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We have a Strongswan VPN server on Ubuntu 20.04 in AWS. It is used to establish a site-to-site VPN tunnel with a client.

Here's a simple topology diagram: enter image description here

Our VPN server will terminate VPN and NAT traffic to the RDS database server. In order for it to response correctly our VPN server will SNAT the packet with its address, so we basically lost track of the client computer's IP address. And there are multiple client computers.

In this case, how do we properly NAT the response to client?

Thank you for your time.

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  • If I understand you correctly, you already have a NAT that maps client IPs to the VPN server's internal IP. So what do you mean with "properly NAT the response to client"?
    – ecdsa
    Jun 10 at 16:20
  • Why are you doing NAT? Jun 10 at 16:37
  • @ecdsa the RDS server is not inside of the VPN network so it has to send all return traffic to our VPN server, from there we are supposed to NAT the traffic to client. However at this point we have lost track of the source(client's) IP address, so the server does not know where to send it.
    – Cal
    Jun 10 at 16:56
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I think do you want to use Inverted Split Tunnel VPN? OR inverted full tunnel to your client? so the load and the Nat will be in your client side.

what kind of vpn you use? Site To Site?

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The SNAT will create an entry in the connection tracking table for each flow it translates. The data from this table is automatically applied to the return traffic.

The individual connection is identified by the ephemeral port number assigned to the flow by the NAT translation of the first packet, and the DB server addresses its replies to this port, which is sufficient for finding the original connection data.

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