Chapter 18 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 159 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 159 on page 280, 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 104 VPN and NAT
SECURITY PROTOCOL | MODE | NAT |
AH | Transport | N |
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AH | Tunnel | N |
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ESP | Transport | Y* |
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ESP | Tunnel | Y |
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Y* - This is supported in the ZyXEL Device if you enable NAT traversal.
18.8 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
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