ZyXEL Communications Internet Security Gateway ZyWALL 2 Series - Manual (Page 150)

ZyWALL 2 Series User’s Guide
10-8 Firewalls
all communications to the Internet that originate from the LAN, and blocks all traffic to the LAN that
originates from the Internet. In summary, stateful inspection:
Allows all sessions originating from the LAN (local network) to the WAN (Internet).
Denies all sessions originating from the WAN to the LAN.
Figure 10-5 Stateful Inspection
The previous figure shows the ZyWALL’s default firewall rules in action as well as demonstrates how
stateful inspection works. User A can initiate a Telnet session from within the LAN and responses to this
request are allowed. However other Telnet traffic initiated from the WAN is blocked.
10.5.1 Stateful Inspection Process
In this example, the following sequence of events occurs when a TCP packet leaves the LAN network
through the firewall's WAN interface. The TCP packet is the first in a session, and the packet's application
layer protocol is configured for a firewall rule inspection:
1. The packet travels from the firewall's LAN to the WAN.
2. The packet is evaluated against the interface's existing outbound access list, and the packet is
permitted (a denied packet would simply be dropped at this point).
3. The firewall inspects packets to determine and record information about the state of the packet's
connection. This information is recorded in a new state table entry created for the new connection. If
there is not a firewall rule for this packet and it is not an attack, then the setting in the Firewall
Summary screen determines the action for this packet.