Stateful Inspection Process; Figure 56 Stateful Inspection - ZyXEL Communications G.SHDSL.bis 4-port Security Gateway P-793H User Manual

are allowed in. The ZyXEL Device uses stateful packet inspection to protect the private LAN
from hackers and vandals on the Internet. By default, the ZyXEL Device's stateful inspection
allows 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 56 Stateful Inspection

The previous figure shows the ZyXEL Device'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 settings in the Firewall General screen determine 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.
5 The outbound packet is forwarded out through the interface.
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Chapter 8 Firewalls
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