Ipsec And Nat; Table 12-1 Vpn And Nat - ZyXEL Communications ZyWALL 5 User Manual

ZyWALL 5 Internet Security Appliance
Outside header: The outside IP header contains the destination IP address of the VPN
gateway.
Inside header: The inside IP header contains the destination IP address of the final system
behind the VPN gateway. The security protocol appears after the outer IP header and before
the inside IP header.

12.4 IPSec and NAT

Read this section if you are running IPSec on a host computer behind the ZyWALL.
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 (see section 13.7 for details).
12-4

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