ZyXEL Communications AMG1302-T11C, Руководство пользователя, Страница: 175 / 307

Chapter 15 Firewall
AMG1302-T11C User’s Guide
175
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 89
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.
15.5.1.1 Threshold Values
If everything is working properly, you probably do not need to change the threshold settings as the
default threshold values should work for most small offices. Tune these parameters when you
believe the AMG1302-T11C has been receiving DoS attacks that are not recorded in the logs or the
logs show that the AMG1302-T11C is classifying normal traffic as DoS attacks. Factors influencing
choices for threshold values are:
1
The maximum number of opened sessions.
2
The minimum capacity of server backlog in your LAN network.
3
The CPU power of servers in your LAN network.
4
Network bandwidth.
5
Type of traffic for certain servers.
Reduce the threshold values if your network is slower than average for any of these factors
(especially if you have servers that are slow or handle many tasks and are often busy).
• If you often use P2P applications such as file sharing with eMule or eDonkey, it’s recommended
that you increase the threshold values since lots of sessions will be established during a small
period of time and the AMG1302-T11C may classify them as DoS attacks.
15.5.2 Configuring Firewall Thresholds
Click Security > Firewall > DoS > Advanced to display the following screen.