The default behavior of a Network ACL (Network Access Control List) in AWS is to deny all inbound and outbound traffic. This means that when you create a new Network ACL and associate it with a subnet within your Virtual Private Cloud (VPC), it starts with no rules explicitly allowing traffic.
Key Points about Default Behavior:
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Implicit Deny:
- When you create a new Network ACL, it contains an implicit "deny all" rule for both inbound and outbound traffic.
- This means that if you do not explicitly define rules to allow specific types of traffic (based on IP protocols, port numbers, and IP ranges), all traffic attempting to enter or leave the associated subnet will be blocked by default.
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Initial Setup:
- Upon creation, you need to configure the Network ACL with rules that permit the desired inbound and outbound traffic.
- Until you define these rules, all traffic will be denied by the Network ACL.
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Rule Evaluation Order:
- Rules within a Network ACL are evaluated in order, starting with the lowest numbered rule.
- An explicit allow or deny rule can override subsequent rules in the list.
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Associating with Subnets:
- When you associate a Network ACL with a subnet in your VPC, the rules defined in the Network ACL apply to all traffic entering and leaving that subnet.
- Each subnet can have only one Network ACL associated with it at any given time.
Setting Up Rules:
To allow specific types of traffic through a Network ACL, you must explicitly create rules. These rules typically include:
- Inbound Rules: Specify which incoming traffic (e.g., HTTP, SSH, ICMP) is allowed into the subnet from specific IP addresses or ranges.
- Outbound Rules: Define which outgoing traffic is allowed from instances in the subnet to specific destinations (e.g., internet, other subnets).
Example Scenario:
Imagine you have a subnet in your VPC where you want to allow HTTP traffic from any source but block all other inbound and outbound traffic initially:
- Network ACL Rules:
- Rule 100: Allow inbound HTTP (port 80) traffic from 0.0.0.0/0.
- Rule 200: Deny all inbound traffic (implicit deny after Rule 100).
- Rule 100: Allow outbound traffic to 0.0.0.0/0 on port 443 (HTTPS).
- Rule 200: Deny all outbound traffic (implicit deny after Rule 100).
In this scenario:
- Inbound HTTP traffic (port 80) from any source (0.0.0.0/0) is allowed.
- All other inbound and outbound traffic is denied by default because no rules explicitly permit it.
Management and Configuration:
- You can manage and configure Network ACLs using the AWS Management Console, AWS CLI (create-network-acl, replace-network-acl-entry), or AWS SDKs.
- Always review and test your Network ACL rules to ensure they meet your security and operational requirements before applying them to production environments.
Understanding the default behavior of Network ACLs is crucial for effectively managing and securing network traffic within your AWS VPC environments.
