How does a packet filter determine whether to allow or block a packet?

A packet filter determines whether to allow or block a packet primarily through the use of pre-defined security rules. These rules dictate how incoming and outgoing packets should be treated based on specific attributes, such as source and destination IP addresses, port numbers, and the protocol used (TCP, UDP, ICMP, etc.).

When a packet arrives at a network device that employs this filtering technique, the packet filter inspects the packet header and compares it against its list of rules. If the packet matches a rule that allows it, the packet proceeds; if it matches a rule that blocks it, the packet is discarded. This mechanism grants network administrators the ability to enforce security policies effectively based on the traffic patterns and characteristics they define in the rules.

Other options, while potentially relevant in broader contexts (such as deep packet inspection or analyzing packet size), do not accurately represent how a standard packet filter operates. A packet filter primarily relies on rules set by the administrator, which makes the approach efficient and straightforward for allowing or blocking traffic.