Pf - OpenBSD packet filter

Packet filtering is the selective passing or blocking of data packets as they pass through a network interface. The criteria that pf(4) uses when inspecting packets are based on the Layer 3 (IPv4 and IPv6) and Layer 4 (TCP, UDP, ICMP, and ICMPv6) headers. The most often used criteria are source and destination address, source and destination port, and protocol.