How to connect via IPIP tunnel?

Question

linux

Hello, I have 2 machines connected via ip tunnel. (left filtering, right receiving)
So far, it is possible to switch between them only through the local IP 192.168.168.1 and 192.168.168.2
How to send packets from the third machine through the filtering one so that they get to the receiving one?

Answer 1

Using NAT on the filter machine.
Or by setting up routing along the entire path of the packets.