In this article, we will look at the most common causes of errors when connecting to an L2TP/IPSec VPN server from Windows 10/11 or Windows Server 2019/2016. If you cannot establish an L2TP VPN connection from Windows devices, take a close look at the error code and description. The reason for the VPN error is often given there.
Can’t Connect to L2TP/IPSec VPN on Windows
An error may occur during a connection attempt from a Windows client to an L2TP VPN server:
Can’t connect to your_L2TP-IPsec-VPN-Server.hostname The network connection between your computer and the VPN server could not be established because the remote server is not responding. This could be because one of the network devices (e.g. firewalls, NAT, routers, etc) between your computer and the remote server is not configured to allow VPN connections. Please contact your Administrator or your service provider to determine which device may be causing the problem.
The most common related VPN error codes are 800, 794, or 809.
If this error occurs, check that you have entered the correct VPN server address, that the VPN server is accessible, and that all necessary ports are open in the firewalls.
The following ports must be open to connect the L2TP/IPsec VPN server:
- UDP port 1701 (Layer 2 Forwarding Protocol (L2F) and Layer 2 Tunneling Protocol (L2TP));
- UDP port 500 (IKE, manage encryption keys);
- ESP 50 protocol (Encapsulating Security Payload) for IPSec;
- If the VPN server is located behind a NAT, you will also need to open the UDP port 4500 (protocol NAT-T, IPSec Network Address Translator Traversal).
Use the built-in Windows VPN Client to connect to the L2TP VPN server. Check your VPN connection settings. Make sure that your VPN connection uses an L2TP tunnel and a pre-shared key (PSK) or certificate to authenticate. You can use PowerShell to list VPN connection settings:
How to Connect to L2TP/IPSec VPN Server Behind NAT
If the destination L2TP VPN server is behind NAT, you cannot connect to it from a Windows computer with the default settings. The problem is that the IPsec protocol doesn’t support NAT. To bypass this restriction, NAT-T is used, which encapsulates IPsec packets over UDP/4500.
NAT-T is enabled by default in almost all operating systems (iOS, Android, Linux) except Windows.
If the L2TP/IPsec VPN server is behind NAT, it is necessary to make a registry change on both the server and client to allow UDP packet encapsulation for L2TP and support (NAT-T) for IPsec.
- Open the Registry Editor (
regedit.exe) and go to the following registry key:
- Windows 11/10/8.1 and Windows Server 2022/2019/2016/2012R2/2008R2 — HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent
- Windows XP/Windows Server 2003 — HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IPSec
- Create a DWORD parameter with the name AssumeUDPEncapsulationContextOnSendRule and the value 2;Note. Possible AssumeUDPEncapsulationContextOnSendRule values are:
- 0 – (a default value) assumes the VPN server is directly connected to the Internet (without NAT);
- 1 – the VPN server is located behind a NAT device;
- 2 — both the VPN server and client are behind a NAT.
You can use the PowerShell cmdlet to make changes to the registry:
Set-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Services\PolicyAgent" -Name "AssumeUDPEncapsulationContextOnSendRule" -Type DWORD -Value 2 –Force;
After enabling NAT-T support, you will be able to connect to the VPN server from the client via NAT (including double NAT). In this case, an encrypted IPsec tunnel is created before an L2TP connection is established (using the IKE protocol: UDP/500 and NAT-T: UDP/4500). An L2TP tunnel is then established inside IPsec on UDP port 1701. This means that if the VPN server is behind NAT, you do not need to forward UDP port 1701 to it from your perimeter router/firewall.
L2TP VPN Connection Attempt Failed on Windows
There is another annoying VPN bug in Windows. If you have more than one Windows computer on your LAN, you won’t be able to connect to an external L2TP/IPSec VPN server at the same time. If you try to connect to the same VPN server from another computer while there is an active VPN tunnel from your network, you will receive an error code 809 or 789:
Error 789: The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remove computer.
According to TechNet, the problem relates to an incorrect implementation of the L2TP/IPSec client in Windows (not fixed for many years).
To fix this bug, you need to modify two registry parameters in the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RasMan\Parameters and restart the computer:
- AllowL2TPWeakCrypto – change to 00000001 (allows weak encryption algorithms; L2TP/IPSec uses the MD5 and DES algorithms);
- ProhibitIPSec – change to 00000000 (enables IPsec encryption, which is often disabled by some third-party VPN clients or system tools).
Run the following command to apply these registry changes:
reg add "HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Rasman\Parameters" /v AllowL2TPWeakCrypto /t REG_DWORD /d 1 /f
reg add "HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Rasman\Parameters" /v ProhibitIpSec /t REG_DWORD /d 0 /f
This enables support for multiple concurrent L2TP/IPSec connections on Windows over a shared public IP address.
Here are some more tips to fix the L2TP VPN connection issue on Windows:
- Check that only the authentication protocol supported by the VPN server is enabled in the L2TP VPN connection settings. Open
ncpa.cpl-> your VPN connection properties -> Security tab. Select the allowed authentication protocols. Usually, only MS-CHAP v2 should be checked here;
- Got to the Options tab -> PPP Settings -> check Enable LCP Extension;
- Check that IKE and AuthIP IPsec Keying Modules and IPsec Policy Agent services are running on Windows:
- Start the Device Manager and remove the WAN Miniport (L2TP) and WAN Miniport (IKEv2) virtual adapters. Reboot the computer. After restarting, Windows will restore these virtual adapters with default settings; For more information, see “How to fix an error VPN connection cannot be established“.
- In some cases, it helps to disable IPSec and use L2TP only. To do this, change the value of the
1in the reg key HKLM\SYSTEM\CurrentControlSet\Services\RasMan\Parameters.