ZyXEL Communications ZyWALL USG 300 User Manual page 309

NAT Loopback Policy Route
Without a NAT loopback policy route, the LAN user SMTP traffic goes to the LAN
SMTP server with 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
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 203 Triangle Route
LAN
Configure a policy route to use the IP address of the ZyWALL's ge1 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 204 NAT Loopback Policy Route
Source 192.168.1.1
3. Even if the packets go through the ZyWALL, they only undergo layer 2 switching, not NAT.
ZyWALL USG 300 User's Guide
Source 192.168.1.21
SMTP
192.168.1.21
SMTP
LAN
192.168.1.21
192.168.1.89
NAT
Source 192.168.1.89
SMTP
192.168.1.89
Chapter 16 Virtual Servers
3
. This creates
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