ZyXEL P-662H-67 User Guide - Page 226
Remote DNS Server
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Chapter 16 VPN Screens Normally you cannot set up an IKE SA with a NAT router between the two IPSec routers because the NAT router changes the header of the IPSec packet. 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. In Figure 130 on page 225, when IPSec router A tries to establish an IKE SA, IPSec router B checks the UDP port 500 header, and IPSec routers A and B build the IKE SA. For NAT traversal to work, you must: • Use ESP security protocol (in either transport or tunnel mode). • Use IKE keying mode. • Enable NAT traversal on both IPSec endpoints. • Set the NAT router to forward UDP port 500 to IPSec router A. Finally, NAT is compatible with ESP in tunnel mode because integrity checks are performed over the combination of the "original header plus original payload," which is unchanged by a NAT device. The compatibility of AH and ESP with NAT in tunnel and transport modes is summarized in the following table. Table 88 VPN and NAT SECURITY PROTOCOL MODE NAT AH Transport N AH Tunnel N ESP Transport Y* ESP Tunnel Y Y* - This is supported in the ZyXEL Device if you enable NAT traversal. 16.8 Remote DNS Server In cases where you want to use domain names to access Intranet servers on a remote network that has a DNS server, you must identify that DNS server. You cannot use DNS servers on the LAN or from the ISP since these DNS servers cannot resolve domain names to private IP addresses on the remote network The following figure depicts an example where three VPN tunnels are created from ZyXEL Device 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 ZyXEL Device 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. 226 P-662H/HW-D Series User's Guide