ZyWALL User's Guide

Chapter 16 Virtual Servers
ZyWALL USG 100/200 Series User’s Guide
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NAT Loopback Policy Route
Without a NAT loopback policy route, the LAN1 user SMTP traffic goes to the LAN1 SMTP
server with the LAN1 computer’s IP address as the source. The source address is in the same
subnet, so the LAN1 SMTP server replies directly. The return traffic uses the SMTP server’s
LAN1 IP address as the source address
3
. This creates a triangle route since the source does not
match the original destination address (1.1.1.1). The user’s computer shuts down the session.
Figure 225 Triangle Route
Configure a policy route to use the IP address of the ZyWALL’s LAN1 interface, 192.168.1.1
as the source address of the traffic going to the LAN1 SMTP server from the LAN1 users. This
way the LAN1 SMTP server replies to the ZyWALL and the ZyWALL applies NAT.
Figure 226 NAT Loopback Policy Route
Click Network > Routing > Policy Route > Add and create the policy route as shown next.
Be careful of where you create the route as routes are ordered in descending priority. This
policy route applies source NAT to traffic sent from LAN1 to the SMTP server.
3. Even if the packets go through the ZyWALL, they only undergo layer 2 switching, not NAT.
192.168.1.21
LAN1
192.168.1.89
Source 192.168.1.21
SMTP
192.168.1.21
LAN1
192.168.1.89
Source 192.168.1.89
SMTP
NAT
Source 192.168.1.1
SMTP