ZyXEL Communications ZyWALL5UTM 4.0 11.10.2.1 TCP Maximum Incomplete and Blocking Time, low, high, SECURITY, FIREWALL, Threshold, one-minute, one-minute, Blocking Time, Blocking Time

ZyWALL 5/35/70 Series User’s Guide

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.

11.10.2.1 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.

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

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

1If the Blocking Time timeout is 0 (the default), then the ZyWALL deletes the oldest existing half-open session for the host for every new connection request to the host. This ensures that the number of half-open sessions to a given host will never exceed the threshold.

2If the Blocking Time timeout is greater than 0, then the ZyWALL blocks all new connection requests to the host giving the server time to handle the present connections. The ZyWALL continues to block all new connection requests until the Blocking Time expires.

The ZyWALL also sends alerts whenever TCP Maximum Incomplete is exceeded. The global values specified for the threshold and timeout apply to all TCP connections. Click SECURITY, FIREWALL and then the Threshold tab to bring up the next screen.

Chapter 11 Firewall Screens 224