Packet filtering
PF is a packet filtering tool with simple syntax, user-friendliness, and extensive features. The packet filtering takes place in the kernel. A pseudo-device, /dev/pf, allows userland processes to control the behaviour of the packet filter (ioctl interface). There are commands to enable and disable a filter, load rule sets, add and remove individual rules or state table entries, and retrieve statistics. The most commonly used functions are covered by pfctl.