VPC Network Access Control Lists

- June 03, 2020

VPC Network Access Control Lists (NACL) functions as a firewall in that it contains inbound and outbound rules to allow traffic based on a source or destination CIDR, protocol, and port. Also, each VPC has a default NACL that can’t be deleted.

But the similarities end there. A NACL differs from a security group in many respects. Instead of being attached to an ENI, a NACL is attached to a subnet. The NACL associated with a subnet controls what traffic may enter and exit that subnet. This means that NACLs can’t be used to control traffic between instances in the same subnet. If you want to do that, you have to use security groups. A subnet can have only one NACL associated with it. When you create a new subnet in a VPC, the VPC’s default NACL is associated with the subnet by default. You can modify the default NACL, or you can create a new one and associate it with the subnet. You can also associate the same NACL with multiple subnets, provided those subnets are all in the same VPC as the NACL. Unlike a security group, which is stateful, a NACL is stateless, meaning that it doesn’t track the state of connections passing through it. This is much like an access control list (ACL) on a traditional switch or router. The stateless nature of the NACL is why each one is preconfigured with rules to allow all inbound and outbound traffic, as discussed in the following sections,VPC Network Access Control Lists.

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Inbound Rules

Inbound rules determine what traffic is allowed to ingress the subnet. Each rule contains the following elements:

Rule number
Protocol
Port range
Source
Action

VPC Network Access Control Lists, The default NACL for a VPC with no IPv6 CIDR comes prepopulated with the two inbound rules listed in

NACL rules are processed in ascending order of the rule number. Rule 100 is the lowest numbered rule, so it gets processed first. This rule allows all traffic from any source. You can delete or modify this rule or create additional rules before or after it. For example, if you wanted to block only HTTP (TCP port 80), you could add the following rule:

before or after it. For example, if you wanted to block only HTTP (TCP port 80), you could add the following rule:

This rule denies all TCP traffic with a destination port of 80. Because it’s the lowest numbered rule in the list, it gets processed first. Any traffic not matching this rule would be processed by rule 100, which allows all traffic. The last rule in Table 4.5 is the default rule. It’s designated by an asterisk (*) instead of a number and is always the last rule in the list. You can’t delete or otherwise change the default rule. The default rule causes the NACL to deny any traffic that isn’t explicitly allowed by any of the preceding rules. Complete Exercise 4.6 to create a custom NACL.
Also Read :- Overview of the TCP/IP Networking Model

Outbound Rules

As you might expect, the outbound NACL rules follow an almost identical format as the inbound rules. Each rule contains the following elements:

Rule number
Protocol
Port range
Destination
Action

Each default NACL comes with the outbound rules listed in Table 4.6. Notice that the rules are identical to the default inbound rules except for the Destination element.

In most cases you will need these rules whether you use the default NACL or a custom one. Because a NACL is stateless, it won’t automatically allow return traffic. Therefore, if you permit HTTPS traffic with an inbound rule, you must also explicitly permit the return traffic using an outbound rule. In this case, rule 100 permits the return traffic. If you do need to restrict access from the subnet—to block Internet access, for example—you will need to create an outbound rule to allow return traffic over ephemeral ports. Ephemeral ports are reserved TCP or UDP ports that clients listen for reply traffic on. As an example, when a client sends an HTTPS request to your instance over TCP port 80, that client may listen for a reply on TCP port 36034. Your NACL’s outbound rules must allow traffic to egress the subnet on TCP port 36034. The range of ephemeral ports varies by client operating system. Many modern operating systems use ephemeral ports in the range of 49152–65535, but don’t assume that allowing only this range will be sufficient. The range for TCP ports may differ from the range for UDP, and older or customized operating systems may use a different range altogether. To maintain compatibility, do not restrict outbound traffic using a NACL. Use a security group instead. If your VPC includes an IPv6 CIDR, AWS will automatically add inbound and outbound rules to permit IPv6 traffic.

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