ZyXEL Communications P-3202HN-Ba, User Manual

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Chapter 12 Firewalls
IAD User’s Guide
151
12.4 The Firewall Threshold Screen
For DoS attacks, the IAD 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 69 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.
12.4.1 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 IAD has been receiving DoS
attacks that are not recorded in the logs or the logs show that the IAD 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.