ZyXEL Communications eircom D1000, Руководство пользователя

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156
eircom D1000 modem U
ser’s Guide
Chapter 13 Firewall
13.5 The DoS Screen
Use this screen to enable DoS protection. Click Security > Firewall > Dos to display the following
screen.
Figure 106
Security > Firewall > DoS
The following table describes the labels in this screen.
Table 61
Security > Firewall > DoS
LABEL DESCRIPTION
Denial of Services Enable this to protect against DoS attacks. The Device will drop sessions that surpass
maximum thresholds.
Apply Click this to save your changes.
Cancel Click this to restore your previously saved settings.
Advanced Click this to go to a screen to specify maximum thresholds at which the Device will start
dropping sessions.
13.5.1 The DoS Advanced Screen
For DoS attacks, the Device uses thresholds to determine when to start dropping sessions that do
not become fully established (half-open sessions). These thresholds apply globally to all sessions.
For TCP, half-open means that the session has not reached the established state-the TCP three-way
handshake has not yet been completed. Under normal circumstances, the application that initiates
a session sends a SYN (synchronize) packet to the receiving server. The receiver sends back an ACK
(acknowledgment) packet and its own SYN, and then the initiator responds with an ACK
(acknowledgment). After this handshake, a connection is established.
Figure 107
Three-Way Handshake
For UDP, half-open means that the firewall has detected no return traffic. An unusually high number
(or arrival rate) of half-open sessions could indicate a DOS attack.