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Hritik Raj
Hritik Raj

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🚨 AWS 130: Routing the Private Way - Implementing a NAT Instance

#aws #networking #ec2 #100daysofcloud

AWS

🛠️ The Cost-Effective Gateway: Providing Internet Access via NAT Instance

Hey Cloud Builders 👋

Welcome to Day 30 of the #100DaysOfCloud Challenge!
Today, we are helping the Nautilus team connect a private server to the outside world. To keep costs low, we are passing on the managed NAT Gateway and building our own NAT Instance. This allows our private instance to securely upload files to S3 without being exposed to the public internet.

This task is part of my hands-on practice on the KodeKloud Engineer platform, which I highly recommend for anyone looking to master real-world DevOps scenarios.

🎯 Objective

  • Create a public subnet named datacenter-pub-subnet.
  • Launch a specialized NAT Instance named datacenter-nat-instance.
  • Configure a custom Security Group to allow internal traffic to flow through the NAT.
  • Update private route tables to point to the NAT Instance.
  • Verify connectivity by checking for successful file uploads in the datacenter-nat-31923 S3 bucket.

💡 Why NAT Instances Matter

A private subnet has no direct path to the internet. A NAT (Network Address Translation) device acts as a "middleman" that sends requests out on behalf of the private server.

🔹 Key Concepts

  • NAT Instance vs. Gateway: A NAT Instance is a regular EC2 instance configured to perform routing. It is cheaper than a managed NAT Gateway but requires manual setup and management.

  • Source/Destination Check: By default, EC2 instances only accept traffic meant for them. To act as a NAT, we must disable this check so the instance can forward traffic from other sources.

  • Public Subnet Requirement: The NAT Instance must live in a public subnet with a route to an Internet Gateway (IGW) to reach the outside world.

🛠️ Step-by-Step: The NAT Workflow

We’ll move from Network Setup → Instance Configuration → Routing.

🔹 Phase A: Prepare the Public Network

  • Create Subnet: Add datacenter-pub-subnet to the existing VPC.
  • Internet Access: Ensure this public subnet has a route to the VPC's Internet Gateway (IGW) in its route table.

🔹 Phase B: Launch and Configure the NAT Instance

  • AMI Selection: Use an Amazon Linux 2 AMI.
  • Security Group: Create a custom SG that allows Inbound HTTP/HTTPS (and ICMP for testing) from the private subnet's CIDR range.
  • Disable Source/Dest Check: This is the most critical step! Select the instance → Actions → Networking → Change source/destination check → Set to Stop.

[Image of the AWS EC2 console showing the dialog to disable Source/Destination check on an instance]

🔹 Phase C: Update Private Routing

  • Route Table Edit: Locate the route table for datacenter-priv-subnet.
  • Add Route: * Destination: 0.0.0.0/0
    • Target: Select Instance and choose your datacenter-nat-instance.

✅ Verify Success

  • Wait for the Cron: The private instance datacenter-priv-ec2 is already running a script to upload datacenter-test.txt.
  • Check S3: Navigate to the S3 bucket datacenter-nat-31923.
  • Confirm: 🎉 If the file appears in the bucket, your NAT Instance is successfully translating and forwarding traffic!

📝 Key Takeaways

  • 🚀 Custom Routing: The NAT Instance becomes the "Default Gateway" for the private subnet.
  • 🛡️ IP Masquerading: The S3 bucket sees the traffic coming from the NAT Instance's public IP, not the private IP of the server.
  • 💸 Cost Efficiency: For low-traffic dev environments, a NAT instance can be significantly cheaper than the hourly cost of a NAT Gateway.

🚫 Common Mistakes

  • Leaving Source/Dest Check ON: The instance will drop any traffic that isn't addressed to its own IP, breaking the NAT functionality.
  • Security Group Tightness: Forgetting to allow traffic from the private CIDR into the NAT Instance's security group.
  • Routing Loop: Pointing the private route table to an instance that doesn't have internet access itself.

🌟 Final Thoughts

You’ve just manually built a core networking component! Understanding NAT instances gives you deep insight into how Linux routing and AWS VPC networking interact. This is foundational knowledge for troubleshooting complex connectivity issues.

🌟 Practice Like a Pro

If you want to try these tasks yourself in a real AWS environment, check out:
👉 KodeKloud Engineer - Practice Labs

It’s where I’ve been sharpening my skills daily!

🔗 Let’s Connect

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