Figure 12-5 Stateful Inspection - ZyXEL Communications Prestige 652H series User Manual

Prestige 652H/HW Series User's Guide
Denies all sessions originating from the WAN to the LAN.
The previous figure shows the Prestige'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.
12.5.1 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
Default Policy screen determines the action for this packet.
4. Based on the obtained state information, a firewall rule creates a temporary access list entry that is
inserted at the beginning of the WAN interface's inbound extended access list. This temporary
access list entry is designed to permit inbound packets of the same connection as the outbound
packet just inspected.
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Figure 12-5 Stateful Inspection

Firewalls