Firewall Thresholds; Threshold Values; Figure 59 Three-Way Handshake - ZyXEL Communications ZyWALL 2Plus User Manual

ZyWALL 2 Plus User's Guide

8.10 Firewall Thresholds

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 59 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.

8.10.1 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:
1 The maximum number of opened sessions.
2 The minimum capacity of server backlog in your LAN network.
3 The CPU power of servers in your LAN network.
4 Network bandwidth.
5 Type of traffic for certain servers.
Reduce the threshold values if your network is slower than average for any of these factors
(especially if you have servers that are slow or handle many tasks and are often busy).
144 Chapter 8 Firewall Screens