1. Install packages
aptitude install libreswan network-manager-l2tp network-manager-l2tp-gnome ike-scan
2. Check encryption
service xl2tp stop
ike-scan 10.20.30.40
Starting ike-scan 1.9.4 with 1 hosts (http://www.nta-monitor.com/tools/ike-scan/)
10.20.30.40 Main Mode Handshake returned HDR=(CKY-R=0305c49ff43d987b) SA=(Enc=3DES Hash=SHA1 Auth=PSK Group=2:modp1024 LifeType=Seconds LifeDuration(4)=0x00007080)
3. Add VPN Connection
Layer 2 Tunneling Protocol (L2TP)
Name: My VPN
Gateway: 10.20.30.40
User name: username@domain.com
Password: *****
4. IPsec Settings
[+] Enable IPsec tunnel to L2TP host
[Advanced]
Phase1 Algorithms: 3des-sha1-modp1024
Phase2 Algorithms: 3des-sha1
[OK]
5. Disable xl2tp service
sudo systemctl disable xl2tpd
6. Connect to VPN
7. If not connecting
sudo aptitude purge libreswan sudo aptitude install strongswan
Źródło: https://askubuntu.com/questions/904217/unable-to-connect-l2tp-ipsec-vpn-from-ubuntu-16-04