1. Install
vpnc
apt install vpnc
2. Create config file e.g.
/etc/my-vpn.conf
IPSec gateway <vpn.host>
IPSec ID <group id>
IPSec secret <group secret>
Xauth username <user name>
# Not provided <password> will be asked upon connecting
# Xauth password <password>
Note
For FritzBox VPN: <group id> == <user name>
3. Connect
vpnc my-vpn