Intro to NACL and Security Groups

A Network Access Control List (NACL) is a security feature in Amazon Web Services (AWS) that acts as a virtual firewall for controlling inbound and outbound traffic at the subnet level within a Virtual Private Cloud (VPC). NACLs are stateless, meaning they don’t keep track of the connection state like stateful firewalls do. They evaluate traffic based on rules that you define and allow or deny traffic accordingly.

Network Access Control Lists (NACLs) and Security Groups are both important components of network security in Amazon Web Services (AWS). They provide different levels of control and operate at different layers of the networking stack.

NACLs provide an additional layer of network security in AWS by allowing you to control traffic at the subnet level. They are useful in various scenarios, such as segmenting traffic between different application tiers, implementing specific security policies, or restricting access from certain IP ranges.


However, it’s important to note that NACLs are stateless and operate at a lower level than security groups, so they should be used in conjunction with other security measures, such as security groups, to provide comprehensive network security for your AWS resources.

Key points about NACLs:


  1. Subnet-Level Security: NACLs are associated with subnets in a VPC. They operate at the subnet level, allowing you to control the traffic entering and leaving that specific subnet.

  2. Rule-Based Filtering: NACLs use rules to determine whether to allow or deny traffic. Each rule consists of a rule number, an action (allow or deny), a protocol (such as TCP, UDP, or ICMP), and conditions like source/destination IP addresses or port ranges.

  3. Evaluation Order: NACL rules are processed in a sequential order based on their rule numbers. They are evaluated from the lowest to highest number until a match is found. If no rule matches, the default action (allow or deny) defined in the NACL is applied.

  4. Inbound and Outbound Rules: NACLs allow you to define separate inbound and outbound rules. This means you can control both the traffic coming into the subnet and the traffic leaving the subnet.

  5. Numbered Rules: NACLs use numbered rules to define the order of evaluation. Each rule has a unique rule number, and you can add, modify, or remove rules without affecting the other rules’ numbers.

  6. Logging: NACLs can be configured to log network traffic that matches specific rules. This allows you to monitor and analyze the traffic flow, identify any unauthorized or suspicious activity, and troubleshoot connectivity issues.

  7. Associating with Subnets: Each subnet in a VPC is associated with a default NACL by default. However, you can assign a custom NACL to a subnet to implement specific security requirements for that subnet.


The following is a detailed summary of NACLs and Security Groups, including their key features, use cases, and comparisons.

Network Access Control Lists (NACLs)
    • NACLs are stateless firewalls that operate at the subnet level within an Amazon Virtual Private Cloud (VPC).
    • They evaluate inbound and outbound traffic based on rules you define, allowing or denying traffic accordingly.
    • NACLs are ordered sets of numbered rules that are processed in a sequential manner, with the first matching rule being applied.
    • Each rule in an NACL consists of a rule number, rule action (allow or deny), and conditions to match the traffic, such as source/destination IP, protocol, or port.
    • NACLs provide granular control and can be associated with multiple subnets within a VPC.
    • They are useful for scenarios where you require subnet-level access control, such as segregating traffic between different application tiers or implementing specific security policies.
    • NACLs are stateless, meaning that you need to explicitly allow inbound and outbound traffic separately.
    • They offer basic logging capabilities to monitor traffic and detect any unauthorized or suspicious activities.
Security Groups
    • Security Groups are stateful firewalls that operate at the instance level within a VPC.
    • They control inbound and outbound traffic based on rules that you define, with the ability to dynamically allow return traffic related to allowed inbound traffic.
    • Security Groups use security group IDs and are associated with instances or ENIs (Elastic Network Interfaces).
    • Rules in security groups define allowed inbound and outbound traffic based on source/destination IP, protocol, and port.
    • Security Groups are evaluated in a permissive manner, where all traffic is denied by default, and only explicitly allowed traffic is permitted.
    • They are highly flexible and can be dynamically updated, making them suitable for dynamic environments or scenarios where instances frequently change.
    • Security Groups are ideal for application-level access control, where you need to specify rules based on the specific requirements of individual instances or ENIs.
    • They provide an additional layer of security beyond NACLs, offering more fine-grained control and advanced features.
Use Cases for NACLs
  1. Segmentation of Subnets: NACLs are useful for segmenting traffic between different subnets within a VPC. You can create separate NACLs for each subnet and define rules to control the traffic flow between them, providing an additional layer of isolation and security.

  2. Implementing Strict Access Controls: NACLs allow you to define fine-grained rules to enforce strict access controls at the subnet level. For example, you can block specific IP addresses or ranges, limit access to specific ports or protocols, or apply stricter rules for sensitive subnets that contain critical data or resources.

  3. Traffic Filtering and Inspection: NACLs provide the ability to filter and inspect inbound and outbound traffic at the subnet level. This can be useful for monitoring and analyzing network traffic, detecting and blocking unauthorized access attempts, or identifying potentially malicious activity.

  4. Compliance and Regulatory Requirements: NACLs can help meet compliance and regulatory requirements by allowing you to enforce specific network security policies and access controls. They enable you to define and enforce security measures that align with industry-specific regulations or internal security policies.

Use Cases for Security Groups
  1. Instance-Level Access Control: Security Groups are primarily used for controlling access to instances (or ENIs) within a VPC. You can define rules to allow or deny traffic based on source IP, port, and protocol, providing instance-level security.

  2. Dynamic and Scalable Environments: Security Groups are highly flexible and suitable for dynamic environments where instances frequently change. They automatically adjust to changes in the infrastructure, allowing you to easily add or remove instances without modifying security group configurations.

  3. Application-Level Filtering: Security Groups are well-suited for implementing application-level access controls. You can define rules based on the specific requirements of each application, allowing or denying traffic to specific ports or protocols required by the application.

  4. Multi-tier Architectures: Security Groups enable the implementation of multi-tier architectures by controlling traffic flow between different layers or tiers of an application. You can create security groups for each tier and define rules to allow necessary communication while blocking unauthorized access.

NACLSecurity Group
NACLs operate at the subnet levelSecurity Groups operate at the instance/ENI level
NACLs are stateless, requiring separate rules for inbound and outbound trafficSecurity Groups are stateful, automatically allowing return traffic
NACLs are evaluated in a specific orderSecurity Groups are evaluated in a permissive manner
NACLs offer broader control over traffic, allowing filtering based on IP, protocol, and portSecurity Groups focus on instance-level access control
NACLs provide basic logging capabilitiesSecurity Groups do not have native logging but can be integrated with other AWS services for logging and monitoring

In summary, NACLs and Security Groups are both crucial for securing your AWS infrastructure. NACLs offer subnet-level control and operate as stateless firewalls, while Security Groups provide instance-level control and operate as stateful firewalls. Understanding their differences and capabilities allows you to design effective network security architectures to protect your applications and data within AWS.


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