ZyXEL P-2802HWL-I1 User Guide - Page 190

Remote DNS Server

Page 190 highlights

Chapter 14 VPN Screens NAT is not normally compatible with ESP in transport mode either, but the ZyXEL Device's NAT Traversal feature provides a way to handle this. NAT traversal allows you to set up an IKE SA when there are NAT routers between the two IPSec routers. Figure 108 NAT Router Between IPSec Routers 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 108 on page 190, 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 72 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. 14.8 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 190 P-2802H(W)(L)-I Series User's Guide

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Chapter 14 VPN Screens
P-2802H(W)(L)-I Series User’s Guide
190
NAT is not normally compatible with ESP in transport mode either, but the ZyXEL Device’s
NAT Traversal
feature provides a way to handle this. NAT traversal allows you to set up an
IKE SA when there are NAT routers between the two IPSec routers.
Figure 108
NAT Router Between IPSec Routers
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 108 on page 190
, 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.
Y* - This is supported in the ZyXEL Device if you enable NAT traversal.
14.8
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
Table 72
VPN and NAT
SECURITY PROTOCOL
MODE
NAT
AH
Transport
N
AH
Tunnel
N
ESP
Transport
Y*
ESP
Tunnel
Y