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What Does a Static Packet Filter Do?
The overall idea is that you need to very carefully control what passes between the Internet and the machine you have connected directly to the Internet. On the external interface to the Internet, you will individually filter what's coming in from the outside and what's going out from the machine as exactly and as explicitly as possible.
A packet filtering firewall consists of a list of acceptance and denial rules. These rules explicitly define which packets will and will not be allowed through the network interface. The firewall rules use the IP packet header fields to decide whether to route a packet through to its destination or not.