19.1. Introduction to Firewalling with IPtables
A firewall is a system that protects itself and other hosts on a network from attackers on untrusted networks, such as the Internet. It can block packets and connections based on a variety of criteria, such as the source address, destination address, port, and protocol. Typically a firewall is also a router, forwarding packets between a secure local network and the untrusted Internet—however, it is also possible for a system to protect just itself.
A firewall system can also be configured to hide multiple hosts behind a single IP address, using a process known as NAT (Network Address Translation). Typically, the hidden hosts are on an internal LAN using a private IP network (such as 192.168.0.0) ...
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