Chapter 24 Firewall

9The firewall rule appears in the firewall rule summary.

Figure 320 Firewall Example: Doom Rule in Summary

24.2 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.

1A computer on the LAN 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.

 

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ZyWALL USG 2000 User’s Guide