ZyWALL 2 Series User’s Guide

When there is outbound traffic with no inbound traffic, the ZyWALL automatically

drops the tunnel after two minutes.

14.7 NAT Traversal

NAT traversal allows you to set up a VPN connection when there are NAT routers between IPSec routers A and B.

Figure 14-3 NAT Router Between IPSec Routers

Normally you cannot set up a VPN connection with a NAT router between the two IPSec routers because the NAT router changes the header of the IPSec packet. In the previous figure, IPSec router A sends an IPSec packet in an attempt to initiate a VPN. The NAT router changes the IPSec packet’s header so it does not match the header for which IPSec router B is checking. Therefore, IPSec router B does not respond and the VPN connection cannot be built.

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. IPSec router B checks the UDP port 500 header and responds. IPSec routers A and B build a VPN connection.

14.7.1 NAT Traversal Configuration

For NAT traversal to work you must:

Use ESP security protocol (in either transport or tunnel mode). Use IKE keying mode.

Enable NAT traversal on both IPSec endpoints.

In order for IPSec router A (see the figure) to receive an initiating IPSec packet from IPSec router B, set the NAT router to forward UDP port 500 to IPSec router A.

14-6

VPN Screens