Internet Security Appliance User's Guide

Chapter 13 Firewall Screens
ZyWALL 5/35/70 Series User’s Guide
274
From VPN To VPN Packet Direction
From VPN To VPN firewall rules apply to traffic that comes in through one of the
ZyWALL’s VPN tunnels and terminates at the ZyWALL (like for remote management) or
goes out through another of the ZyWALL’s VPN tunnels (this is called hub-and-spoke VPN,
see Section 19.13 on page 385 for details). The ZyWALL decrypts the traffic and applies the
firewall rules before re-encrypting it or allowing the traffic to terminate at the ZyWALL.
In the following example, the From VPN To VPN default firewall rule silently blocks the
traffic that the ZyWALL receives from any VPN tunnel (either A or B) that is destined for the
other VPN tunnel or the ZyWALL itself. VPN traffic destined for the DMZ is allowed
through.
Figure 161 From VPN to VPN Example
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.
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.