule takes effect.
Default NACLs:
- When you create a VPC, AWS provides a default NACL that initially allows all inbound and outbound traffic.
- You can customize the default NACL or create new NACLs as per your security requirements. You cannot delete default NACLs.
Key Considerations:
- Statelessness: Due to their stateless nature, if you allow inbound traffic on a specific port, you also have to explicitly allow the corresponding outbound traffic (usually using ephemeral ports).
- Subnet Level: NACLs are subnet-based, thus you need to ensure they are defined with subnets CIDR blocks in mind.
- Best Practice: It's a best practice to use NACLs in addition to Security Groups to create defense in depth. NACLs act as a first line of defense at the subnet, while Security Groups provide a second layer of protection at the instance level.
In essence, the AWS Network ACL Association is the vital link that connects a security policy, defined in a NACL, to a specific subnet, therefore controlling traffic flow in and out of that subnet.