packet filter (PF)

Packet filters are software programs in Internetworking and security components such as routers, firewalls or proxy servers. They are used to select digital signals and can filter out data packets according to specified bit patterns. The filtered out data packets can be used for synchronization, determine the beginning and end of a message, support routing or be used to block data packets.

In security systems, packet filtering is one of the simpler firewall concepts. In this concept, a more or less complex set of rules is used to decide for each IP packet arriving at the firewall host whether it should be forwarded, discarded or rejected. For this purpose, certain data packet attributes such as the source address, destination address, packet length or port number are checked.

The oldest firewall concept is static packet filtering ( SPF), also known as stateless. In this concept, the bit constellations or network addresses are fixed. No data packets are inspected, only the security of a connection is determined. And there is the concept with dynamic pallet filtering, with Stateful Packet Inspection( SPI), where the routing information is filtered from the header of the data packet. A more far-reaching method is deep packet inspection( DPI), in which the payload is also examined for its information. For secure protection, the German Federal Office for Information Security( BSI) recommends the three-stage PAP model with two packet filters and an intermediate Application Layer Gateway( ALG).