Use VPN for only one RDP connection in Ubuntu

Question

I'm not an expert in networking stuff. I have a CISCO VPN connection set up, to connect my computer at my work, using a VPN connection, and Remmina. This works fine, but all my internet traffic goes through this VPN connection -> sites are blocked, etc.

Is it possible to route only this specific RDP connection through the VPN, and the rest not?

These are the results of running netstat -rn before the VPN connection:

Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window   irtt Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG        0 0           0 wlan0
192.168.1.0     0.0.0.0         255.255.255.0   U         0 0           0 wlan0

and afterwards:

Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window   irtt Iface
0.0.0.0         0.0.0.0         0.0.0.0         U         0 0          0 tun0
128.131.240.4   192.168.1.1     255.255.255.255 UGH       0 0          0 wlan0
192.168.1.0     0.0.0.0         255.255.255.0   U         0 0          0 wlan0

These are the results of running netstat-rn after suggested things:

Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window   irtt Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG        0 0           0 wlan0
128.131.39.64   0.0.0.0         255.255.255.255 UH        0 0           0 tun0
128.131.240.4   192.168.1.1     255.255.255.255 UGH       0 0           0 wlan0

Answer 1

I figured it out how to do it:

in the network manager go to the VPN-configuration, then to IPv4, settings and Routing.

Enable: Use this connection only for ressources of this network.

Then add the ip-adress (e.g. 128.131.39.64) the subnet mask (255.255.255.255) and the gateway (0.0.0.0). The netstat -rn looks similar but not exactly as in the suggested answer from Paul

0.0.0.0         192.168.1.1     0.0.0.0         UG        0 0          0 wlan0
128.131.39.64   0.0.0.0         255.255.255.255 UH        0 0          0 tun0
128.131.240.4   192.168.1.1     255.255.255.255 UGH       0 0          0 wlan0
192.168.1.0     0.0.0.0         255.255.255.0   U         0 0          0 wlan0

remote desktop connection works, but surfing is not routed through the VPN-connection

Answer 2

Once the tunnel us up, issue the following.

Remove the default route:

sudo ip route del default dev tun0

This was pushing everything down the tunnel. Next, put it back where it was:

sudo ip route add default via 192.168.1.1

Next add a specific route for the RDP server you want to get to:

sudo ip route add <rdp server ip address>/32 dev tun0

So if the RDP server was 10.10.10.10, this would look like:

sudo ip route add 10.10.10.10/32 dev tun0

The success of this depends a bit on how deeply the vpn client embeds itself in the network stack, but it should work...