ZyXEL Communications ZyXEL ZyWALL 2WG - Manual (Page 211)

Chapter 11 Firewall
ZyWALL 2WG User’s Guide
211
• The first row allows the LAN computer at IP address 192.168.1.7 to access the IRC
service on the WAN.
• The second row blocks LAN access to the IRC service on the WAN.
• The third row is (still) the firewall’s default policy of allowing all traffic from the LAN to
go to the WAN.
The rule for the CEO must come before the rule that blocks all LAN to WAN IRC traffic. If
the rule that blocks all LAN to WAN IRC traffic came first, the CEO’s IRC traffic would
match that rule and the ZyWALL would drop it and not check any other firewall rules.
11.6 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).
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 IP alias to put the ZyWALL and the
backup gateway on separate subnets.
11.6.1 Asymmetrical Routes and IP Alias
You can use IP alias instead of allowing asymmetrical routes. IP Alias allow you to partition
your network into logical sections over the same interface.
By putting your LAN and Gateway A in different subnets, all returning network traffic must
pass through the ZyWALL to your LAN. The following steps describe such a scenario.
1 A computer on the LAN 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 the LAN in Subnet 1.