Otdfbt

When openvpn client connects to a vpn server it creates a route for server ip with old default gateway. How can I tell openvpn not to use old default gateway but use my provided custom gateway ip.

I.e. when I have vpn server in another vpn network which is not default route on my machine.

Adding this two line to the Server Config File will force all traffic to go through your VPN

push "redirect-gateway def1"
push "dhcp-option DNS 8.8.8.8"

If you want to set a next-hop router for a specific subnet after both VPNs are established, use something like

ip -4 route add $DESTINATION_SUBNET via $NEXT_HOP dev $INTERFACE_NAME

for example

ip -4 route add 10.0.0.0/8 via 172.16.0.1 dev eth0

You don't need to modify either default gateway; the more specific route will be chosen over the default route.

The redirect-gateway option may or may not change the gateway for reaching the OpenVPN server, depending on the flags provided to it.

According to man page of openvpn:

Option flags:

local -- Add the local flag if both OpenVPN servers are directly connected via a common subnet, such as with wireless. The local flag will cause step 1 above (Creation of a static route for the --remote address which forwards to the pre-existing default gateway) to be omitted.

So, just add a redirect-gateway local option in client config file, to override the pushed option. This will cause openvpn to just add a new default route, and not a specific route for the server. So the current route being used to connect to ther server will continue to be used.

If you want this behavior for all clients, edit the server config file and change the option push "redirect-gateway" to push "redirect-gateway local".

I also don't want the first VPN to be default gateway. I just need it
only for certain subnet.

On the client side you can ignore any routes from vpn server with the following options

script-security 2
route-noexec
route-up setup-routing.sh

and configure your own routes with route-up script, that could be something like this

$ cat setup-routing.sh 

#!/bin/bash

ip ro add 192.168.10.0/24 via $route_vpn_gateway

Where route_vpn_gateway is environment variable that points to the default gateway used by --route options, as specified in either the --route-gateway option or the second parameter to --ifconfig when --dev tun is specified.