Recently some readers came across an error message with nat -t regedit. This problem can occur due to many factors. We will discuss this below.

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    This DWORD value allows Windows to establish security associations while the VPN server and its Windows VPN client computer can be behind NAT devices. HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesPolicyAgent. RegValue: AssumeUDPEncapsulationContextOnSendRule. Type: double word.

    This article describes how to set up a new L2TP/IPsec server behind NAT-T on a specific device.

    Applies to: Windows 10 (most editions), Windows Server 2012 R2
    Original KB number: 926179

    Overview

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  • By default, Windows Vista and Windows Server 2008 do not support Internet Protocol Security (IPsec) or Network Address Traversal (NAT) Security (NAT-T) for servers behind a NAT device. If the Truly Private Network (VPN) server is only a NAT device, a computer running a Windows Vista or Windows Server 2008 VPN application cannot establish a Layer 3 Tunneling Protocol (L2TP/IPsec) connection to the VPN server. This scenario includes VPN servers running Windows Server 2008 and Windows Server 2003.

    Due to how NAT products translate network traffic, you might expect unexpected results in this scenario:

  • You are placing the server behind a NAT device.
  • You are using a nat-t IPsec environment.
  • If for sv If you need to use IPsec, use public IP addresses for servers that you can connect to from the Internet. If you place the server behind a NAT device and then use an IPsec NAT-T environment, you can allow communication by changing the registry value on the VPN client computer and you will see the VPN server.< /p>

    Set The AssumeUDPEncapsulationContextOnSendRule Registry Key

    How do I add AssumeUDPEncapsulationContextOnSendRule?

    Set the PC registry key AssumeUDPEncapsulationContextOnSendRule. Select Start > All Programs > Accessories > Run, type regedit, and click OK. If the User Account Control dialog box appears on the screen, users” prompting you to elevate your admin token, select Continue.

    Follow these steps to create and configure the AssumeUDPEncapsulationContextOnSendRule registration rate:

    1. Log on to your current Windows Vista client as almost any user who is a member of the Administrators group.

    2. Select Start > All Programs > Accessories > Run, select regedit and click OK. When the 18-meter box of the User Account Control dialog box appears on the screen, and you are prompted to increase the supervisor token, select Continue.

    3. Find the following subsection:

      How do I set up L2TP?

      From the Windows 10 Start Menu, click Settings.Click Network and Internet.Click VPN in the navigation menu on the left.Click Add VPN Connection.In the VPN provider text box, Windows (Built-in) is often selected.In the Connection name text box, enter a name for the mobile VPN (for example, “VPN L2TP”).

      HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesPolicyAgent

      Note

      What is NAT traversal in IPsec?

      NAT-T (NAT traversal or UDP encapsulation) almost guarantees that IPsec VPN connections remain open when traffic passes through gateways in addition to devices using NAT. When an IP packet passes through a network address resolver, it is modified in a way that is not necessarily fully compatible with IPsec.

      Your registry may also use the AssumeUDPEncapsulationContextOnSendRule DWORD value if you want toDo not use a VPN client computer running Microsoft Windows XP Service Pack 2 (SP2). To do this, locate and select the registry subkey HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesIPSec.

    4. On the Edit Point menu, click New, in this case select DWORD (32-bit) Value.

    5. Type AssumeUDPEncapsulationContextOnSendRule, then press Enter.

    6. Right-click AssumeUDPEncapsulationContextOnSendRule and select Edit.

    7. In the value data package, enter one of the fan base values:

    8. This is the default value. If set to 0, Windows will not be able to use mapping servers behind NAT devices.

    9. 1

      If set to 1, Windows may well establish secure connections to servers behind NAT devices.

    10. nat-t regedit

      2

      When set to 2, Windows often enforces security by leaving the exact server/VPN client mappings of certain computers (based on Windows Vista or Windows Server 2008) behind NAT devices. Ok,

    11. Select and close the Registry Editor.

    12. nat-t regedit

      Restart your computer.

  • 2 seconds to read
  • This section, method, or task steps explains how to edit the registry. However, if the registry is changed incorrectly, big problems can occur. So be sure to follow these steps carefully. For additional protection, you should usually make a backup before modifying the registry. You can then repair the registry if you find a problem. For more information about backing up and restoring the registry, see How to back up and restore the Windows registry in Windows.

    You can also apply the AssumeUDPEncapsulationContextOnSendRule DWORD value type to a VPN client computer running Microsoft Windows XP SP2. Look at this, then select the actual registry key HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesIPSec.

    Because PPTP disables VPN support on iOS, one new customer decided to reconfigure the VPN server from PPTP to L2TP/IPSec on Windows Server 2012 R2. Internal LAN VPN clients connect to the VPN server without problems, but external Windows clients receive an 809 error when they try to connect.Connections to the L2TP VPN server:

    On other versions of Windows, attachment errors 800, 794, or 809 may indicate the same issue.

    Note that the VPN host is behind a NAT and the router is usually set to L2TP port forwarding:

  • UDP 1701 – Layer 2 Transfer Protocol (L2F) and Tunneling Protocol 2 (L2TP)
  • UDP 500
  • UDP 4500 NAT-T – IPSec Network Address Translator Bypass
  • Protocol 50 ESP
  • These domains are also open in all Windows Firewall rules for VPN connections. The classic configuration is used. The vpn client built into Windows is used to connect.

    If you connect via PPTP to the same VPN server, the connection will succeed.

    VPN Error 809 For L2TP/IPSec Due To Windows Behind Nat

    It turns out that the problem is generally known and is described in the article https://support.microsoft.com/en-us/kb/926179. The built-in Windows VPN client does not support standard L2TP/IPsec connections over NAT. This is because ipsec uses ESP (Encapsulating Security Payload) to encrypt packets, in combination with ESP it does not support PAT (Port Address Translation). If you want to use IPSec for communication, Microsoft recommends usingUse the public IP addresses of the VPN server.

    But there is a general workaround. You can fix this deficiency by enabling project NAT-T support, which allows ESP 50 packets to be encapsulated as UDP packets on port 4500. NAT-T is enabled by default on almost all Sprint systems (iOS, Android, Linux) except Windows.

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