ZyXEL 202H 91-003-194001B User Manual

P-202H Plus v2 User’s Guide

83

Chapter 8 Firewalls

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.

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 

) 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.

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.

9 When the connection terminates or times out, the connection's state table entry is deleted 

and the connection's temporary inbound access list entries are deleted.

Additional rules may be defined to extend or override the default rules. For example, a rule 
may be created which will:

• Block all traffic of a certain type, such as IRC (Internet Relay Chat), from the LAN to the 

Internet.