Linux System Calls Question:

Iptables is the userspace command line program used to
configure the Linux 2.4.x and 2.6.x IPv4 packet filtering
ruleset. Iptables allows administrators to configure the
operating system so that it allows applications and clients
to connect through the network and stop unwanted
applications and clients from communicating and corrupting
the operating system.
It is not specific to Redhat. It is available in all linux
2.4.x and 2.6.x kernels.