ZyXEL Communications ZyWall USG 50-H Series, User Manual

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Chapter 18 Firewall
ZyWALL USG 50-H User’s Guide
297
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.
1 A computer on 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.
4 The ZyWALL then sends it to the computer on LAN1 in Subnet 1 .
Figure 230 Using Virtual Interfaces to Avoid Asymmetrical Routes
18.2.1 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 intra-zone traffic blocking (see the chapter about zones), the firewall
automatically creates (implicit) rules to deny packet passage between the interfaces in the
specified zone.
• Besides configuring the firewall, you also need to configure virtual servers (NAT port
forwarding) to allow computers on the WAN to access LAN devices. See Chapter 14 on
page 257 for more information.
• The ordering of your rules is very important as rules are applied in sequence.