Well, this is a rather awkward inquiry and I'm pretty pessimistic about it, but lets see what the community has to say.
The scenario:
A server connecting to multiple VPN endpoints doing all sort of stuff.
The problem:
The same IP can be assigned to multiple devices.
The challenge:
Be able to route traffic successfully through each of these devices with the same IP address.
In the following scenario:
tun0: 192.168.200.10 192.168.200.1
tun1: 192.168.200.11 192.168.200.1
tun2: 192.168.200.12 192.168.200.1
No problems. Using Linux's sources based routing things work like a charm. However, since I don't control from the client side the IP address I'm going to be assigned, I might get:
tun0: 192.168.200.10 192.168.200.1
tun1: 192.168.200.10 192.168.200.1
tun2: 192.168.200.10 192.168.200.1
And here comes the problem. Is there a way to control routing tables per interface regardless of IP?
Currently each interface that goes up gets another entry in /etc/iproute2/rt_tables such as:
100 tun0
101 tun1
102 tun2
Then the following for each device:
ip rule add from 192.168.200.X iif tunX table tunX
ip route add default via 192.168.200.1 table tunX
In order to test it I'm using 'curl --interface tunX'.
The first interface more or less "wins" the routing decision.
I want the right interface to win.
Theoretically I'm well aware I'm doing something many would consider wrong. However I'd like to know if there is a way around it, because I think there is.
Any ideas about it? Perhaps iptables fwmark could help if at all?