This tutorial is for setting up L2TP / IPSec VPN server on SonicWALL TZ300 with Firmware version 6.5. This should work for majority of other SonicWALL firewalls as well.

• Switch to Classic Navigation Mode if not already in Classic Mode (hamburger icon in bottom left).
• Go to VPN > Settings and Enable WAN GroupVPN.
  
  
  
  • Click Edit button next to WAN GroupVPN
    • General: Set Shared Secret
    • Advanced: Enable Accept Multiple Proposals for Clients.
    • All other settings can be left as they are.
      
• Go to VPN > L2TP Server 
  • Check “Enable L2TP Server” and click Configure
    
    
    • L2TP Server:
      • Enter DNS Servers (normally your internal LAN DNS)
    • L2TP Users:
      • Select “Use the Local L2TP IP pool” and enter IP Address range to be issued to VPN clients (this doesn’t need to be within your internal LAN subnet).
      • User group for L2TP users: select Trusted Users
        
• Go to Users > Local Users & Groups > Add User…
  • Settings: enter Name and Password for your VPN user.
  • Group: Everyone and Trusted Users.
  • VPN Access: LAN Subnets
    

That should be it. You can now login from any L2TP / IPSec supporting client.

Update:

I noticed that Windows 10 VPN clients seemingly takes long time to connect to SonicWALL L2TP Server. It stays on “Completing the connection” stage for 20-30 seconds.

In practice this doesn’t seem to affect functionality though – VPN connection is actually established very quickly (2-3 seconds). So you can just ignore “Completing the connection” status and start connecting to internal resources straight away.

April 2018
SonicWALL TZ300