Creating a NAT instance with Ubuntu

AWS moved from Amazon Linux to Amazon Linux 2, and now they’ve stopped supporting Amazon-Linux-based AMIs. Also, it seems that they won’t release an AMI for a NAT instance based on Amazon Linux 2.

You now need to configure your instance by yourself to use it as a NAT instance.

I’ve created a NAT instance based on Ubuntu 20.04 LTS AMI, and here is what I’ve done.

  1. Create an EC2 instance from Ubuntu 20.04 LTS AMI
  2. Disable “Source / destination check”
  3. Login to the instance

Now you can configure your instance using ufw. It’s a wrapper around good old iptables. I was kind of surprised that it still uses iptables after 25 years since I first launched a linux-based NAT box at home.

First, allow ssh by sudo ufw allow ssh.

Next, edit /etc/default/ufw and change DEFAULT_FORWARD_POLICY from DENY to ACCEPT.

Next, edit /etc/ufw/sysctl.conf and uncomment net/ipv4/ip_forward=1.

Now, edit /etc/ufw/before.rules to add NAT rules. You’ll add these rules at the end.

# NAT
*nat
:POSTROUTING ACCEPT [0:0]
-F
-A POSTROUTING -s 10.0.0.0/16 -j MASQUERADE
COMMIT

Don’t forget to change 10.0.0.0/16 to your network address.

Once you’ve done all of them, sudo ufw enable to enable these rules.