How can I route only a particular subnet to the StrongSwan VPN but not my whole traffic on linux?

I have no particular competences on networking, so I'll do my best to explain my needs. On my Linux laptop I'm running StrongSwan (with NetworkManager) to connect to a particular VPN with IPsec. This VPN lets me reach these kind of IPs 10.*.*.*.

Now, my problem is that when I run the VPN all the traffic goes through the VPN but I would prefer to route to the VPN only packets addressed to those IPs (10.*.*.*).

How can I do it? Can someone provide me a simple guide, or share the necessary configurations and how to apply them?

Solution 1:

Many Thanks to @ecdsa for the completeness of his answer.

I'm a newbie in networking and I struggled so much to implement the solution although it was trivial.

The following solution adopt the fourth suggestion and require the following steps:

1. Switch on your VPN through NetworkManager

2. Run the following command to discover the route created from NetworkManager

   user@laptop:~$ ip route list table 220
   default via 192.168.1.1 dev enp0s31f6 proto static src 172.26.199.15
   

3. Take note of the interface (enp0s31f6) and virtual ip (172.26.199.15)

4. Flush the current route, because you want to use a custom route, with the following command

   sudo ip route flush table 220
   

5. Add your custom route with the following command

   sudo ip route add 10.0.0.0/8 dev enp0s31f6 via 172.26.199.15 table 220
   

Now only packets addressed to 10.0.0.0/8 will be routed throught the VPN.