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IPSec and NAT
12.4
Read this section if you are running IPSec on a host computer behind the Contivity 221.
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
switch, 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 (see section 13.7 for details).
Table 12-1 VPN and NAT
SECURITY PROTOCOL
AH
AH
ESP
ESP
MODE
NAT
Transport
N
Tunnel
N
Transport
N
Tunnel
Y
Introduction to IPSec 12-5
Contivity 221 VPN Switch User's Guide
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