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Prestige 334 User’s Guide
Chapter 14 Introduction to IPSec
154
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. When using
AH
protocol, packet contents
(the data payload) are not encrypted.
A NAT device in between the IPSec endpoints will rewrite either the source or destination
address with one of its own choosing. The VPN device at the receiving end will verify the
integrity of the 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, although NAT traversal provides a way to use
Transport
mode
ESP
when there is a
NAT router between the IPSec endpoints (
for details).
Table 45
VPN and NAT
SECURITY PROTOCOL
MODE
NAT
AH
Transport
N
AH
Tunnel
N
ESP
Transport
N
ESP
Tunnel
Y