Figure 186 Triangle Route; Figure 187 Nat Loopback Policy Route - ZyXEL Communications ZyWALL USG 300 User Manual

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
match the original destination address (1.1.1.1). The user's computer shuts down the session.

Figure 186 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 187 NAT Loopback Policy Route

Source 192.168.1.1
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.
3. Even if the packets go through the ZyWALL, they only undergo layer 2 switching, not NAT.
ZyWALL USG 300 User's Guide
3
Source 192.168.1.21
SMTP
192.168.1.21
SMTP
LAN
192.168.1.21
. This creates a triangle route since the source does not
192.168.1.89
NAT
Source 192.168.1.89
192.168.1.89
Chapter 16 Virtual Servers
SMTP
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