Chapter 21 IPSec
VSG1435-B101 Series User’s Guide
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Transport mode ESP with authentication is not compatible with NAT.
21.4.6 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 124 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 the above figure, 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.
Table 89 VPN and NAT
SECURITY PROTOCOL MODE NAT
AH Transport N
AH Tunnel N
ESP Transport N
ESP Tunnel Y
A
B
