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Designing Secure Communications Between Networks
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A PPTP VPN client can be placed behind a Network Address Translation (NAT) server if the NAT server has a PPTP NAT editor. (A NAT editor is a software addition that understands a specific protocol.) Most NAT servers have a PPTP NAT editor. L2TP/ IPSec clients cannot be placed behind a NAT server unless the VPN client and VPN server understand IPSec NAT Traversal (NAT-T). NAT-T is an IETF standard-track addi tion that describes how the IPSec protocol can be used with a NAT server. Windows Server 2003, Microsoft L2TP/IPSec VPN client software, and the L2TP/IPSec NAT-T update for Windows XP and Windows 2000 support NAT-T. To understand why the VPN client and server must understand NAT-T, you must exam ine the way that IPSec encrypts the packets and understand how NAT works. Figure 7-5 showed the portion of the IPSec-protected packet that is encrypted which is just about everything. Not encrypted is the IP address of the source and destination com puters. When a NAT server receives a packet to forward to the Internet, it replaces the source IP address. This does not cause a problem for most IP traffic because any responses will be directed to the NAT server and the NAT server can match the response with the original sending computer and forward it. However, within the encrypted IPSec-protected packet are checksums that are calculated when the packet contained the original source IP address. Because NAT cannot decrypt and then reencrypt the packet with a new checksum, the packet is interpreted by IPSec as being corrupt or modified. This is why an unmodified Windows 2000 computer cannot support an L2TP/IPSec VPN when NAT is part of the equation. Windows Server 2003 solves that problem with NAT-T. NAT-T uses UDP encapsulates of the IPSec packet and allows it to pass through NAT. Internet Key Exchange (IKE) can detect whether NAT-T is present and uses UDP-ESP encapsulation. Windows Server 2003 supports NAT-T. The following Windows clients, as updated or configured, also support NAT-T. You can use any of the following clients to create a VPN connection using L2TP/IPSec even if they are behind a NAT server as long as the VPN server is running Windows Server 2003:
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Exam Tip A Windows 2000 server that has been updated with the L2TP/IPSec update cannot become a Routing and Remote Access VPN server. The update is only meant for a client.
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Windows XP and the L2TP/IPSec update Windows 2000 and the L2TP/IPSec update Windows NT 4.0 and the L2TP/IPSec VPN client Windows 98 and the L2TP/IPSec VPN client
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Lesson 1
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Selecting Protocols for VPN Access
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See Also For more information about the L2TP/IPSec VPN client, see Microsoft L2TP/ IPSec VPN Client on the Windows 2000 page of the Microsoft Web site at http: //www.microsoft.com/windows2000/server/evaluation/news/bulletins/l2tpclient.asp.
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For information about the L2TP/IPSec update for Windows XP and Windows 2000, see Microsoft Knowledge Base Article 818043 on the support page of the Microsoft Web site at http://support.microsoft.com/default.aspx scid=kb;en-us;818043.
Guidelines for Selecting a VPN Protocol for a Specific Communication Task
Follow these guidelines for selecting a VPN protocol for a specific communication task:
In general, if a public key infrastructure is not available, select PPTP. If the highest level of security is required, select L2TP/IPSec. L2TP/IPSec provides not only mutual authentication of users, but also the computer and VPN endpoint. L2TP/IPSec also provides stronger end-to-end encryption than PPTP does. Use IPSec tunnel mode only when interoperability issues require you to. If DES or 3DES is required by the organization s security policy for encryption, use L2TP/IPSec. Because of the CPU overhead incurred by using IPSec on VPN servers, consider either installing IPSec offload hardware or using PPTP instead. Use PPTP when testing VPN connectivity. It is easier to use and configure. If the VPN works with PPTP but does not work with L2TP/IPSec, the problem is related to the protocol or protocol configuration there will be fewer things to troubleshoot. Because PPTP does not require certificates for computer authentication and NAT is less likely to cause a problem with a PPTP VPN, using PPTP to test your VPN will allow you to verify or troubleshoot VPN network connectivity. You can then test the IPSec protocol.
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