User's Manual

Chapter 14 Firewall 162
For TCP, half-open means that the session has not r
eached 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 106 Th
ree-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.
14.5.1.1 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 Device has been receiving DoS attacks that are not recorded in the logs or the logs show
that the Device is classifying normal traffic as DoS attacks. Factors influencing choices for threshold
values are:
1 The
maximum number of opened sessions.
2 The minimum ca
pacity of server backlog in your LAN network.
3 T
he CPU power of servers in your LAN network.
4 Net
work bandwidth.
5 T
ype of traffic for certain servers.
Reduce the threshold values if your network is slower than average for any of these factors
(e
specially if you have servers that are slow or handle many tasks and are often busy).
• If you often use P2P applications su
ch 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 Device may classify them as DoS attacks.