ZyXEL Communications ZyXEL ZyWALL USG-1000, User Manual

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Chapter 20 Firewall
ZyWALL USG 2000 User’s Guide
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The firewall rule appears in the firewall rule summary.
Figure 203 Firewall Example: MyService Example Rule in Summary
20.2 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.
1
A computer on the LAN initiates a connection by sending a SYN packet to a
receiving server on the WAN.
2
The ZyWALL reroutes the packet to gateway A, which is in Subnet 2.
3
The reply from the WAN goes to the ZyWALL.