https://developer.android.com/reference/android/net/VpnServi...
"There can be only one VPN connection running at the same time. The existing interface is deactivated when a new one is created."
Note this is not about routing some traffic to the VPN and other traffic to the clear net. This is about running two VPN connections simultaneously.
On Linux, I have no problem running either bare wireguard or tailscale alongside Forticlient. On Windows and macOS it's a bit more janky, specifically the DNS resolution, but I don't daily drive these platforms so I may be missing some kind of knowledge to fix this.
Aka what I want to achieve is (my-machine + tail/wireguard) --> (server with tailscale/wireguard + forti vpn) --> Corporate network. So wireguard or tailscale to receive traffic and forward it through forti.
Or another option (my machine fortivpn over tail/wireguard) --> (server as exit node) --> corporate network
Rather than using the official forticlient I am using https://github.com/adrienverge/openfortivpn. It has some options to configure custom pppd/routes/dns etc if necessary, which I have not touched as I don't know enough :P DNS resolution is not important for my usecase, only traffic.
In theory, as long as the Forti VPN does not overlap with the Tailscale IP address range, the simplest solution is to just run Tailscale and openfortivpn on a single node. You can then advertise the Forti VPN subnets within Tailscale, that's effectively what my image does as well in a nutshell, except that it's parsing the WireGuard config and setting up firewall rules for convenience.
Tailscale does NAT automatically by default, so it will look like all traffic is coming from the openfortivpn client itself.
With openfortivpn, you can usually ignore whatever routes you receive and set up your own. I haven't tried the specific set up you talk about, but I don't see why it wouldn't work. However, you would most likely need to NAT on the machine running the Fortinet client.
> However, you would most likely need to NAT on the machine running the Fortinet client.
Could you please elaborate a little more here? NAT from where to where?