ZyXEL Communications Prestige 652H series, User Manual

Page: 186 / 583

Prestige 652H/HW Series User’s Guide
13-24 Firewall Screens
13.9 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 a rule is matched in the
Edit Rule screen (see Figure 13-5)
.
Configure the Log
Settings screen to have the Prestige send an immediate e-mail message to you when an event generates an
alert. Refer to the chapter on logs for details.
13.10 Configuring Attack Alert
Attack alerts are the first defense against DOS attacks. In the Threshold 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.
13.10.1 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 normal small offices with ADSL bandwidth. 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.
13.10.2 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 12-2 ).
For UDP, “half-open” means that the firewall has detected no return traffic.