ZyXEL Communications 5 Series User Manual

 Chapter 13 Firewall Screens

ZyWALL 5/35/70 Series User’s Guide

3 The reply from the WAN goes to the ZyWALL. 
4 The ZyWALL then sends it to the computer on the LAN in Subnet 1. 

Figure 162   Using IP Alias to Solve the Triangle Route Problem

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 163   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. 

If everything is working properly, you probably do not need to change the threshold settings as 
the default threshold values should work for most small offices. Tune these parameters when 
you believe the ZyWALL has been receiving DoS attacks that are not recorded in the logs or 
the logs show that the ZyWALL is classifying normal traffic as DoS attacks. Factors 
influencing choices for threshold values are: