Chapter 19 Firewall
19.2 The Firewall Screen
Asymmetrical Routes
If an alternate gateway on LAN1 has an IP address in the same subnet as the ZyWALL’s LAN1 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 LAN1 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 LAN1. The following steps and figure describe such a scenario.
1A computer on LAN1 initiates a connection by sending a SYN packet to a receiving server on the WAN.
2The ZyWALL reroutes the packet to gateway A, which is in Subnet 2.
3The reply from the WAN goes to the ZyWALL.
4The ZyWALL then sends it to the computer on LAN1 in Subnet 1.
Figure 246 Using Virtual Interfaces to Avoid Asymmetrical Routes
19.2.1 Configuring the Firewall Screen
Click Firewall to open the Firewall screen. Use this screen to enable or disable the firewall and asymmetrical routes, set a maximum number of sessions per host, and display the configured firewall rules. Specify from which zone packets come and to which zone packets travel to display only the rules specific to the selected direction. Note the following.
•If you enable
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