Routing VPN traffic through another network interface

Question

I have an OpenVPN server set up on Ubuntu 18.04. Clients connect to the VPN and all traffic is routed through the VPN. The outgoing IP is the same IP the clients connect to.

Now I've added another network interface on a private subnet. I want the clients' outgoing traffic to be routed through that interface so that the outgoing IP will be the external ip of the NAT gateway on the private subnet.

I'm a bit at a loss as I don't know how to configure the routing. If I just increase the metric of eth1 to put it above eth0 then I can no longer connect to the VPN or SSH to the server.

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         ip-172-30-0-1.e 0.0.0.0         UG    100    0        0 eth0
default         ip-172-30-10-1. 0.0.0.0         UG    200    0        0 eth1
10.8.0.0        0.0.0.0         255.255.255.0   U     0      0        0 tun0
172.30.0.0      0.0.0.0         255.255.255.0   U     0      0        0 eth0
ip-172-30-0-1.e 0.0.0.0         255.255.255.255 UH    100    0        0 eth0
172.30.10.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1
ip-172-30-10-1. 0.0.0.0         255.255.255.255 UH    200    0        0 eth1

Green arrows are what I want to achieve, red is what happens now.

Answer 1

It seems like a problem of asymmetric routing, I've had these in multi-homed servers before. One reason you can no longer ssh to your server is that the packet comes from on interface but the response goes thru another. If I remember correctly, when sending simple ping to IP A, I received a reply from IP B and it doesn't go well with secure protocols/applications. I solved this using "policy routing"

That article helped me do the trick: 4.1. Simple source policy routing

And here is good article about Asymmetric Routing on Multi-Homed Servers