“Although Iptables gloms most of the glory, since it is the
packet filter included in the Linux kernel, there is an alternative
worth considering that is at least equally powerful, and easier to
use: pf. pf is developed and maintained on OpenBSD, but the easiest
way to get acquainted with it is by running m0n0wall.“m0n0wall is a specialized implementation of FreeBSD + pf
designed for routers and firewalls. It weighs in at well under 10
megabytes, while still delivering a complete operating system, a
firewall, Web administration, traffic shaping services, a DNS and a
DHCP server, SNMP, support for DynDNS updates, and a whole lot
more…”