ZyXEL Communications 1000 Manuale Utente

 Chapter 24 Firewall

ZyWALL USG 1000 User’s Guide

The firewall rule appears in the firewall rule summary.

Figure 320   Firewall Example: Doom Rule in Summary   

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 LAN initiates a connection by sending a SYN packet to a 
receiving server on the WAN.

The ZyWALL reroutes the packet to gateway A, which is in Subnet 2. 

The reply from the WAN goes to the ZyWALL.