ZyXEL Communications NBG420N User Manual

 Chapter 13 Firewall

NBG420N User’s Guide

If you click Add or the Modify icon on an existing rule, the Add Firewall Rule screen is 
displayed. Use this screen to add a firewall rule or to modify an existing one.

Do not respond to 

requests for 

unauthorized 

services

Select this option to prevent hackers from finding the NBG420N by probing for 

unused ports. If you select this option, the NBG420N will not respond to port 

request(s) for unused ports, thus leaving the unused ports and the NBG420N 

unseen. By default this option is not selected and the NBG420N will reply with an 

ICMP Port Unreachable packet for a port probe on its unused UDP ports, and a 

TCP Reset packet for a port probe on its unused TCP ports. 
Note that the probing packets must first traverse the NBG420N's firewall 

mechanism before reaching this anti-probing mechanism. Therefore if the firewall 

mechanism blocks a probing packet, the NBG420N reacts based on the firewall 

policy, which by default, is to send a TCP reset packet for a blocked TCP packet. 

You can use the command "sys firewall tcprst rst [on|off]" to change this policy. 

When the firewall mechanism blocks a UDP packet, it drops the packet without 

sending a response packet.

Firewall Rule

#

This is your firewall rule number. The ordering of your rules is important as rules 

are applied in turn. Use the Move button to rearrange the order of the rules.

Active

This icon is green when the rule is turned on. The icon is grey when the rule is 

turned off. 

Service Name

This field displays the services and port numbers to which this firewall rule applies.

IP

This field displays the IP address(es) the rule applies to.

Schedule

This field displays the days the firewall rule is active.

Log

This field shows you whether a log will be created when packets match the rule 

(Match) or not (No).

Modify

Click the Edit icon to modify an existing rule setting in the fields under the Add 

Firewall Rule screen.
Click the Remove icon to delete a rule. Note that subsequent firewall rules move 

up by one when you take this action.

Add

Click the Add button to display the screen where you can configure a new firewall 

rule. Modify the number in the textbox to add the rule before a specific rule 

number.

Move

The Move button moves a rule to a different position. In the first text box enter the 

number of the rule you wish to move. In the second text box enter the number of 

the rule you wish to move the first rule to and click the Move button.

Misc setting

Bypass Triangle 

Route

Select this check box to have the NBG420N firewall ignore the use of triangle 

route topology on the network. 

Max NAT/Firewall 

Session Per User

Type a number ranging from 1 to 2048 to limit the number of NAT/firewall sessions 

that a host can create.

Apply

Click Apply to save the settings. 

Reset

Click Reset to start configuring this screen again. 

Table 58   Security > Firewall > Services