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ZyWALL 2 Series User’s Guide

Introduction to IPSec

13-5

13.4 IPSec and NAT

Read this section if you are running IPSec on a host computer behind the ZyWALL.

NAT is incompatible with the

protocol in both

Transport

and

Tunnel

mode. An IPSec VPN using the

protocol digitally signs the outbound packet, both data payload and headers, with a hash value appended

to the packet. When using

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

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