ZyXEL Communications Prestige 661H Series, User Manual

Page: 162 / 547

Prestige 661H/HW Series User’s Guide
Chapter 12 Firewall Configuration 162
12.12.1 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 most small offices. Factors influencing
choices for threshold values are:
• The maximum number of opened sessions.
• The minimum capacity of server backlog in your LAN network.
• The CPU power of servers in your LAN network.
• Network bandwidth.
• 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.
12.12.2 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 58 on page 134 ). 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 delete half-open sessions as necessary, until the rate of new
connection attempts drops below another threshold (
one-minute low ). The rate is the number
of new attempts detected in the last one-minute sample period.
12.12.2.1 TCP Maximum Incomplete and Blocking Time
An unusually high number of half-open sessions with the same destination host address could
indicate that a Denial of Service attack is being launched against the host.