ZyXEL Communications P-2602HW(L) Series - Manual (Page 226)

P-2602H(W)(L)-DxA Series User’s Guide
226 Chapter 17 VPN Screens
17.7 VPN, NAT, and NAT Traversal
NAT is incompatible with the AH protocol in both transport and tunnel mode. An IPSec VPN
using the AH protocol digitally signs the outbound packet, both data payload and headers,
with a hash value appended to the packet, but a NAT device between the IPSec endpoints
rewrites the source or destination address. As a result, the VPN device at the receiving end
finds a mismatch between the hash value and the data and assumes that the data has been
maliciously altered.
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 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 226 , 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 82 VPN and NAT
SECURITY PROTOCOL MODE NAT
AH Transport N
AH Tunnel N
ESP Transport Y*
ESP Tunnel Y