ZyXEL p-2802h-i1 Manual Do Utilizador

 Chapter 11 Firewalls

P-2802H(W)(L)-I Series User’s Guide

The following table describes the labels in this screen. 

Table 65   Firewall: Threshold

Denial of Service 

Thresholds

The ZyXEL Device 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.

One Minute Low

This is the rate of new half-open sessions per minute that causes the firewall to 

stop deleting half-open sessions. The ZyXEL Device continues to delete half-

open sessions as necessary, until the rate of new connection attempts drops 

below this number.

One Minute High

This is the rate of new half-open sessions per minute that causes the firewall to 

start deleting half-open sessions. When the rate of new connection attempts rises 

above this number, the ZyXEL Device deletes half-open sessions as required to 

accommodate new connection attempts.
For example, if you set the one minute high to 100, the ZyXEL Device starts 

deleting half-open sessions when more than 100 session establishment attempts 

have been detected in the last minute. It stops deleting half-open sessions when 

the number of session establishment attempts detected in a minute goes below 

the number set as the one minute low.

Maximum 

Incomplete Low

This is the number of existing half-open sessions that causes the firewall to stop 

deleting half-open sessions. The ZyXEL Device continues to delete half-open 

requests as necessary, until the number of existing half-open sessions drops 

below this number.

Maximum 

Incomplete High

This is the number of existing half-open sessions that causes the firewall to start 

deleting half-open sessions. When the number of existing half-open sessions 

rises above this number, the ZyXEL Device deletes half-open sessions as 

required to accommodate new connection requests. Do not set Maximum 

Incomplete High to lower than the current Maximum Incomplete Low number.
For example, if you set the maximum incomplete high to 100, the ZyXEL Device 

starts deleting half-open sessions when the number of existing half-open 

sessions rises above 100. It stops deleting half-open sessions when the number 

of existing half-open sessions drops below the number set as the maximum 

incomplete low.

TCP Maximum 

Incomplete 

An unusually high number of half-open sessions with the same destination host 

address could indicate that a DoS attack is being launched against the host. 
Specify the number of existing half-open TCP sessions with the same destination 

host IP address that causes the firewall to start dropping half-open sessions to 

that same destination host IP address. Enter a number between 1 and 256. As a 

general rule, you should choose a smaller number for a smaller network, a slower 

system or limited bandwidth. The ZyXEL Device sends alerts whenever the TCP 

Maximum Incomplete is exceeded. 

Action taken when 

TCP Maximum 

Incomplete 

reached threshold

Select the action that ZyXEL Device should take when the TCP maximum 

incomplete threshold is reached. You can have the ZyXEL Device either:
Delete the oldest half open session when a new connection request comes.
or
Deny new connection requests for the number of minutes that you specify 

(between 1 and 255).

Apply

Click Apply to save your changes back to the ZyXEL Device.

Reset Click 

Reset to begin configuring this screen afresh.