iptables is the userspace command line program used to set up, maintain, and
inspect the tables of IPv4 packet filter rules in the Linux kernel. It's a
part of packet filtering framework which allows the stateless and stateful
packet filtering, all kinds of network address and port translation, and is a
flexible and extensible infrastructure with multiple layers of API's for 3rd
party extensions. The iptables package also includes ip6tables. ip6tables is
used for configuring the IPv6 packet filter.
Note that some extensions (e.g. imq and l7filter) are not included into
official kernel sources so you have to patch the sources before installation.