In short: you can’t! Official crappy FortiClient VPN package for Linux doesn’t support IPSec VPNs. But there’s an alternative package which supports IKEv1.

ShrewSoft VPN client, despite not being developed since 2013, works fine. Maybe because Forticlient still uses aged protocols… 🙂

I followed this XModulo guide which is basically good except for few things which needs to be actualized.

Let’s go straight to the compilation and installation

apt install cmake libssl-dev libedit-dev flex bison qt4-default qt4-qmake
wget https://www.shrew.net/download/ike/ike-2.2.1-release.tgz -P /tmp
tar -xvzf ike-2.2.1-release.tgz -C /tmp
cd /tmp/ike
cmake -DCMAKE_INSTALL_PREFIX=/usr -DQTGUI=YES -DETCDIR=/etc -DNATT=YES
make
sudo make install
sudo ldconfig
sudo mv /etc/iked.conf.sample /etc/iked.conf

Now the package should be installed. Launch

sudo iked

to start the backend daemon, and for the GUI:

qikea

As a side note, ShrewSoft VPN is also available as SNAP package, but it’s currently buggy and won’t work.