ZyXEL ZyWALL USG 50 User Manual - Page 365
The Firewall Screen
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9 The firewall rule appears in the firewall rule summary. Figure 221 Firewall Example: Doom Rule in Summary Chapter 22 Firewall 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. 1 A computer on the LAN1 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. ZyWALL USG 50 User's Guide 365
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