Create Custom NAT Instance in AWS VPC

Purpose

Build a NAT instance using CentOS to propose internet access for private subnet in AWS VPC.
In this tutorial, we assume that we already have a VPC with public and prvate subnet like belo picture.

Preparation

[root@nat-server ~]# yum update

Configure System

In /etc/sysctl.conf enable ip forwarding :

[root@nat-server ~]# vi /etc/sysctl.conf

Add this :

# For NAT Server
net.ipv4.ip_forward = 1

Reboot now for good measure :

[root@nat-server ~]# reboot

Test our config :

[centos@nat-server ~]$ cat /proc/sys/net/ipv4/ip_forward
1

Set Iptables for Masquerade

Issue iptables command below :

[centos@nat-server ~]$ iptables -t nat -A POSTROUTING -o eth0 -s 172.31.158.0/24 -j MASQUERADE

Edit the /etc/rc.local file to make masquerade will automatically enable at boot time :

[centos@nat-server ~]$ vi /etc/rc.local

And add this iptables command before "exit 0" :

...

iptables -t nat -A POSTROUTING -o eth0 -s 172.31.158.0/24 -j MASQUERADE
exit 0

...

Run 'chmod +x /etc/rc.d/rc.local' to ensure the script will be executed during boot :

[centos@nat-server ~]$ chmod +x /etc/rc.d/rc.local

Create Custom Route to Associate the Private Subnet

1. Create route tables (attach to the VPC)

   Route

   Destination : 172.31.0.0/16 ; target : local

   Destination : 0.0.0.0/0 ; target : nat instance

   Subnet association

   172.31.2.0/24