ZyXEL 4-port Prestige 792H Router 91-004-342001 User Manual

Prestige 792H G.SHDSL Router 

8-8  

Firewalls 

 

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

 

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. 

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.