Jimmy shows you how to
configure NAT on your
VPC CIDR for VPN traffic

I have an AWS virtual private network (VPN) connection to a network or Amazon Virtual Private Cloud (Amazon VPC) where the network CIDRs overlap or I want to expose only a single IP. How do I configure network address translation (NAT) for my AWS VPN?

AWS VPN does not currently provide a managed option to apply NAT to VPN traffic. Instead, you can manually configure NAT using a software-based VPN solution, of which there are several options in the AWS Marketplace. You can also manually configure NAT on an Amazon Elastic Compute Cloud (EC2) Linux instance running a software-based VPN solution along with iptables.

This example configuration uses two VPCs. The first is an AWS managed VPN and the second is a software-based VPN solution that is used as the customer gateway.

Before you begin, be sure that you set up an AWS VPN connection. Be sure to install your chosen VPN solution on the EC2 Linux instance by using your distribution's package manager.

Allow VPN Traffic

Configure your VPC route table, security groups, and NACLs to allow VPN traffic:

  1. Enter the route towards the destination network into your route table. Set the elastic network interface of your software VPN EC2 instance as the target.
  2. Be sure that your route table has a default route with a target of an internet gateway.
  3. Allow inbound traffic using UDP port 500 (ISAKMP) and 4500 (IPsec NAT-Traversal) in the instance's security group rules.
  4. Disable source/destination checks to allow the instance to forward IP packets.

Configure VPN Connection

Configure the VPN connection based on the solution you chose. AWS offers several downloadable example configuration files based on device vendor and model.

Configure iptables

Configure your iptables rules based on the type of NAT you want to perform.

For source NAT, use the following string, filling in appropriate values in place of the brackets:

sudo iptables -t nat -A POSTROUTING -d <Destination address or CIDR> -j SNAT --to-source <Your desired IP address>

For destination NAT, use the following string, filling in appropriate values in place of the brackets:

sudo iptables -t nat -A PREROUTING -j DNAT --to-destination <Your desired IP address>

To save your running iptables configuration to a file, use this command:

sudo iptables-save > /etc/iptables.conf

To load this configuration on boot, place the following line in /etc/rc.local before the exit 0 statement:

iptables-restore < /etc/iptables.conf

Optional: Test your AWS VPN connection. You should now see that traffic is appropriately translated based on your iptables configuration.

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Published: 2018-07-17