User`s guide

Chapter 9 Firewall Screens
LAN-Cell 2 User’s Guide
207
Figure 122 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 LAN-Cell has been receiving DoS attacks that are not recorded in the logs or
the logs show that the LAN-Cell 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 LAN-Cell may classify them as DoS attacks.
Security Considerations
" Incorrectly configuring the firewall may block valid access or introduce security
risks to the LAN-Cell and your protected network. Use caution when creating
or deleting firewall rules and test your rules after you configure them.
Consider these security ramifications before creating a rule:
1 Does this rule stop LAN users from accessing critical resources on the Internet? For
example, if IRC is blocked, are there users that require this service?
2 Is it possible to modify the rule to be more specific? For example, if IRC is blocked for
all users, will a rule that blocks just certain users be more effective?