ZyXEL Communications P-661H Series - Manual (Page 208)

P-661H/HW Series User’s Guide
208 Chapter 13 VPN Screens
Figure 122 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 122 on page 208 , 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.
13.8 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 79 VPN and NAT
SECURITY PROTOCOL MODE NAT
AH Transport N
AH Tunnel N
ESP Transport Y*
ESP Tunnel Y