Network between 2 instances of OpenVPN

There are some reasons when you have two or more instances on one server.
Assume that:

Clients of first instance are in 10.8.0.0/24
Clients of second in 10.8.1.0/24
Server IPs is 10.8.0.1 (tun0 interface) and 10.8.1.1 (tun1).
Each network has clients 10.8.0.2 and 10.8.1.2.

Task is allow ping between 10.8.0.2 and 10.8.1.2 .

Add routes

On 10.8.1.2 we need to add route to 10.8.0.0/24 network via IP 10.8.1.1.

Knowing this OS will forward packets with IP 10.8.0.ххх to 10.8.1.1, so they will go to server and appear on tun1.

1st way to make it manually, e.g. if Client 2 run Windows:

route -p add 10.8.0.0 mask 255.255.255.0 10.8.1.1

Same Same thig with Client 1, e.g. if it run Linux:

route add -net 10.8.1.0 netmask 255.255.255.0 gw 10.8.0.1

This is not cool way because it needs performing manual actions on clients and after reboot changes will be lost.

2nd and correct way is use automatic routes configuring with push "route х.х.х.х mask" option.

For 10.8.1.0/24 network openvpn config we need to add:

push "route 10.8.0.0 255.255.255.0"

In this case, when Client 2 will connect to OpenVPN it will automaticaly do things that we performed manually. Add same line but with route 10.8.1.0 to another config.

IP Forward in sysctl

Add (or uncomment) next to /etc/sysctl.conf

net.ipv4.ip_forward=1

and reload it:

sysctl --system

iptables

The last thing change iptables on server:

iptables -A FORWARD -i tun1 -o tun0 -s 10.8.1.0/24 -d 10.8.0.0/24 -j ACCEPT
iptables -A FORWARD -i tun0 -o tun1 -s 10.8.0.0/24 -d 10.8.1.0/24 -j ACCEPT

Also take care about persistent saving iptables settings.