ZyXEL Communications ZyWALL 1000 User Manual

 Chapter 6 Tutorials

ZyWALL USG 1000 User’s Guide

Without a NAT loopback policy route, the LAN user SMTP traffic goes to the LAN SMTP 
server has the LAN computer’s IP address as the source. The source address is in the same 
subnet, so the LAN SMTP server replies directly. The return traffic uses the SMTP server’s 
LAN IP address as the source address

1

. 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 100   Triangle Route 

Configure a policy route to use the IP address of the ZyWALL’s ge1 (LAN) interface, 
192.168.1.1 as the source address of the traffic going to the LAN SMTP server from the LAN 
users. This way the LAN SMTP server replies to the ZyWALL and the ZyWALL applies 
NAT. 

Figure 101   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 the LAN to the SMTP server.

1.

Even if the packets go through the ZyWALL, they only undergo layer 2 switching, not NAT.