ZyXEL Communications 792H manual Stateful Inspection Process

Prestige 792H G.SHDSL Router

Allows all sessions originating from the LAN (local network) to the WAN (Internet). Denies all sessions originating from the WAN to the LAN.

Figure 8-5 Stateful Inspection

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.

8.5.1 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 packet is inspected by a firewall rule 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 default action for packets not matching following rules field (see Figure 10-4) determines the action for this packet.

8-8 Firewalls