The Firewall Screen - ZyXEL Communications ZYWALL USG 20 Manual

The firewall rule appears in the firewall rule summary.
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Figure 232 Firewall Example: Doom Rule in Summary

22.2 The Firewall Screen

Asymmetrical Routes
If an alternate gateway on the LAN has an IP address in the same subnet as the
ZyWALL's LAN IP address, return traffic may not go through the ZyWALL. This is
called an asymmetrical or "triangle" route. This causes the ZyWALL to reset the
connection, as the connection has not been acknowledged.
You can have the ZyWALL permit the use of asymmetrical route topology on the
network (not reset the connection). However, allowing asymmetrical routes may
let traffic from the WAN go directly to the LAN without passing through the
ZyWALL. A better solution is to use virtual interfaces to put the ZyWALL and the
backup gateway on separate subnets. Virtual interfaces allow you to partition your
network into logical sections over the same interface. See the chapter about
interfaces for more information.
By putting LAN 1 and the alternate gateway (A in the figure) in different subnets,
all returning network traffic must pass through the ZyWALL to the LAN. The
following steps and figure describe such a scenario.
A computer on the LAN1 initiates a connection by sending a SYN packet to a
1
receiving server on the WAN.
The ZyWALL reroutes the packet to gateway A, which is in Subnet 2.
2
The reply from the WAN goes to the ZyWALL.
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ZyWALL USG 20/20W User's Guide
Chapter 22 Firewall
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