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A network ACL applies to a subnet as a whole. Only one network ACL per subnet is allowed. The rules in a network ACL are stateless (i.e., return traffic must be explicitly allowed). Rules can be for inbound or outbound traffic, they can allow or deny traffic, and they are evaluated starting from the lowest rule number. After the first match the search is terminated. The default network ACL allows all inbound and outbound IP traffic.
A security group can be seen as a virtual firewall that controls the traffic for one or more instances. When you launch an instance, you must assign a security group to the instance. If no custom security group is specified, a default security group will be created and associated with the instance. You can add rules to each security group that allow traffic to or from its associated instances. The rules of a security group can be modified at any time, and the modifications are automatically applied to all instances that are associated with the security group. If there is more than one security group associated with an instance, the rules of all groups are combined.
Security groups in a VPC are associated with network interfaces. Changing an instance's security groups changes the security groups associated with the primary network interface (eth0). Additional security groups can be associated with any other network interfaces added to an instance.
Points to note with respect to security groups:
- By default, all outbound traffic is allowed.
- Rules in a security group always define what is permitted. They cannot be used to deny specific traffic.
- Response traffic to traffic that was permitted by a rule is always allowed (connection tracking).
- A security group cannot allow more permissive access to a subnet than the permitted traffic defined in the network ACL of the subnet.
Please see the relevant AWS documentation for more information and configuration details.
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