Chapter 12 IPSec VPN
P-2812HNU-51c User’s Guide
271
value, and complain that the hash value appended to the received packet doesn't
match. The VPN device at the receiving end doesn't know about the NAT in the
middle, so it assumes that the data has been maliciously altered.
IPSec using ESP in Tunnel mode encapsulates the entire original packet
(including headers) in a new IP packet. The new IP packet's source address is the
outbound address of the sending VPN gateway, and its destination address is the
inbound address of the VPN device at the receiving end. When using ESP protocol
with authentication, the packet contents (in this case, the entire original packet)
are encrypted. The encrypted contents, but not the new headers, are signed with
a hash value appended to the packet.
Tunnel mode ESP with authentication is compatible with NAT because integrity
checks are performed over the combination of the "original header plus original
payload," which is unchanged by a NAT device.
Transport mode ESP with authentication is not compatible with NAT.
12.5.6 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 P-
2812HNU-51c’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 137 NAT Router Between IPSec Routers
Table 77 VPN and NAT
SECURITY PROTOCOL MODE NAT
AH Transport N
AH Tunnel N
ESP Transport N
ESP Tunnel Y
A
B
