Nat Traversal; Figure 87 Vpn Host Using Intranet Dns Server Example; Figure 88 Nat Router Between Ipsec Routers - ZyXEL Communications ADSL VoIP IAD with 802.11g Wireless 2602HW Series User Manual

Prestige 2602HW Series User's Guide
The following figure depicts an example where three VPN tunnels are created from Prestige
A; one to branch office 2, one to branch office 3 and another to headquarters. In order to
access computers that use private domain names on the headquarters (HQ) network, the
Prestige at branch office 1 uses the Intranet DNS server in headquarters. The DNS server
feature for VPN does not work with Windows 2000 or Windows XP.

Figure 87 VPN Host using Intranet DNS Server Example

If you do not specify an Intranet DNS server on the remote network, then the VPN host must
use IP addresses to access the computers on the remote network.

17.8 NAT Traversal

NAT traversal allows you to set up a VPN connection when there are NAT routers between
the two IPSec routers.

Figure 88 NAT Router Between IPSec Routers

Normally you cannot set up a VPN connection with a NAT router between the two IPSec
routers because the NAT router changes the header of the IPSec packet. In the previous figure,
IPSec router A sends an IPSec packet in an attempt to initiate a VPN. The NAT router changes
the IPSec packet's header so it does not match the header for which IPSec router B is
checking. Therefore, IPSec router B does not respond and the VPN connection cannot be built.
NAT traversal solves the problem by adding a UDP port 500 header to the IPSec packet. The
NAT router forwards the IPSec packet with the UDP port 500 header unchanged. IPSec router
B checks the UDP port 500 header and responds. IPSec routers A and B build a VPN
connection.
206 Chapter 17 VPN Screens