Zyxel-communications Internet Security Gateway ZyWALL 100 Manual do Utilizador descarregar pdf (Página 167)

ZyWALL 100 Internet Security Gateway

Using the ZyWALL Web Configurator 15-7

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.

15.4.2 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 13-2).

For UDP, "half-open" means that the firewall has detected no return traffic.

The ZyWALL 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

ZyWALL starts deleting half-open sessions as required to accommodate new connection requests. The

ZyWALL 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 ZyWALL starts

deleting half-open sessions as required to accommodate new connection requests. The ZyWALL 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.

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.

Whenever the number of half-open sessions with the same destination host address rises above a threshold

(TCP Maximum Incomplete), the ZyWALL starts deleting half-open sessions according to one of the

following methods:

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Zyxel-communications Internet Security Gateway ZyWALL 100 Manual do Utilizador Página 167