Figure 162 Using Ip Alias To Solve The Triangle Route Problem; Figure 163 Three-Way Handshake - ZyXEL Communications 5 Series User Manual

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

DoS Thresholds
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.
Threshold Values
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:
ZyWALL 5/35/70 Series User's Guide
Chapter 13 Firewall Screens
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