NAT Gateway vs egress-only Security Group

Question

Hi, I am wondering what the security benefits are of an resource in a private subnet behind a NAT Gateway vs a resource in a public subnet where the resource has a Security Group without any ingress rules.

The main function of NAT gateway is to only allow outbound-initiated communications, but the security group without ingress rules achieves this too. NAT gateway is of course more expensive (security groups being free) so there must be reasons which make the NAT gateway more secure else nobody would use it.

I can see the following security benefits:
- If only a security group, someone could remove or modify it accidentally/without realising the implications, in such a way as to permit inbound-initiated comms.
- More declarative; it's clear to developers that services in a private subnet shouldn't receive inbound-initiated comms.

There are also non-security related benefits, eg if you need other people to allowlist you then it's easier to allow a single IP (that of the NAT Gateway).

Have I missed any benefits, in particular security benefits? Because the benefits I have listed don't seem to explain why NAT gateway is so greatly preferred?

Thanks, Ben

Tavares F · Accepted Answer

Hi Ben L,

You are correct in the concerns about using Elastic IPs in Instances.

I can point  some additions to the points 2 and 3 as you asked.
In terms of IP address obfuscation, once the outbound IP is not attached directly to the instance, if an external resource gets your IP when you start a communication, they will not able to reach your instance directly to explore any security breach, besides it is not possible to know how many instances are behind the NAT Gateway so a external observer will not know of you are using  one, two or twenty instances to reach their service, so if you have several internal clients you can obfuscate them behind the NAT Gateway bringing more privacy.

About centralized control, when you provide a single pont of egress you can control the outbound data easier because when you have an IGW, in all instance that have elastic IPs you have to manage security group to avoid the egress communication. With a NAT Gateway you can manage only the Nat Gateway subnet NACL to  avoid any host to access the NAT Gateway.

You also will be charged by every EIP (public IPv4) you have in your ec2 instances or ECS tasks.

Regards

re:Post Agent · Answer

You've identified some key benefits of using a NAT Gateway over a security group without ingress rules, but there are indeed additional security advantages to using a NAT Gateway:

1. Network-level isolation: A NAT Gateway provides network-level isolation by placing resources in a private subnet. This adds an extra layer of security beyond just the security group rules.

2. IP address obfuscation: The NAT Gateway hides the private IP addresses of your instances from the internet, making it harder for potential attackers to map your internal network structure.

3. Centralized control: NAT Gateways provide a single point of egress for multiple instances, allowing for easier monitoring and control of outbound traffic.

4. Reduced attack surface: By keeping instances in a private subnet, you eliminate the need for public IP addresses on those instances, reducing the overall attack surface.

5. Protection against misconfiguration: As you mentioned, it's harder to accidentally expose instances to the internet when they're in a private subnet behind a NAT Gateway.

6. Stateful filtering: NAT Gateways perform stateful translation, allowing only return traffic that corresponds to outbound requests.

7. Scalability and high availability: NAT Gateways are managed by AWS, offering better scalability and availability compared to manually configured security groups.

8. Compliance requirements: Some compliance standards may require the use of NAT Gateways for outbound internet access from private resources.

While security groups are powerful and flexible, NAT Gateways offer these additional layers of security and control that make them a preferred choice for many organizations, especially those with strict security requirements or complex network architectures. The cost is often justified by the enhanced security posture and simplified management of outbound internet access for private resources.  
**Sources**  
[EC2 access to internet in private subnet | AWS re:Post](https://repost.aws/questions/QUmaicWxiZSDiXYZ18FaiwIQ/ec2-access-to-internet-in-private-subnet)  
[NAT Gateway for outbound request | AWS re:Post](https://repost.aws/questions/QU9RYFVH0ARXWaNvUt9LB2-w/nat-gateway-for-outbound-request)  
[NAT gateways - Amazon Virtual Private Cloud](https://docs.aws.amazon.com/vpc/latest/userguide/vpc-nat-gateway.html)

NAT Gateway vs egress-only Security Group

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