ZyXEL Communications Corporation P660HNT1AV2 User Manual

Chapter 14 Firewall

P-660HN-Tx(A) v2 User’s Guide

Use this screen to enable DoS protection. Click Security > Firewall > Dos to display the following 
screen.

Security > Firewall > Dos

The following table describes the labels in this screen.

For DoS attacks, the ADSL Router 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. 

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. 

Security > Firewall > Dos

Denial of Services Enable this to protect against DoS attacks. The ADSL Router 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 ADSL Router will 
start dropping sessions.