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Chapter 17 VPN
EMG5324-D10A User’s Guide
212
incoming packet by computing its own hash 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.
17.6.3 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 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 111 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 111 on page 212, 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:
Table 71 VPN and NAT
SECURITY PROTOCOL MODE NAT
AH Transport N
AH Tunnel N
ESP Transport N
ESP Tunnel Y
A
B