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Chapter 9 Firewalls
Figure 90 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.
9.4.1 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 ZyXEL Device has been receiving DoS attacks that are not recorded in the
logs or the logs show that the ZyXEL Device 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).
• If you often use P2P applications such as file sharing with eMule or eDonkey, it's
recommended that you increase the threshold values since lots of sessions will be
established during a small period of time and the ZyXEL Device may classify them as
DoS attacks.
9.4.2 Configuring Firewall Thresholds
The ZyXEL Device also sends alerts whenever TCP Maximum Incomplete is exceeded. The
global values specified for the threshold and timeout apply to all TCP connections.
Click Firewall > Threshold to bring up the next screen.
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P-660HN-Fx User's Guide
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Troubleshooting
