ZyXEL Communications ZyWALL USG 300 User Manual page 310

Chapter 16 Virtual Servers
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 LAN
to the SMTP server.
Figure 205 Create a Policy Route
Now the LAN SMTP server replies to the ZyWALL's LAN IP address and the ZyWALL
changes the source address to 1.1.1.1 before sending it to the LAN user's
computer. The source in the return traffic matches the original destination address
(1.1.1.1) and the LAN user can use the LAN SMTP server.
Figure 206 NAT Loopback Successful
NAT
Source 192.168.1.21
Source 1.1.1.1
SMTP
SMTP
LAN
192.168.1.21 192.168.1.89
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ZyWALL USG 300 User's Guide