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• Stateful Packet Inspection
This is called a “stateful” packet inspection because it examines the
contents of the packet to determine the state of the communications; i.e., it
ensures that the stated destination computer has previously requested the
current communication. This is a way of ensuring that all communications
are initiated by the recipient computer and are taking place only with
sources that are known and trusted from previous interactions. In addition
to being more rigorous in their inspection of packets, stateful inspection
firewalls also close off ports until connection to the specific port is
requested.
When particular types of traffic are checked, only the particular type of
traffic initiated from the internal LAN will be allowed. For example, if the
user only checks “FTP Service” in the Stateful Packet Inspection section,
all incoming traffic will be blocked except for FTP connections initiated
from the local LAN.
Stateful Packet Inspection allows you to select different application types
that are using dynamic port numbers. If you wish to use the Stateful Packet
Inspection (SPI) to block packets, click on the Yes radio button in the
“Enable SPI and Anti-DoS firewall protection” field and then check the
inspection type that you need, such as Packet Fragmentation, TCP
Connection, UDP Session, FTP Service, H.323 Service, or TFTP Service.
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