ZyXEL Communications 792H - Manual (Page 138)

Prestige 792H G.SHDSL Router
9-4 Firewall Configuration
9.4.1 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 Alert screen (Figure 9-3 - select the Generate alert when
attack detected checkbox) or when a rule is matched in the Edit Rule screen (see Figure 10-5)
. 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).
9.4.2 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.
9.4.3 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 8-2).
For UDP, "half-open" means that the firewall has detected no return traffic.
The Prestige measures both the total number of existing half-open sessions and the rate of session
establishment attempts. Both TCP and UDP half-open sessions are counted in the total number and rate
measurements. Measurements are made once a minute.
When the number of existing half-open sessions rises above a threshold (max-incomplete high), the Prestige
starts deleting half-open sessions as required to accommodate new connection requests. The Prestige
continues to delete half-open requests as necessary, until the number of existing half-open sessions drops
below another threshold (max-incomplete low).
When the rate of new connection attempts rises above a threshold (one-minute high), the Prestige starts
deleting half-open sessions as required to accommodate new connection requests. The Prestige continues to