Zyxel-communications 70 Series Manual de usuario descargar pdf (Pagina 228)

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ZyWALL 5/35/70 Series User’s Guide

Chapter 11 Firewall Screens 228

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

a threshold (TCP Maximum Incomplete), the ZyWALL starts deleting half-open sessions

according to one of the following methods:

1 If 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.

2 If 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.

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Zyxel-communications 70 Series Manual de usuario Pagina 228

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