How to avoid asynchronous routing in Ubuntu?

Question

VPN

Greetings. There is approximately such a tricky topology: The

question arose - how can you access the server with ubuntu (6.3) from the router (192.168.6.100) without asynchronous routing. Those. how to tell Ubuntu that it would respond through the same interface to which the request came, despite the fact that all connections should always be active. On the server, you get 3 routes to the same network through different gateways and through different devices. All this economy is connected among themselves through vpn. All routers, if anything - microtics.
It is impossible to natit traffic from three routers in the middle.
You can't change the address either.
On the upper router, in turn, the middle connections change randomly, i.e. then 6.1 is not available, then 6.4, then all are available.

Answer 1

Mark the packets at the entrance and, according to the marking, throw them according to different tables at the exit.
https://habr.com/post/108690/

Answer 2

In the current configuration, all hosts are in the same broadcast range. It's more like switching than routing. The scheme needs to be redone, and not further aggravate the situation.
You must always use only 1 of the available connections by configuring routing and, for example, OSPF.