Cutting NAT Gateway Costs on AWS

AWS NAT Gateway fees are a common source of unexpected costs and can easily give you a nasty surprise on your monthly bill.

I experienced this firsthand recently. My NAT Gateway charges shot past $1,000 in just a few days, forcing me to scramble for ways to reduce them without compromising the reliability of my applications. It was a stressful wake-up call.

In this article, I’ll share the strategies—from low-hanging fruits to architectural changes—that helped me get those costs back under control.

Why do you need a NAT Gateway anyway?

In short: security.

A NAT (Network Address Translation) Gateway allows instances in a private subnet to connect to the internet (for updates, patching, or external API calls) while preventing the internet from initiating connections inward to those instances.

For a few years, I used public subnets for almost all my workloads just to avoid the headaches (and costs) of NAT Gateways. But as my infrastructure grew, I realized the importance of isolating critical resources, like databases or application servers, in private subnets to reduce the attack surface.

Visualizing the Difference: Private vs Public Subnets

Let's put aside the complex networking jargon for a moment. The difference really comes down to one question: How exposed do you want your server to be?

The Public Subnet: Direct Access

Think of a Public Subnet as the "front door" of your house. When you launch an EC2 instance here, you typically assign it a Public IP address.

• The Reality: Your instance is directly connected to the internet. It can download updates or reply to web requests freely via the Internet Gateway (which is free).
• The Risk: Since it has a public address, it is visible to the entire world. This increases your attack surface significantly because anyone can attempt to connect to it (unless your Security Groups are impeccable).

(Note: You could technically put an instance here without a Public IP, but it would be an edge case: it would be completely stranded with no way to reach the internet or be reached, rendering it mostly useless for standard workloads).

The Private Subnet: The "Safe Zone"

This is where you should keep your crown jewels. Instances here do not have Public IP addresses and are hidden behind the VPC network.

• The Advantage: Massive security boost. You can host your Internal Load Balancers, Databases, and Backend Workers here. The outside world literally cannot initiate a connection to them, reducing your attack surface to near zero.
• The Problem: Since they have no direct route to the outside, they are stuck. If your private database needs to download a security patch from the internet, it can't.

This is where the NAT Gateway comes in. It acts as a secure middleman, allowing your private instances to "phone out" for updates without letting anyone from the outside "phone in."

Here is a simple breakdown:

To visualize this, imagine the Internet Gateway in the public subnet is a wide-open door. The NAT Gateway for the private subnet is a security guard: he lets your staff out to run errands, but he physically stops strangers from walking in.

Understanding NAT Gateway Costs (Where the money goes)

Before fixing the problem, we need to understand exactly how AWS charges for this service. It’s not just a flat fee; it's a two-part pricing model that can get expensive fast.

1. The Hourly "Rental" Charge

You pay a fixed hourly rate just for having the NAT Gateway provisioned and available, regardless of whether you use it.

• Cost roughly: ~$0.045 per hour (varies slightly by region).
• Impact: That’s about $32 per month, per NAT Gateway.

2. The Data Processing Charge (The hidden killer)

This is where my bill exploded. You are charged for every gigabyte of data that passes through the NAT Gateway.

• Cost roughly: ~$0.045 per GB processed.

Here is the crucial trap: This includes data going out to the internet and the response data coming back in.

Furthermore, if your EC2 instance is in one Availability Zone (e.g., us-east-1a) and your NAT Gateway is in another (e.g., us-east-1b), you might be hit with Inter-AZ Data Transfer costs before the data even reaches the NAT Gateway to be processed.

In summary: high traffic volume + crossing Availability Zones = skyrocketing bills.

💸 What kind of traffic causes high NAT costs?

Any workload that involves significant outbound internet traffic from private subnets can lead to high NAT Gateway costs. You might think "I don't browse the web from my servers," but here is where the data actually goes.

1. 🗄️ External Managed Databases (The "Data Hogs")

This is often the biggest culprit. We frequently use managed database services hosted outside our VPC, such as MongoDB Atlas, Redis Cloud (Upstash), or ClickHouse Cloud.

The Cost Mechanism: Every single query to a service hosted outside your VPC goes through the NAT Gateway and incurs data processing fees.

2. ☁️ "Internal" AWS Services (The S3 Trap)

This is the most counter-intuitive category. Many assume traffic to Amazon S3 or DynamoDB stays "internal" to the AWS network and is free.

⚠️ Warning: It is not free by default. By default, S3 and DynamoDB are accessed via public endpoints. If your private instances push gigabytes of backups to S3 or pull large datasets, all that traffic routes through the NAT Gateway.

I've seen bills double just because of a nightly backup job running from a private subnet.

3. 📦 Docker Images & Software Updates

Every time your server starts, it likely initiates downloads. This becomes critical in auto-scaling environments.

• 🐳 Docker Pulls: If you use massive Docker images (e.g., 1GB+) and have an auto-scaling group that frequently spins up new instances, you are downloading that gigabyte through the NAT every single time.
• 🔄 OS Updates: Running apt-get update or yum install fetches packages from public repositories via the NAT.

4. 📡 Third-Party APIs and Firehoses

While a few requests to Stripe or SendGrid are negligible, high-volume connections can hurt significantly.

Think about logging agents (like Datadog or Splunk) or analytics tools:

• If you ship terabytes of logs to an external provider via the internet...
• You are paying the "NAT tax" on every single log line.

Summary: Where is the money going?

Get accurate stats with VPC Flow Logs

To truly understand where your data is flowing and how much is passing through your NAT Gateway, enable VPC Flow Logs. This will give you detailed insights into the traffic patterns within your VPC, helping you identify the biggest offenders.

Analyzing Flow Logs:

I have created a package vpc-flowlogs-egress-analyzer that can help you to understand your egress traffic better by parsing VPC Flow Logs stored in S3 and generating a report of the top destinations and data volumes.

A result.json file will be generated and will contain detailed information about the top egress destinations, including the total bytes transferred to each destination IP address. This can help you identify which external services are consuming the most bandwidth through your NAT Gateway.

Cost Optimization Strategies

FCK NAT: The (f)easible (c)ost (k)onfigurable NAT!

The fck nat project is a game-changer for anyone looking to cut down on NAT Gateway costs. It provides a self-managed NAT solution that can be deployed on EC2 instances. The idea is to route traffic from private subnets to the network interface of an EC2 instance acting as a NAT. You will just have to pay for the EC2 instance hours and the data transfer fees, which are often significantly lower than NAT Gateway costs.

In my case, I switched our dev environment to use fck nat on a t3.medium instance, which cost me around $30/month. This change alone cut our NAT costs by over 70%. You just need to ensure you have proper monitoring and scaling in place to handle traffic spikes. I created a specific grafana dashboard to monitor critical metrics from this instance like Bandwidth allowance exceeded, Packet drops etc...

Thoughts on using fck nat in production:

While fck-nat is extremely effective for development and staging environments, I have not yet adopted it in production.

In theory, the project supports a production-grade setup: you can deploy it behind an Auto Scaling Group for high availability, benefit from the instance’s natural bandwidth scaling model (larger instances = higher network throughput).

That being said, NAT is a critical piece of network infrastructure, and I have not seen enough real-world production references or large-scale case studies to confidently replace AWS’s managed NAT Gateway in mission-critical systems. I believe it can absolutely be made production-ready with the proper tooling, monitoring, failover automation, and redundancy — but it requires more operational discipline and ownership than what many teams expect.

Using fck nat alongside NAT Gateway

Something I want to experiment with in the future is a hybrid approach: using fck-nat for non-critical egress traffic in production while keeping the managed NAT Gateway for critical workloads that require high availability and AWS support. For example if you pull large amount of non-critical data from third parties, you could route that traffic through fck-nat while keeping your database connections and other essential services on the managed NAT Gateway. Is it something you can try and report back on? I’d love to hear your experiences!

PrivateLink Endpoints for AWS Services

One of the most effective strategies for reducing NAT Gateway costs is to leverage AWS PrivateLink endpoints for the AWS services your workloads interact with the most. After running the flow log analysis described earlier, it becomes clear which services generate the largest share of outbound traffic — notably S3, ECR, DynamoDB, and other internal AWS APIs.

By creating Interface Endpoints (PrivateLink) for these services inside your VPC, traffic stays entirely within the AWS network instead of routing through the NAT Gateway. This alone can eliminate a substantial amount of NAT data processing charges.

AWS documentation

A simple way to verify that PrivateLink is working correctly is to check DNS resolution from within your cluster or EC2 instances. For example, for ECR "DKR" (image layer downloads), you can run:

dig <ACCOUNT_ID>.dkr.ecr.eu-west-3.amazonaws.com +short

If the endpoint is configured correctly, you should see private IPs associated with your VPC’s PrivateLink ENIs, such as:

192.168.156.183
192.168.164.84
192.168.112.7

This confirms that the traffic is routed internally, bypassing the NAT Gateway entirely.

PrivateLink for Third-Party Services (MongoDB Atlas, etc.)

Another discovery that had a major impact on my NAT bill was the fact that several third-party providers now support PrivateLink as well. For example, MongoDB Atlas and many others provide the ability to expose their services through AWS PrivateLink endpoints.

In my case, enabling PrivateLink for MongoDB Atlas alone removed several gigabytes per day of egress traffic that previously flowed through the NAT Gateway. The cost reduction was immediate and substantial.

If your flow log analysis shows significant outbound traffic to a third-party SaaS or managed database, it is absolutely worth checking whether they support PrivateLink — it can be a game-changer both financially and operationally.

Conclusion

I hope this little deep dive helped you make sense of where your NAT Gateway costs are coming from and what you can actually do about them. When it happened to me, it felt both stressful and confusing — like money was leaking out of the VPC and I had no idea why. But once you know where to look, the problem becomes much easier to understand (and to fix).

If you’re facing a sudden spike or just want to get ahead of future surprises, I really encourage you to turn on VPC Flow Logs, run an analysis, and start with the simple wins: PrivateLink for AWS services, checking whether your third-party providers support PrivateLink, and optimizing the big data movers. Even small adjustments can have a huge impact.

And if you try out my analyzer or discover something interesting in your own logs, feel free to reach out — I’d love to hear about it. Hopefully this article saves you some time, some stress, and maybe even a chunk of your AWS bill. 😉