Building a Custom NAT Server on AWS: A Step-by-Step Guide

Network Address Translation (NAT) servers are essential components in a cloud infrastructure, allowing instances in a private subnet to connect to the internet or other AWS services while preventing the internet from initiating a connection with those instances. This blog provides a detailed guide on setting up a NAT server from scratch in an AWS cloud environment.

Step 1: Launching an AWS Instance

Start a t1.micro instance:

• Navigate to the AWS Management Console.
• Select the EC2 service and choose to launch a t1.micro instance.
• Pick an Amazon Machine Image (AMI) that suits your needs (commonly Amazon Linux or Ubuntu).
• Configure instance details ensuring it's in the same VPC as your private subnet but in a public subnet.

Step 2: Configuring the Instance

Disable "Change Source / Dest Check":

• Right-click on the instance from the EC2 dashboard.
• Navigate to "Networking" and select "Change Source / Dest Check."
• Disable this setting to allow the instance to route traffic not specifically destined for itself.

Security Group Settings:

• Ensure the Security Group associated with your NAT instance allows the necessary traffic.
• Typically, it should allow inbound traffic on ports 80 (HTTP) and 443 (HTTPS) for updates and patches.

Step 3: Configuring the NAT Server

Access your instance via SSH and perform the following configurations:

Enable IP Forwarding:

1. Edit the /etc/sysctl.conf file to enable IP forwarding. This setting allows the instance to forward traffic from the private subnet to the internet.

   sed -i "s/net.ipv4.ip_forward.*/net.ipv4.ip_forward = 1/g" /etc/sysctl.conf
   

2. Activate the change immediately:

   echo 1 > /proc/sys/net/ipv4/ip_forward
   

3. Confirm the change:

   cat /etc/sysctl.conf | grep net.ipv4.ip_forward
   

   Expected output: net.ipv4.ip_forward = 1

Configure iptables:

1. Set up NAT using iptables to masquerade outbound traffic, making it appear as if it originates from the NAT server:

   iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
   

   This command routes all connections reaching eth0 (the primary network interface) to all available paths.

2. Allow traffic on ports 80 and 443 for updates and external access:

   iptables -A INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT
   iptables -A INPUT -m state --state NEW -p tcp --dport 443 -j ACCEPT
   iptables -A FORWARD -i eth0 -j ACCEPT
   

Step 4: Routing Configuration

Configure Route Tables:

• In the AWS Console, go to the VPC Dashboard and select Route Tables.
• Modify the route table associated with your private subnet:
  • Add a route where the destination is 0.0.0.0/0 (representing all traffic), and the target is the instance ID of your NAT server.
• Modify the route table associated with your NAT instance:
  • Ensure there's a route where the destination is 0.0.0.0/0, and the target is the internet gateway of your VPC.

Conclusion

With these steps, you've successfully created a NAT server in your AWS environment, allowing instances in a private subnet to securely access the internet for updates and communicate with other AWS services. This setup is crucial for maintaining a secure and efficient cloud infrastructure. Always monitor and maintain your NAT server to ensure it operates smoothly and securely. Currently there are managed NAT server services from AWs which we can use for production grade environments.