ZyXEL Communications 2 Plus User Manual

Chapter 11 Firewall

ZyWALL 2 Plus User’s Guide

The following table describes the labels in this screen.  

For DoS

 

attacks, the ZyWALL uses thresholds to determine when to start dropping sessions 

that do not become fully established (half-open sessions). These thresholds apply globally to 
all sessions.
For TCP, half-open means that the session has not reached the established state-the TCP three-
way handshake has not yet been completed. Under normal circumstances, the application that 
initiates a session sends a SYN (synchronize) packet to the receiving server. The receiver 
sends back an ACK (acknowledgment) packet and its own SYN, and then the initiator 
responds with an ACK (acknowledgment). After this handshake, a connection is established. 

Figure 140   Three-Way Handshake

For UDP, half-open means that the firewall has detected no return traffic. An unusually high 
number (or arrival rate) of half-open sessions could indicate a DOS attack. 

Table 52   SECURITY > FIREWALL > Anti-Probing

Respond to PING 

on

Select the check boxes of the interfaces that you want to reply to incoming Ping 

requests. 
Clear an interface’s check box to have the ZyWALL not respond to any Ping 

requests that come into that interface.

Do not respond to 

requests for 

unauthorized 

services.

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

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

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

unseen. If this option is not selected, the ZyWALL 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 ZyWALL's firewall rule 

checks before reaching this anti-probing mechanism. Therefore if a firewall rule 

stops a probing packet, the ZyWALL reacts based on the firewall rule to either 

send a TCP reset packet for a blocked TCP packet (or an ICMP port-unreachable 

packet for a blocked UDP packets) or just drop the packets without sending a 

response packet.

Apply

Click Apply to save your changes back to the ZyWALL.

Reset

Click Reset to begin configuring this screen afresh.