How to forward packets without changing the sender address?

Question

linux

Explanation.
When working with an "iron" router (even if it's Dlink) ... It has 2 ports. VAN and LAN. All packets coming to the VAN are pulled onto the LAN port. At the same time, the machines behind the router located on the LAN port see the external IP addresses of the senders of these packets.

Yes, I have Linux) AND IPTABLES.
2 interfaces. VAN and VPN (on which other machines sit)
And in fact 3 rules.
1. Forward
2. D-NAT (PreRoute)
3. S-NAT (PostRoute)

Well, machines in the VPN network see incoming packets with the IP address of the VPN interface of the Linux router.
And here is the question from the title ... How to forward packets without changing the sender address?
For everything that I do so far either stops working altogether, or works the way it works. )

Thank you.