ZyXEL Communications Prestige 652, User Manual, Page: 149 / 521

Prestige 652 Series User’s Guide
12-2 Firewall Configuration
12.3 Attack Alert
Attack alerts are real-time reports of DoS attacks. In the Attack Alert screen, shown later, you may choose
to generate an alert whenever an attack is detected. For DoS attacks, the Prestige uses thresholds to determine
when to drop sessions that do not become fully established. These thresholds apply globally to all sessions.
You can use the default threshold values, or you can change them to values more suitable to your security
requirements.
12.3.1 Alerts
Alerts are reports on events, such as attacks, that you may want to know about right away. You can choose to
generate an alert when an attack is detected in the
Attack Alert screen (Figure 12-2 - select the Generate
alert when attack detected checkbox) or when a rule is matched in the Rule Config screen (see Figure
13-4)
.
When an event generates an alert, a message can be immediately sent to an e-mail account that you
specify in the Log Settings screen (see the chapter on logs).
12.3.2 Threshold Values
Tune these parameters when something is not working and after you have checked the firewall counters.
These default values should work fine for most small offices. 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.
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), then the default values should be reduced.
You should make any changes to the threshold values before you continue configuring firewall rules.
12.3.3 Half-Open Sessions
An unusually high number of half-open sessions (either an absolute number or measured as the arrival rate)
could indicate that a Denial of Service attack is occurring. For TCP, "half-open" means that the session has
not reached the established state-the TCP three-way handshake has not yet been completed (see
Figure 11-2 ).
For UDP, "half-open" means that the firewall has detected no return traffic.