Wuxi MitraStar Technology Co. Ltd DSL100HNUT1V3 User Manual

Chapter 14    Firewall

162

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 106 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. 

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 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 Device may classify them as DoS attacks.