Stateful Inspection And The Zywall - ZyXEL Communications ZyXEL ZyWALL 2WE User Manual

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 16-3) 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.
5. The outbound packet is forwarded out through the interface.
6. Later, an inbound packet reaches the interface. This packet is part of the connection previously
established with the outbound packet. The inbound packet is evaluated against the inbound access
list, and is permitted because of the temporary access list entry previously created.
7. The packet is inspected by a firewall rule, and the connection's state table entry is updated as
necessary. Based on the updated state information, the inbound extended access list temporary
entries might be modified, in order to permit only packets that are valid for the current state of the
connection.
8. Any additional inbound or outbound packets that belong to the connection are inspected to update
the state table entry and to modify the temporary inbound access list entries as required, and are
forwarded through the interface.
When the connection terminates or times out, the connection's state table entry is deleted and the
9.
connection's temporary inbound access list entries are deleted.

13.5.2 Stateful Inspection and the ZyWALL

Additional rules may be defined to extend or override the default rules. For example, a rule may be created
which will:
i. Block all traffic of a certain type, such as IRC (Internet Relay Chat), from the LAN to the Internet.
ii. Allow certain types of traffic from the Internet to specific hosts on the LAN.
iii. Allow access to a Web server to everyone but competitors.
iv. Restrict use of certain protocols, such as Telnet, to authorized users on the LAN.
Firewalls
ZyWALL 2 and ZyWALL 2WE
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